Theories, Techniques and Materials for Sealing Coalbed Methane Extraction Boreholes in Underground Mines: A Review

Abstract

To further enhance the intelligent technology, platformisation, and systematisation of coalbed methane extraction sealing technology, this paper analyses the research progress of theories, technologies, and sealing materials related to coalbed methane extraction sealing and systematically summarises the latest achievements of the basic theories, key technologies, and sealing materials of coalbed methane extraction. Considering the increasing mining depth, advancements in intelligent technology, and the evolving landscape of coalbed methane development, it is particularly important to establish a more comprehensive coalbed methane extraction borehole sealing system. Based on this, future development trends and research prospects are proposed: In terms of coalbed-methane-extraction-related theories, there should be a stronger focus on fundamental research such as on gas flow within the coal matrix. For coalbed methane extraction borehole sealing technologies and devices, efforts should be made to enhance research on intelligent, platform-based, and systematic approaches, while adapting to the application of directional long borehole sealing processes. In terms of coalbed methane extraction borehole leakage detection, non-contact measurement and non-destructive monitoring methods should be employed to achieve dynamic monitoring and early warning of methane leaks, integrating these technologies into coalbed methane extraction system platforms. For coalbed methane extraction borehole sealing materials, further development is needed for liquid sealing materials that address borehole creep and the development of fractures in surrounding rock, as well as solid sealing materials with Poisson’s ratios similar to that of the surrounding rock mass.

1. Introduction

The “Statistical Review of World Energy 2024” reports that global primary energy consumption increased by 2% in 2023 compared to 2022, reaching 620 EJ (Figure 1). Of this, coal accounted for 31.61% of global primary energy consumption. In 2023, coal remained a major fuel for power generation, maintaining a stable share of about 35%. Global coal production reached a historic high of 179 EJ (Figure 2), with the Asia-Pacific region contributing nearly 80% of the global coal production, primarily concentrated in Australia, China, India, and Indonesia. In 2023, global coal consumption surpassed 164 EJ for the first time, a 1.6% increase from 2022 and seven times the average growth rate of the past decade. Although China is the largest coal consumer, accounting for 56% of global consumption, India’s coal consumption for the first time exceeded the combined total of Europe and North America in 2023 [1]. Gas, as a byproduct of the coal formation process, poses widespread risks due to the reliance of most coal production capacity on underground mining [2,3,4,5]. This year, gas accidents have occurred in almost all major coal-producing countries. For instance, on 29 June 2024, a gas explosion occurred at the Grosvenor mine of Anglo American in Queensland, Australia; on 26 June 2024, a gas explosion happened at the Solikamsk mine in Perm Krai, Russia; on 10 March 2024, a gas explosion at the Hoshth mine in Balochistan, Pakistan, resulted in 12 fatalities; and on 1 February 2024, a gas explosion at a coal mine in Cundinamarca, Colombia, led to 2 deaths.

In China, greenhouse gas emissions from energy use, industrial processes, and methane amounted to 12.6035 billion tons in 2023, representing a 6% increase and accounting for 31.2% of global emissions. Under China’s “carbon peak and carbon neutrality” goals, coal consumption still constitutes 55.3% of total energy consumption. It is projected that by the end of the 14th Five-Year Plan, the share of coal in energy consumption will remain around 50% [6]. During the 2022 “Two Sessions” of China, President Xi Jinping emphasised that China’s energy structure is predominantly coal-based, and this fundamental reality will be difficult to change in the short term. Currently, China has over 3000 coal mines, most of which see gas concentration in extraction boreholes drop below 10% after two months of extraction. With the increase in mining depth, the pressure and content of coal seam gas rise, intensifying the risks of gas outbursts and explosions [7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23]. From January 1 to 19 March 2023, there was one coal and gas outburst accident, four dangerous coal and gas outburst incidents, and forty-eight high gas concentration exceedance accidents across China. As of 15 July 2024, three coal and gas outburst accidents and one gas explosion accident have occurred in China in 2024, resulting in 29 fatalities.

The frequent gas accidents worldwide pose significant challenges to the safe development of the coal industry. Therefore, effectively preventing and controlling gas-related accidents has become a top priority for the sustainable development of the global coal industry [2,3,4,5,7,8,9,10,11,12,17,18,19,20,24,25,26].

In order to achieve the “dual-carbon” goal as soon as possible, China has incorporated new requirements, such as the digital and intelligent transformation of traditional industries, into the process of energy revolution and high-quality development. China’s National Development and Reform Commission (NDRC) and Bureau of Energy (BEA), among others, have successively promulgated the “14th Five-Year Plan for Scientific and Technological Innovation in the Energy Sector”, the “14th Five-Year Plan for a Modern Energy System”, and the “Benchmarking Levels in Key Areas of Clean and Efficient Utilisation of Coal (2022)” and other documents. At the same time, with the development of science and technology, gas is receiving more and more attention as a clean energy source. “The Notice of the Comprehensive Department of the National Energy Administration on Organising and Carrying Out Demonstration Work of Efficient Extraction and Utilisation of Coal Mine Gas and Coalbed Methane Exploration and Development” and “the Implementation Plan of Green and Low Carbon Advanced Technology Demonstration Project” both take the efficient extraction and utilisation of coal mine gas as a key direction of the green and low-carbon advanced technology demonstration project. According to “the 14th Five-Year Plan for Coal”, it is expected that by 2025, the amount of coalbed methane (CBM) developed and utilised in China will reach 10 billion cubic metres.

At present, high-gas mines, prominent mines, and high-gas areas of low-gas mines in China are mostly treated or utilised by pre-extraction. However, according to incomplete statistics, more than 80% of the coalbed methane extraction concentration of down-hole down-grade boreholes in China will decay to 6–20% in a short time, and the pre-extraction rate of coal seam gas is less than 23% [7,9,10,12,27,28,29]. How to improve the coalbed methane extraction rate will directly affect the effectiveness of gas management and utilisation and consequently the timeframe for achieving China’s “double-carbon” goal.

In this context, this paper compiles the research history of coalbed methane extraction hole sealing theory, technology, and materials, summarises the gas-flow-related theory and the current status of research on coalbed methane extraction borehole sealing technology and sealing materials, and puts forward the future development direction to explain the research progress in related fields and provide references to prove the effect of coalbed methane extraction.

2. Coalbed Methane Flow Theory

Coalbed methane extraction is the process of pumping free gas and adsorbed gas desorbed by the extraction process from the coal seam into the coalbed methane extraction pipeline under negative pressure through surface drilling or downhole drilling [30]. The engineering theoretical basis of coalbed methane extraction is the theory of coal seam gas flow.

The coal body is divided into countless substrates by a large number of crisscrossing fracture networks, and a large number of pores exist within the coal substrate, forming a kind of dual-media structure [31,32,33,34], which makes the process of coal seam gas flow very complicated (Figure 3). Coal seam gas flow theory, also known as coal seam gas transport theory, mainly includes adsorption–desorption, diffusion, and seepage, which are the basis for describing the flow or transport law of gas in coal seams. Different coal seam gas flow laws are based on different models of gas flow in homogeneous or non-homogeneous media [33,35,36,37,38,39,40]. In terms of the mechanism of gas flow, although there is a basic consensus that the gas flow law follows Darcy’s law in fissures, there is still a great deal of controversy about the theory of gas flow in coal matrix, especially the driving force of gas flow [31,33,41,42,43,44]. Most scholars believe that the driving force for gas transport in coal matrix is the concentration gradient, and therefore Fick’s law is used to describe the gas diffusion process. However, some scholars believe that the driving force for gas transport in the coal matrix is the pressure gradient and argue that gas flow in the coal matrix is more in line with Darcy’s law; and some scholars believe that Darcy flow and Fick’s diffusion coexist in gas flow within the coal matrix [31,33]. In order to facilitate the differentiation, this paper divides the gas adsorption and desorption law of coal matrix into two parts according to the spatial distance: “Gas adsorption and desorption law near the surface of coal matrix” and “gas migration theory away from the coal matrix surface”.

2.1. Gas Adsorption–Desorption Pattern near the Surface of Coal Matrix

The gas in the coal body is divided into free and adsorbed states. In the coalbed methane extraction process, the free state gas is firstly extracted under negative pressure, and the adsorbed state gas is subsequently desorbed to become extractable gas. The gas adsorption and desorption process of coal matrix, especially the desorption process, is the key step of gas output. The study of gas adsorption and desorption laws is an important theoretical basis for constructing gas flow models and predicting the effect of coalbed methane extraction. Gas adsorption desorption models mainly include theoretical models and semi-empirical/empirical models [33].

2.1.1. Adsorption and Desorption Modelling

Coalbed methane adsorption models or desorption models are used to describe the adsorption and desorption processes of gas in coal and have attracted extensive attention from scholars, and a variety of gas adsorption and desorption models have been proposed [33,45,46,47,48,49,50,51,52,53]. The gas diffusion model of coal is an important tool for theoretical description of the adsorption and desorption process of gas in coal. With the deepening of research, new diffusion models have been proposed continuously.

In 1951, Barrer [54] derived the classical diffusion model by studying the diffusion of natural gas in zeolites and derived the exact solution of the diffusivity and its simplified equation. Nandi et al. [55,56] carried out a study of gas diffusion in coal and calculated the diffusion coefficients by using the classical diffusion model. Ruckenstein et al. [57] proposed a double-diffusion model to describe gas adsorption and desorption on the basis of the classical model. Smith et al. [52] fitted the measured gas diffusion data of coal with a dual-porosity model and a single-pore model, respectively, and found that the dual-porosity model had a higher fitting accuracy. Clarkson et al. [58] proposed an improved dual-porosity model in order to enhance the adaptability of the dual-porosity diffusion model. The classical model and the dual-porosity model explain the gas diffusion phenomenon better due to their strict derivation process and clear physical meaning and have been widely recognised and applied. Some scholars also introduced time-dependent or pressure/concentration-dependent kinetic diffusion coefficients and combined them with the Fickian diffusion model to describe the desorption process over the whole timescale, which resulted in a better match with the experimental desorption data [33].

Most of the above models assume that the gas flow in coal particles is driven by a concentration gradient, and it is valid to use Fick’s law to describe the gas diffusion in the coal matrix. In addition, a mathematical model for coalbed methane adsorption and desorption based on Darcy’s law has also been developed in the literature, and the numerical solution and experimental results of the gas adsorption and desorption model were compared through fixed-pressure and variable-pressure adsorption and desorption experiments, and it was found that the computational results of Darcy’s model were more consistent with the experimental data than the Fick’s classical model, which led to the assumption that Darcy’s law is more effective than Fick’s law in describing gas diffusion. Due to the extraordinarily complex structure of the pore system within the coal matrix, the pore size range extends from the angstrom scale to the micrometre scale [33,57,59,60,61,62,63,64]. The medium flow mechanisms in pores of different scales may be different, so the process of gas diffusion in coal is a multi-scale and multi-mechanism flow phenomenon, and the validity of describing gas diffusion using only Fick’s law is questionable. Many scholars believe that gas transport in coal matrix is a multi-mechanism process. Lunarzewski [65] argued that only the application of Darcy’s law can effectively describe gas transport in lump coals unless the coal suffers from excessive damage.

Gas diffusion refers to the movement of gas molecules from an area of higher concentration to an area of lower concentration, driven by the concentration gradient. It occurs on a molecular level and is not influenced by external forces like pressure differences or the presence of cracks.

Gas seepage refers to the slow movement of gas through porous materials or small openings, often driven by pressure differences. Seepage usually occurs in materials like rock or soil, where gas passes through tiny voids or pores, following Darcy’s law in most cases.

Gas leakage refers to the uncontrolled escape of gas through larger openings or cracks, usually due to a defect or failure in the sealing system. Leakage is often more rapid than seepage and typically results from structural flaws or pressure differences.

In summary, diffusion is molecular movement due to concentration differences, seepage is slow movement through porous materials due to pressure differences, and leakage is uncontrolled gas escape through visible cracks or openings.

Lu et al. [66] argued that the release of methane from coal matrix is a combination of gas diffusion and gas seepage, and that either diffusion or seepage dominates the process, which is mainly dependent on coal pore structure characteristics within the matrix. Experimental studies have shown that two types of pores exist within the coal matrix, one is diffusion pores that control methane desorption and diffusion, and the other is permeation pores that control methane permeation [67,68,69,70]. The triple-porosity/dual-permeability model assumes that methane in the coal matrix is transported through desorption and diffusion from micropores into meso/macropores, and then transported through meso/macropores and fissures by Darcy flow, and this type of model exhibits higher accuracy than the double-porosity/single-permeability model, suggesting that not only diffusive flow but also Darcy flow exist in the process of gas flow in the pore space of the coal matrix [33,67,68,69,70,71].

2.1.2. Semi-Empirical/Empirical Mathematical Modelling

In addition to the above mathematical models of gas adsorption and desorption derived from the basic theory, researchers have proposed a number of relational equations to represent the change in adsorption and desorption with time based on measured gas adsorption and desorption data or measured field data in order to facilitate the rapid calculation of gas adsorption and desorption. These empirical or semi-empirical relational equations mainly have two forms: Power function form and exponential form.

(1)

Power function equation

Barrer [54] obtained a simplified equation for gas adsorption versus time through adsorption experiments on zeolites based on the analytical solution of Fick’s classical model. This relationship equation usually has a power function form and can describe the variation of gas adsorption with time:

$${\displaystyle \frac{Q_t}{Q_{\infty }}}={\displaystyle \frac{2s}{V}}\sqrt{{\displaystyle \frac{Dt}{\pi }}}$$

(1)

where Q_t is the cumulative gas desorption volume at time t, cm³/g; Q_∞ is the limiting adsorption volume, cm³/g; s is the external specific surface area of the coal sample, cm²/g (the specific surface area refers to the total surface area of a material per unit mass. It can be categorised into two types: External surface area and internal surface area. The external specific surface area refers to the area of the outer surface of a solid material, which is the part of the solid in contact with the external environment. The external specific surface area does not include the pores or microporous structures inside the solid and only accounts for the outer surface); V is the volume per unit mass of the coal sample, cm³/g; D is the diffusion coefficient, cm/min; t is the adsorption time, min.

Nandi et al. [55,56] and Sevenster et al. [72] found that there are some limitations in the application of Barrer’s formula by fitting the data from adsorption–desorption experiments. In particular, when the adsorption amount is close to or more than half of the limiting adsorption amount, the prediction accuracy of Barrer’s formula will be significantly reduced, and the prediction error increases with time.

To overcome these limitations, Smith et al. [52] referred to Barrer’s formula and proposed an improved relation by studying experimental data:

$${\displaystyle \frac{Q_t}{Q_{\infty }}}={\displaystyle \frac{6}{\sqrt{\pi }}}\sqrt{D_{e}t}$$

(2)

where D_e is the effective diffusion coefficient, cm²/s.

(2)

Exponential

Bolt et al. [73] carried out gas desorption experiments on coal samples with various degrees of coal alteration, and by processing the experimental data, they proposed the equation for the relationship between cumulative gas desorption and time as:

$$Q_t=Q_{\infty }\left(1-A{e}^{-\lambda t}\right)$$

(3)

where A, λ are empirical constants.

Airey [74] argued that coal gas flow can be represented by Darcy’s law and stated that the expression for the amount of gas desorbed from coal versus time is:

$$Q_t=Q_{\infty }\left[1-e^{-{\left(t/t_0\right)}^n}\right]$$

(4)

where t₀ is an empirical constant.

2.2. Theory of Gas Migration Theory Away from the Coal Matrix Surface

Some studies have divided the pores in the coal matrix into an interconnected macroporous system and a disconnected microporous system, suggesting that gas desorbs and diffuses from microporous matrix to macroporous matrix, and then seeps out of the matrix macroporosity into the fissures. This approach uses two sets of equations to describe gas flow processes in the coal matrix [33,57,58,60]. However, this model has difficulties in accurately distinguishing between large and small pores in the coal matrix, the physical significance of the parameters is not sufficiently clear, and the complex calculation of the equations limits its generalised application. Therefore, scholars generally assume that all pores in the coal matrix have the same diameter and adopt a set of gas flow equations to describe the gas flow in the coal matrix. However, even under the assumption of the same pore diameter, there is still controversy about the driving force of gas flow.

2.2.1. Diffusion Theory

Some scholars have argued that the main driving force for gas transport in coal matrix is the concentration gradient, and thus gas flow conforms to Fick’s law [42,55,75,76,77,78,79]. Ertekin et al. developed a pseudo-steady-state gas transport model based on Fick’s first law by assuming that gas concentration varies with the spatial location of the coal matrix, while ignoring the effect of time on concentration [75]. However, gas transport in coal matrix is a non-stationary process related to both space and time, and ignoring the effect of time on concentration may lead to the physical properties of gas transport described by the model not matching the actual situation. Subsequently, some scholars have developed non-stationary gas transport models based on Fick’s second law, which integrates the effects of time and spatial location on gas concentration [33,78,79,80]. Some studies found that the model based on Fick’s law could not accurately describe the whole process of gas diffusion, especially in the initial stage of gas diffusion or during the stage of the maximum cumulative amount, where the issue of being unsuitable for Fick’s law arises [33,77,78,81].

In order to solve these problems, scholars have established a gas transport model based on Fick’s law with diffusion coefficient varying with time through experimental and theoretical studies [33,45,60,82,83,84,85,86,87,88,89,90,91]. Taking the diffusion coefficient as a variable makes the simulation results of the model more consistent with the experimental data and ensures the accuracy of the calculation results. However, the treatment of the diffusion coefficient change with time contradicts the initial assumption of Fick’s law, and scholars have not yet clarified the physical significance of this change. As a result, some scholars have become sceptical about using Fick’s law to describe the gas flow in coal matrix and try to use new theories to describe the gas transport process in coal matrix [33,67,92,93,94,95,96]. For example, based on the fractional dynamics approach, some scholars improved the unsteady gas transport model and established a time–space fractional order anomaly model for gas adsorption and desorption in coal matrix and validated the model by using mercury intrusion porosimetry (MIP), scanning electron microscopy (SEM) techniques, and adsorption and desorption experiments. This model contains several uncertain parameters, and the calculation process is complicated and needs to be determined with the help of a fractal dimension, which is not easy to apply. In addition, Fick’s law considers the total gas content in the coal body as the diffusion concentration of the gas, which leads to the calculated value of the gas transport being higher than the actual value. Liu et al. [97] corrected the defects in Fick’s law by distinguishing the free state gas from the adsorbed state gas in the coal matrix and established a non-linear gas flow model for coal matrix based on the difference of densities.

Meanwhile, simulation tools play an important role in describing the complex transport phenomena associated with CBM. Wang [98] summarised the basic theory of gas transport in nanopores, covering the mechanisms of gas transport in porous media and their simulation methods, including molecular dynamics and lattice Boltzmann simulation methods, as well as pore reconstruction methods (e.g., porosity, pore size, and pore shape) in diffusion and transport applications.

2.2.2. Theory of Seepage

Another group of scholars argued that the main driving force for gas transport in coal matrix is the pressure gradient, and thus the gas flow conforms to Darcy’s law [33,67,96,99,100,101,102]. Earlier, Airey [74] applied Darcy’s law to describe gas flow in a coal body; Zhou et al. [102,103] concluded that the gas flow laws in both coal matrix and fissures follow Darcy’s law and established a mathematical model of gas leakage from drilling holes, which provided theoretical guidance for improving the efficiency of drilling extraction and drilling sealing technology. Liu et al. [103] used different particle sizes and pressures under different conditions for the gas adsorption and desorption experiments, the gas flow models were established by Darcy’s law and Fick’s law, respectively, and the results of the solved model were compared with the experimental data. It was found that the mathematical model based on Darcy’s law was more in line with the experimental results. However, with further research, it was found that when the permeability coefficient of coal grains was inverted [103,104,105], the permeability coefficient decreased exponentially with the increase in pressure, and this change was quite different from the experimental results in the literature [106,107]. That is, the permeability coefficient of the coal matrix gas flow model based on Darcy’s law needs to change with pressure in order to match the experimental results with the simulation results. However, this is similar to the problem of the diffusion coefficient changing with time in Fick’s law, where the key constants in the model should not change with time or pressure.

Therefore, using either Fick’s law or Darcy’s law to describe the coal matrix gas flow process is problematic. In the development of coal matrix gas transport theory, the gas flow mechanism is still controversial, and new theories need to be proposed to accurately describe the coal matrix gas flow process through systematic and comprehensive theoretical and experimental studies and to establish a general, accurate, and easy to promote application of coal matrix gas flow models based on these new theories.

2.3. Solid–Gas Coupled Modelling of Gas Flow

The seepage characteristics of coal seam gas are affected by factors such as ground stress field, ground temperature field, ground electric field, acoustic field, pore pressure, sliding effect, and moisture of the coal body. Currently, there are several models, including “single-pore–single-seepage” model, “dual-porosity–single-seepage” model, “dual-porosity–double-seepage” model, the “single-pore–single-permeability” model, the“dual-porosity–single-permeability” model, the “dual-porosity–double-permeability” model, and the “triple-pore–double-permeability” model (Figure 4). Among them, the triple-porosity/double-permeability model takes into account the simultaneous existence of diffusion flow and Darcy flow during the flow of gas in the pores of the coal matrix and has a higher accuracy [108].

In terms of mathematical models of fluid permeability, scholars have achieved many important results. For example, some scholars proposed the capillary model, which is based on the assumption of capillary bundles and replaces the pore structure of the core with capillary bundles [109,110]; Fatt et al. established the spherical particle stacking model, which stacks spherical particles of certain diameters in a specific way and is used to simulate the pore space of reservoir rocks [111]; and some scholars used the lattice method to study the gas seepage in the Klinkenberg slip effect in gas seepage and the relative permeability curves of oil and water phases [111,112]. In recent years, with the rapid development of computer technology and image scanning and analysis technology, more and more researchers have established pore network models that are closer to the real pore structure of the core by reconstructing the pore network [113], such as the SD permeability model [114].

According to the pore structure of coal, the form of gas diffusion, and seepage characteristics, it can be classified into the following types from the point of view of the idealised model and the form of mutual coupling of coal and gas by the micro- and macrostructure of the coal body [33,66,67,68,69,70,71,74,75,76,77,81,83,115,116,117].

2.3.1. The “Monoporous–Monotonic” Model

The “single-pore–single-permeability” model idealises the pore structure of a gas-containing coal body as a homogeneous coal matrix model and considers only the flow of gas in the pores of the matrix. After the adsorption and desorption dynamic equilibrium of the gas in the coal matrix is broken, the gas is desorbed on the surface of the matrix, and then flows directly in the coal matrix; the gas pressure acts on the surface of the matrix, which affects the deformation of the solid matrix, and the solid–gas coupling model reflects the Darcy seepage law of gas based on the single-pore medium.

2.3.2. Dual-Porosity–Single-Permeability” Model

The “dual-pore–single-seepage” model idealises the coal pore system as a dual-pore medium structure, i.e., the coal matrix and the fissures. In the dual-pore structure, the scale of the fissures is much larger than that of the pores of the coal matrix, so only the seepage of gas in the fissures of the coal body is considered. When the dynamic equilibrium adsorbed on the microporous surface is broken, the gas desorbs on the microporous surface of the coal matrix, and then diffuses from the surface of the high-concentration coal matrix to the low-concentration coal body fissures, where it flows in the fissures. During this process, there is an influence of adsorption-induced deformation and gas pore pressure on coal matrix deformation; conversely, the volumetric and surface forces of the coal matrix also impact the gas pressure. This process is a strongly coupled process in which the deadsorption of gas from the pore surfaces of the coal matrix acts as a source for gas flow in the fractures.

2.3.3. The “Dual-Porosity–Double-Seepage” Model

The “dual-pore–dual-seepage” model idealises the pore structure of the coal body as a dual-pore structure of matrix pores and fissures. The main flow spaces of fluids in the coal seam are matrix pores and coal body fissures, i.e., matrix seepage and coal body fissure seepage are considered at the same time. When the dynamic equilibrium of gas adsorption in the coal body is broken, the gas adsorbed on the surface of the matrix microporosity of the coal body is desorbed, and a part of the desorbed gas flows in the pore space within the coal matrix, while another part of the desorbed gas is transported to the cracks of the coal body through diffusion, and then flows in the cracks. In this process, the interaction between the solid medium and the gas is also strongly coupled, and the desorbed gas from the matrix is the source and sink of both the pore flow of the gas within the matrix and the flow in the coal body fissures.

2.3.4. Triple-Porosity and Dual-Permeability Model

The “triple-porosity and dual-permeability” model idealises the coal body as a porous medium consisting of impermeable matrix, seepage matrix, and fissures. The main seepage space in the coal body is the seepage matrix and fissures, and the main factors affecting the permeability are the seepage pores and fissures.

The gas flow process in the coal body involves gas desorbing from the micropore surfaces of the coal matrix and diffusing into the pores of the flowing matrix under the influence of concentration gradients. The gas then flows through the pores of the coal flowing matrix driven by pressure gradients. Generally, gas always moves towards the space with the least flow resistance. Eventually, most of the free gas flows into the fracture structure of the coal body, and under the pressure gradient in the fractures, it continuously moves towards larger and macropores and is finally extracted to the surface.

Sang [115] proposed a typical triple-porosity and dual-permeability model, which considers a three-porosity medium composed of kerogen matrix (organic matrix), inorganic matrix, and fractures. The gas is mainly transported by diffusion in the casing matrix and seepage transport in the inorganic matrix and fissures; the amount of gas in the casing is then the source–sink term for the gas flow in the inorganic matrix and fissures, and the fissure gas flow is in accordance with the cubic law.

In summary, the research on solid–gas coupling theory has achieved relatively rich results. However, the impermeable matrix, permeable matrix, and fractures have the same computational domain in most of the coupling models, which is somewhat different from the gas diffusion and transport in the actual reservoir. Therefore, it is necessary to establish a solid–gas coupling model that distinguishes the computational domains of fractures and matrix to better investigate the gas flow law in reservoirs.

3. Sealing Technology for Coalbed Methane Extraction

After the coal body is exposed, the fissure is the main path of gas transport, its free gas gushes out under the action of a pressure gradient, the place of storing gas (coal matrix) is affected, and its adsorbed gas is desorbed from the inner surface of the pore and transported to the fissure within the pore space of the matrix and then gushes out of the coal seam [7,8,14,15,16,20,21,22,25,26,27,28,29,36,118,119,120]. Coalbed methane extraction is the process of extracting the free gas from the coal seam by negative pressure (Figure 5). The effect of coalbed methane extraction is not only related to the gas storage law, adsorption and desorption of the coal seam itself, but also related to the sealing effect of the coalbed methane extraction borehole. The level of coalbed methane extraction sealing technology determines the effect of coalbed methane extraction. The key to coalbed methane extraction sealing technology lies in coalbed methane extraction sealing parameters, sealing technology, and sealing devices.

3.1. Theory of Fracture Development in Drilling Surrounding Rock

The development of fissures in the surrounding rock of coalbed methane extraction boreholes directly affects the flow path and flow rate of gas. By studying the theory of fissure development, we can understand the distribution, scale, and connectivity of the fissures in the surrounding rock of the coalbed methane extraction boreholes, so as to optimise the reasonable sealing parameters of the boreholes, to ensure that the gas can be released and flowed smoothly from the coal seam, and to improve the efficiency of coalbed methane extraction.

3.1.1. Current Status of Coal Rock Drilling Perimeter Rock Stability Research

The construction of coalbed methane extraction boreholes is equivalent to the excavation of a small roadway or chamber, and the stress distribution around the borehole is similar to that of a circular roadway. At present, theoretical analysis and numerical simulation methods (Figure 6 and Figure 7) are mainly used to study the stress distribution of the surrounding rock in the excavation body [7,115].

Due to the complex field conditions of the mine, the mechanical properties and damage characteristics of the coal rock body by the roadway excavation and digging, back-mining, and drilling construction are more difficult to obtain, so some researchers have devoted themselves to the methods of obtaining the above parameters under laboratory conditions. To this end, some scholars have carried out research on the mining mechanical behaviour of coal rock bodies under different mining conditions, obtained the change in stress state experienced by the coal body in front of the workings, and investigated the strength characteristics, deformation characteristics, and infiltration evolution law of the coal samples during the loading and unloading process [121,122,123,124,125].

(1)

Classical mechanical analysis based on the Mohr–Coulomb criterion

Hu [121], based on classical elasto-plastic mechanics, equated the borehole stress field to a plane-strain circular hole problem, derived an expression for the stress distribution around the borehole, and combined with the empirical radius of influence, gave a relational equation for the relationship between the plastic zone of the borehole and the radius of influence as well as the diameter of the borehole. Lu et al. [126] conducted a theoretical analysis of the elasto-plastic stresses around the circular roadway. Hashemi [127] investigated the variation of particle displacement of the borehole wall under different factors.

Zhang et al. [128] proposed a new analytical solution for calculating the stress distribution around a circular excavation body. On the basis of the research results on the damage deformation of the roadway and coal rock borehole, the radius of the crushing zone and damage expansion zone is given in combination with the damage expansion characteristics of the coal body, and it is pointed out that it is too conservative to use only the Mohr–Coulomb criterion to determine whether the coal body is in a plastic state or not. Even if the stress state is unchanged, the coal body may be damaged after a certain period of time after the yield state.

(2)

Classical mechanics analysis based on Hoek–Brown criterion

Most of the above studies are based on the linear Mohr–Coulomb criterion, but in engineering practice, due to the inherent non-linear characteristics of rocks and rock bodies, as well as the structural surfaces, the effect of stress state on strength, and anisotropic rock bodies, the linear Mohr–Coulomb criterion is not applicable, especially in the case of coal rock bodies which are mostly jointed, and the damage characteristics tend to be more in line with the non-linear Hoek–Brown criterion [129]. Brown et al. [130] and Lee et al. [131] investigated and obtained numerical solutions for the stress field of strain softening in the chamber enclosure.

Zhou et al. [132], based on the Hoek–Brown criterion, studied the strength damage patterns of sandstone under different unloading rates. They proposed an index to describe the changes in rock strength due to unloading, introducing the concept of the unloading strength damage factor, and modified the Hoek–Brown criterion through engineering examples. Sheng [133] used the non-linear Hoek–Brown criterion with the non-correlated flow method to derive the formulae for the deformation characteristics of the tunnel-enclosing rock and the Romberg method to give the stress solution, and finally the deformation solution of the tunnel-enclosing rock is obtained, and the reasonableness of the theoretical derivation and the final solution is verified by comparing the engineering examples with the Carranza-Torres method.

Tu et al. [134] proposed an elastic–plastic calculation method for deep coal seam borehole collapse pressure based on the radius of the engineering-permissible plastic zone based on the Hoek–Brown criterion and verified the correctness of the method by combining it with engineering examples and comparing with the Hoek–Brown criterion and the Mohr–Coulomb criterion.

At the same time, due to the Hoek–Brown criterion, related parameter selection is difficult, and a large number of scholars have conducted related research and achieved significant results.

Xia et al. [135] gave the formulae of GSI and D parameters in the Hoek–Brown criterion based on rock wave velocity and compared the Hoek–Brown criterion with the rock wave velocity method based on the actual engineering slopes, and the results showed that the rock wave velocity method can give the mechanical parameters of the rock body quickly and accurately in the case of insufficient test data. Yang et al. [136] estimated the rock body properties in the excavation damage zone based on the generalised Hoek–Brown damage criterion and a varying perturbation factor to estimate the rock mass properties in excavation damage zones. The results showed that the perturbation factor was quantified by using acoustic P-wave velocities tested in both undamaged and damaged rock mass, rather than a single constant value selected from a descriptive guide. Chang et al. [137], in order to address the traditional method involved in the determination of the perturbation factor (D) when considering mining damage, which cannot reflect the anisotropic characteristics of jointed rock strength, proposed a numerical analysis method to determine the D value using the jointed rock damage tensor and mining damage evolution simulation.

Sun et al. [138] constructed a new method for numerical modelling of jointed rock bodies based on the geological strength index (GSI) and the principles of damage mechanics. With the help of the basic concept of damage mechanics, the joint equivalence coefficient was proposed, and the relationship between GSI and joint spacing was established. Sun et al. [139] quantitatively described the quantitative relationship between GSI and the integrity coefficient of the rock body and the degree of rock weathering in the Hoek–Brown criterion, and then determined the relevant rock mechanical parameters in the Hoek–Brown criterion, and the parameters obtained by this method were compared with the actual engineering to obtain a small error, which verified the reasonableness of the method. Compared with the actual engineering, the error obtained by this method is relatively small, which verifies the reasonableness of this method. Bertuzzi et al. [140] constructed a new quantitative table of GSI using the interval number theory, which solves problems such as the number of volumetric joints of the rock body being unable to be determined accurately and provides a new basis for the reasonable selection of parameters in indoor rock mechanics experiments. Liu et al. [141] introduced the spacing of the nodal surfaces in the rock body and JRC into the quantitative relationship of GSI in the Hoek–Brown criterion, and then determined the related rock body mechanical parameters in the Hoek–Brown criterion. Brown’s criterion of quantitative GSI grading and parameter values established a quantitative GSI evaluation system and successfully predicted the related rock mechanical parameters by combining with the actual engineering fissured rock. Fischer et al. [142], based on the quantitative GSI perimeter rock rating system and the theory of continuum media, established a mechanical model of post-peak strain softening of fractured rock considering the influence of the perimeter pressure and used the elasto-plasticity. The intrinsic model developed explicit structural models for intact rocks and provided realistic parameter ranges for the explicit structural properties. Some scholars [140,141,142,143,144] quantified the formulae describing the GSI with the volume nodal number of the rock body and the structural surface condition factor, combined them with the Hoek–Brown criterion to estimate the mechanical parameters of the rock body, and verified the reasonableness of the results through specific examples.

Some scholars [143,144] summarised the strength discounting method based on the Hoek–Brown criterion, derived the stress-invariant expression form of the Hoek–Brown criterion, proposed the equal proportional discounting scheme of σci and mb, and proved the correctness of the scheme through numerical simulation.

(3)

Enclosure stability analysis by other methods

Chuanqing Zhang et al. [145] used yield proximity and its phase-complementary covariates as random variables and proposed an inversion algorithm to derive the relevant parameters and proposed an analysis method for the safety of the surrounding rock based on the statistical law of rock strength.

Fu et al. [146] introduced the concept of local energy release rate and established an unloading instability analysis model for the goaf. Based on catastrophe theory, they developed a local energy criterion for identifying rock instability. They concluded that in the goaf, energy is primarily released in the horizontal direction, while energy is concentrated in the vertical direction.

Zhou Hui et al. [147] investigated the relationship between the non-yield point and yield point in the principal stress space for four yield codes, defined the yield proximity index, and gave the expression of yield proximity under the four yield codes, and the research results can be better used in the evaluation of stability of the surrounding rock of underground engineering. Li [148], based on the Mohr–Coulomb criterion, Drucker–Prager criterion, and elastic–plasticity theory, derived the safety factor of tunnel-surrounding rock, and combined with numerical simulation, the corresponding support measures were selected through a tunnel-surrounding rock example. Gaede et al. [149] analysed the stresses around the borehole in anisotropic media.

(4)

Analyses of the stability of the surrounding rock considering the case of drilling or roadway support

Wang et al. [150] derived a formula for calculating the circle of perimeter rock relaxation based on the Hoek–Brown criterion combined with elastic–plasticity theory when the pressure measurement coefficient is not equal to 1 and the support force is considered. The displacement monitoring of the perimeter rock in deep tunnels is close to the theoretical calculation results, which indicates the correctness of the derived formula for calculating the circle of perimeter rock relaxation.

In common construction practices, gas extraction boreholes typically require grouting for sealing. The grouting material in the sealing section provides some support to the borehole, which has a certain impact on the stability of the surrounding rock. A few scholars have also studied the stability of the surrounding rock under the support effect of the sealing section.

Some scholars [151,152,153] analysed the stability of the drilled hole sealing section by drawing on the relevant results of roadway support and introduced the analogy of roadway support in the drilled hole sealing section in the plastic radius of the surrounding rock and the deformation of the surrounding rock to measure the stability of the drilled hole sealing section and ultimately the Mohr–Coulomb criterion to determine that the sealing section is in the plastic zone or the crushing zone. Some other scholars [154,155,156,157], based on the results of roadway support, gas-containing coal constitutive model, elastic–plastic mechanics, etc., derived the expression of effective stress of coal body around the borehole wall of horizontal and inclined boreholes under the consideration of pore pressure and gave the corresponding collapse pressure of the borehole wall by combining it with the Mohr–Coulomb criterion and established the viscoelastic non-linear creep constitutive model of soft coal and combined it with the numerical simulation to analyse the stability of borehole collapse.

3.1.2. Current Status of Research on Fissures in the Surrounding Rocks of Coal Rock Drill Holes

Field practice and experimental research show that the interaction between the ground stress and the surrounding rock of the roadway will produce different sizes of rupture zones. Many researchers mainly use field measurements or indoor tests to study the distribution characteristics of the rupture zones in the surrounding rock of the excavation body.

Kong et al. [158] numerically simulated the development of fissures in actual thin coal mining workings in the void zone based on the discrete element software UDEC 6.0. Liu et al. [159] combined the rock mechanics with the gas flow law of coal rock in the process of full stress–strain and used a deep displacement automatic detector and a fissure channel circuit recorder to obtain the change in gas fissure channel with the working’s back mining, from primary fissure to five stages of fissure channel generation, expansion, maturity, and closure.

Yu et al. [160] studied the generation and development process of perimeter rock fissures in deep roadways, put forward the concepts of plate-like structure and block structure, and classified the perimeter rock fissures into four partitions, namely, macrotensile fissure zone, macroscopic shear fissure zone, high-incidence and microscopic fissure zone, and weakly influenced fissure zone. Xu et al. [161] used UDEC to simulate the distribution characteristics, expansion law, and relative evolution tendency of the perimeter rock fissure field of roadways under different spans. Li et al. [162] analysed the fissure development and destabilisation characteristics of a mudstone roof under different water content conditions by means of borehole peeping equipment, revealing that there is a saturation phenomenon in the fissure development and that controlling the development of the microfissure zone into the fissure development zone is the key to the support of the anchor rod technology. Li et al. [163] studied the deformation characteristics of randomly distributed through fissures generated after the excavation of underground chambers and established a computational model for the deformation of the rock body with randomly distributed through fissures, which showed that the equivalent modulus of elasticity gradually decreased with the increase in the average spacing of the fissures, and the equivalent Poisson’s ratio gradually increased with the small average inclination angle.

Fu et al. [164] introduced the definition and calculation method of fissure strain based on the CD fissure strain model, derived the fissure volumetric strain formula, defined the corresponding characteristic stresses at each stage of coal body fissure development as fissure closure stress, crack initiation stress, damage amplification stress, and peak stress, and concluded that the process of damage amplification of the coal body is the process of gradual compaction of fissures within the coal body, the concentration of stress at the tip that causes the expansion of primary fissures, newborn fissure generation, and then the development, connection, penetration, and formation of macroscopic damage surfaces.

Hu [165] firstly used a similar simulation device in the laboratory to simulate the unloading damage of the surrounding rock around the borehole in the peripressure environment, and the borehole was changed from a round shape to a rugby-ball-like shape, the top and bottom of the borehole collapsed, and the left and right sides were damaged to form a crushed zone. A statistical analysis of the fracture of the borehole was carried out by using a borehole peeper, and three kinds of macro- and microscopic fractures in the borehole were found, namely, straight fractures, curved fractures, and bifurcation fractures.

Some scholars [166,167,168] tested the acoustic emission of coal samples with different degrees of fissure development and non-uniform distribution of fissures during the loading process, and the studies showed that the acoustic emission energy of the coal samples with uniform distribution of fissures occurred at the beginning of the elastic deformation, and then incremented to the vicinity of a stable value when the coal samples were loaded.

Yang et al. [169] estimated the fracture density of rock cores by measuring the P-wave and S-wave velocities. The results showed that there is a negative correlation between elastic wave velocity and fracture density. Additionally, the wave velocity of the rock cores varied significantly at different measurement frequencies, exhibiting strong dispersion characteristics. Yu et al. [170] investigated the fracture damage of coal samples under repeated water immersion and showed that with the increase in the number of immersion times, the closure stress threshold of the coal sample fracture, the threshold of the initial development of the fracture, and the threshold of the fracture damage are linearly reduced.

3.1.3. Current Status of Research on Perimeter Rock Fissures of Coal Rock Drill Holes Considering Fissure Roughness

The above studies mainly focus on theoretical analysis, numerical simulation, on-site monitoring, etc., but all of them consider the roadway or coal rock drilling holes in accordance with the regular shape, while in engineering practice, due to the blasting vibration, drilling rig vibration, and various factors of excavation and the rock body’s own anisotropic, non-homogeneous, uneven distribution of pore space, joint, and other characteristics, which lead to the unevenness of the surface of the coal rock drilling holes and the roadway, the unevenness of the inner surface of the roadway is even higher. As we all know, considering engineering examples, the rock body damage and fracture generation preferentially occurs in a weak surface or local stress concentration point due to the contact surface unevenness and expands and develops in the weak surface or local stress concentration point. The roughness of the roadway or coal rock drill hole has an important influence on the shear strength of rock body damage. Therefore, the unevenness of the surface of the roadway or coal rock drill hole, i.e., the surface roughness, has a crucial influence on the damage of the surrounding rock and the development of the fracture distribution of the coal rock drill hole [119,124,151,152,153,171,172,173,174,175,176,177].

Yang et al. [178] prepared rock-like specimens of non-persistent jointed rock bodies containing circular holes and tested them under uniaxial compression by modelling with discrete element software PFC 4.0, which suggested that cracks were generated at the joint locations and concentrated at larger bulges, and the number of cracks was positively correlated with the JRC and normal stress. The results showed that the peak strength and modulus of elasticity of the non-persistent jointed rock samples varied in a “U” shape with respect to the joint inclination. Noori et al. [179] investigated the fracture behaviour and microcracking process of anisotropic sandstones by means of experimental tests and particle flow code (PFC2D). The fracture behaviour of anisotropic sandstones was modelled by bonding particles’ discrete element model (DEM) and embedding weak surfaces (smooth contact model) in intact rocks and compared with laboratory results to verify the accuracy of the model.

Some experts [180,181] quantitatively characterised the roughness of rock discontinuity surfaces by employing fractal geometry methods, but the existing methods suffer from computational complexity, inaccuracy, and difficulty in characterising undulations and anisotropy. Through 3D laser scanning and ArcGIS data processing, the discontinuous surface morphology of artificial granite samples is quantitatively investigated, and an extended 3D fractal dimension containing three parameters is proposed, which is able to comprehensively characterise the roughness, undulation, and anisotropy and has the advantages of simplicity, high efficiency, and high accuracy. Fan et al. [182], in order to study the three-dimensional joint roughness coefficients (JRCs) by laser scanning technology, a surface morphology test, and direct shear test, proposed a method to convert 3D surface morphology to a 2D mean height profile and calculated JRC values for 15 nodules based on statistical parameters, used the improved Barton empirical formula to calculate peak nodule shear strength, and verified the reliability of the method in laboratory studies.

Chen Shijiang et al. [183] systematically summarised the development of JRC and concluded that the fractal dimension is still an effective tool for quantitative description of JRC and the application of 3D printing technology can effectively promote the study of JRC. Liu Xige et al. [184] used 3D printing technology and PFC to carry out double rough structural surface shear test research and concluded that the reason why the shear strength of a double structural surface is lower than the shear strength of either structural surface is that the rough morphology of the structural surface leads to the destruction of the intermediate interlayer of rock in the first place, which then leads to the overall shear strength of the double structural surface being greatly reduced. Tang et al. [185] proposed a new peak shear strength formula based on Grasselli structural surface roughness coefficients, and the new shear formula conforms to the Mohr–Coulomb criterion and has a clear physical significance. Cai et al. [186] proposed a three-dimensional roughness research index I_pap that can reflect anisotropy and compared it with Grasselli roughness evaluation method in engineering applications. The validity of I_pap was verified.

3.2. Rational Sealing Parameters for Coalbed Methane Extraction

3.2.1. Coalbed Methane Extraction Borehole Leakage

In the case that the coal rock body is relatively intact and not affected by tectonic or external forces to form large joints through the coal rock body, the reasonable sealing parameters of the coalbed methane extraction borehole directly depend on the distribution of the fracture field around the borehole. A schematic diagram of the air leakage fissure field of a coalbed methane extraction borehole is shown in Figure 8 [119].

The degree of air leakage from the borehole is an important indicator of the quality of the sealing hole, and there are three main ways of air leakage, and the actual air leakage is a combination of these three forms:

(1)

High permeability of the sealing material or the existence of cracks, resulting in the flow of air from the sealing material into the borehole, as shown in Figure 9a;

(2)

The hole sealing material cannot seal the fissures developed in the surrounding rock of the borehole, resulting in air leakage through these fissures, as shown in Figure 9b;

(3)

The depth of the sealed hole does not exceed the “zone of penetration between the fissure of the surrounding rock and the free surface”, which leads to the connection of the fissure with the fissure inside the sealed hole section and, consequently, the air leakage, as shown in Figure 9c.

In order to simplify the problem of drilling “air leakage”, it is assumed that the surrounding rock around the drill hole is absolutely dense before excavation, i.e., the maximum pore and fissure scale is smaller than the diameter of gas molecules, and the permeability is zero. Then, the common point of the above three air leakage pathways is that the air leakage channel should eventually communicate with the free surface of the roadway. Even if the perimeter rock fissure of the drill hole has a high degree of development, if it is not ultimately connected with the free surface of the roadway wall, the drill hole will not leak air.

Therefore, it is possible to explore a classification based on whether fractures can connect with the free surface of the roadway, dividing the surrounding rock fractures into a “fracture-connected zone” and a “fracture-non-connected zone” with the free surface. Theoretically, it is difficult to judge whether the fissure is communicating with the free surface of the roadway, but in reality, there is permeability in the surrounding rock, and the gas inside the drill hole may seep into the roadway through the surrounding rock or the sealing material. Therefore, the problem of “air leakage” from the borehole is theoretically unavoidable.

Currently, research on permeability is relatively mature, and it is recommended to consider using permeability as a quantitative criterion for determining whether a borehole is “leaking” or whether the surrounding rock fractures are connected to the free surface. This can be specifically referenced against the critical permeability values used in standards for assessing the difficulty of coalbed methane extraction.

3.2.2. Study on Reasonable Sealing Depth of Coalbed Methane Extraction Boreholes

Reasonable sealing parameters for coalbed methane extraction mainly include sealing depth and sealing length. The fissures around the roadway and coal rock drilling holes are directly related to the sealing depth and sealing length of the underground coal seam gas drilling holes. Reasonable sealing depth and sealing length can block the leakage channel of the drilling holes, save costs, improve the efficiency of coal seam coalbed methane extraction, and ensure the normal development of the coal mine safety production work. Some scholars, based on the theory of loosening circles, unloading zone plasticity zone division, drilling destabilisation damage, and other bases, described the development of fissures in the peripheral rock of the roadway and coal rock drilling holes, and then determine the degree of reasonable sealing depth of coal rock drilling holes underground, and the related research results are abundant [7,18,119,167].

Some scholars think that the fissure system in the plastic zone of the roadway is very developed, that it is difficult to completely seal it, and it is better to choose the starting sealing depth of the drilling hole to avoid the plastic zone of the roadway [7,18,20,22], however, other scholars, on the basis of the distribution of the stress of the peripheral rock of the roadway as well as elasticity and plasticity characteristics of the coal rock body in the stress areas, concluded that the depth of sealing of drilling holes should be greater than the decompression area of the peripheral rock of the roadway but at the same time should be smaller than the depth of the peak stress point of the coal wall [187,188,189,190,191]. Some experts even conclude that air leakage can only be prevented when the sealing length of the drill hole exceeds the stress concentration area of the roadway [192].

When the depth of the sealing hole covers the peak stress point, the peak stress point and the sealing section correspond to the double peaks of the pressure distribution diagram of the surrounding rock of the drilling hole, which indicates that, in addition to the widely recognised rule that “the peak stress point has an adverse effect on coalbed methane extraction in the area, and there is a barrier effect”, there is also a similar effect in the vicinity of the sealing section. The main reason for the unsatisfactory coalbed methane extraction effect is that, near the peak stress point, the sealing section just covers the peak stress point, which is hindered by the blocking effect of the sealing section and the peak stress point on the flow of gas at the same time, and the superimposed effect of the two makes this place rely on the natural discharge of gas from the roadway, which ultimately leads to the formation of a gap in the coalbed methane extraction zone because very little gas can be discharged from the borehole; when the sealing depth of the coalbed methane extraction borehole is deeper than the peak stress point, there is a similar effect in the area. Although there is no superimposed obstruction effect of the sealing section and the peak stress point, the permeability of the coal seam near the peak stress point is low, and there is also a situation of high gas pressure and gradient when gas is discharged naturally only by the roadway.

3.2.3. Research on Reasonable Sealing Length of Coalbed Methane Extraction Boreholes

Based on the reliability of sealing materials, the impact of different sealing lengths on gas drainage effectiveness is minimal. In fact, longer sealing lengths may lead to a reduction in the effective drainage length, which in turn can result in a higher gas pressure gradient near the sealed section, thus adversely affecting the improvement of gas drainage efficacy.

At the same time, the increase in sealing length can be regarded as changing the reliability of the extraction system through the series connection of shorter sealing segments. Although the increase in sealing length can certainly improve the reliability of the extraction system, when the reliability of sealing can be ensured by shorter sealing segments, the significant increase in material and process costs is less effective. To sum up, under the condition of ensuring the reliability of the sealing section, reducing the length of the sealing hole appropriately can make the coalbed methane extraction effect improve.

It should be pointed out that in the current coalbed methane extraction work, it is difficult to ensure that the inner wall of the coalbed methane extraction borehole is completely well supported. As the coalbed methane extraction proceeds, the pore pressure of the coal body gradually decreases, and the unsupported area of the borehole will inevitably undergo damage, creating new fissures, which will affect the sealing effect of the sealed section of the coalbed methane extraction borehole. At the same time, due to the creep characteristics of the coal rock body, the stability of the coalbed methane extraction drilling and sealing section will inevitably deteriorate with the passage of time. The above two factors leading to instability or sealing deterioration of the borehole are difficult to solve by the existing sealing materials and techniques. If the sealing depth is in the unpressurised zone where plastic deformation has already occurred, the peak stress point will continue to move to the depth of the roadway as coalbed methane extraction proceeds, while the surrounding rock in the unpressurised zone of the roadway will be subjected to a stable state of stress and maintained at a small value. Therefore, under the premise of reliable sealing materials, trying to determine the sealing depth in the depressurisation zone may, on the contrary, have some reference value for the improvement of the sealing performance of the borehole.

3.3. Coalbed Methane Extraction Hole Sealing Process

3.3.1. Single-Plug Seal Drilling Technology

Single-plug seals are not common nowadays and can be divided into two forms: Non-grouted and grouted [9,193]. The non-grouting type is mainly seen in traditional clay (yellow mud) sealing or mechanical sealer and friction-type sealer used for shallow hole injection or shallow hole extraction. Polyurethane, cement rolls, capsules, etc. are mainly used as a single plug, and then the plug is placed in a suitable position according to the inclination of the borehole and then grouted. The single plug only relies on the self-weight of the plug and sealing material or the force with the inner wall of the borehole to achieve the sealing of the borehole. Mechanical sealers, single bladders, or polyurethane are used for temporary plugging of holes at the head of a coal mine or at the face of a rock portal for coalbed methane extraction. Mechanical sealers are relatively expensive compared to the bag sealers commonly used today and are difficult to use on a large scale to seal gas pre-extraction holes. The polyurethane foaming speed is faster, the strength is lower, and polyurethane material has a certain degree of toxicity. The “one plug, one injection” sealing method is mostly used for sealing drill holes with a large inclination angle. However, with the development of coalbed methane extraction sealing technology, there are more forms of sealing for inclined drill holes. The “one plug, one injection” sealing method does not allow for a pressure-retaining grouting process, which is not conducive to the penetration of the slurry into the fissure and does not allow for adequate sealing of the borehole.

In the early days, there was no special sealer for cement mortar sealing, and only the sealing cement was injected into the hole to seal it, so the length of the sealing hole was limited. The polyurethane sealing is used to install the chemical reaction “A” and “B” materials on the coil and then install the coil on the extraction pipe and send them into the hole together to seal the hole. Due to the special characteristics of the material, this sealer can only be made on site before use. Before the “two plugs and one injection” sealing process was widely used, most of the commonly used downhole sealers were mechanical sealers. The principle of this kind of hole sealer is to seal the hole opening section of the borehole through the deformation of the device itself and the hole wall to form a squeeze, friction, and other effects of fixing, and the corresponding hole sealers mainly include friction-type hole sealers and hydraulic expansion-type hole sealers. Mechanical hole sealers are simple in structure and easy to operate and can be reused, but the blocking part is mainly the drilled hole section, which cannot block the fissure inside the hole wall, so the blocking ability of the gas drilling hole is limited. In addition, the limitations of the principle and structure of the hole sealer lead to higher requirements for the drilled holes, and the sealing effect is good for smooth and stable drilled holes. Therefore, this type of hole sealer is suitable for temporary sealing and not suitable for long-term work.

3.3.2. “Two Plugs and One Injection” Seal Drilling Technique

The “two plugs and one injection” technique is widely used in the field of coalbed methane extraction and hole sealing and was developed based on the principle of “solid sealing liquid, liquid sealing gas” in the pressure measurement technology of Zhou Shining, an academician of the Chinese Academy of Engineering [194]. “Two plugs” refers to the use of two sealing plugs, which can be either packers or fast-curing sealing materials. “One injection” refers to a single grouting process, where sealing material is injected into the confined space formed between the two plugs and the borehole wall [9]. At this stage, based on building on the “two plugs and one injection” method, advancements have been made by improving aspects such as plugging heads, sealing materials, borehole stress conditions, and automation. By continuously combining related technologies, several techniques have been developed, including “packer-type sealing”, “strong–weak–strong sealing”, “three plugs and two injections”, “integrated packer sealing”, “casing sealing”, “integrated multi-channel sealing”, “solid–liquid coupled fluid-wall sealing”, and “drill–cut–seal” techniques. Additionally, a multi-injection technology has been developed based on the initial single-injection method [9,121,171,193,195].

However, as the extraction continues, the fissures around the drill holes develop, which in turn induce gas leakage from the drill holes. In view of the coal seam coalbed methane extraction leakage and the dynamic characteristics of the fissures around the drill hole, experts and scholars in related fields have proposed the concept of dynamic sealing of coalbed methane extraction boreholes in order to achieve the purpose of uninterrupted repair of the fissures and leakage channels in the boreholes. The difference is that the former section has a limited number of grouting times, generally two times, while in the latter sealing section of the hole, there is a dynamic grouting section. To this end, researchers have developed the “multiple plugs and multiple injections” sealing methods, as shown in Figure 10, Figure 11 and Figure 12, including the “three plugs and one injection” sealing method, the dynamic sealing method, and the integrated sealing method.

3.3.3. Other Seal Drilling Techniques

In addition to the above commonly used coalbed methane extraction borehole sealing techniques, a large number of scholars have attempted to study coalbed methane extraction borehole sealing techniques from other perspectives. Zhou Fubao et al. [8,21,196] proposed the concept of secondary sealing, which is used to plug the air leakage fissure by inputting ultrafine powder particles into the sealed drill holes, thus achieving the purpose of reducing air leakage and increasing gas concentration. Ji et al., by analysing the conventional negative-pressure pumping and nitrogen injection to strengthen the pumping technology, proposed the pulse-pressure nitrogen injection technology and carried out gas replacement experiments, which showed that pulse-pressure nitrogen injection is more efficient than uniform pressure and is more important for the efficient use of gas resources. It can improve coalbed methane extraction efficiency, reduce costs, and is of great significance to improve the prevention and control of gas disasters and the level of resource development [197]. Xiong [198] showed that, for the serious gas leakage of the coal seam borehole, the steps include the use of hydraulic slit cutting equipment to seal the hole section of the original material that is removed, cut slots in the boundary of the roadway stress elevation area, seal the borehole again, and perform grouting sealing again. After grouting and sealing, the sealing section of the borehole and the cement-filled groove around the borehole form an organic whole, which can effectively avoid the leakage of external air into the interior of the extraction borehole. Ren et al. [199], in order to solve the technical problem of low gas concentration and fast decay of the extraction borehole in the bottom extraction lane, analysed the mechanical characteristics of the lane and proposed the through pipe direct connection with pressure sealing technology, which eliminates the leakage of the extraction pipeline interfaces and then greatly improves the concentration of the coalbed methane extraction in the bottom extraction lane. The concentration of coalbed methane extraction in the bottom pumping tunnel has been greatly improved. For the sealing of downstream water-bearing drill holes, Rong et al. [200] proposed a complete set of technical solutions of “sealing water in the area first, then drilling and sealing the holes”, and Zhai et al. [201,202] proposed a multi-hole parallel pressure–air drainage technology, and based on this, they developed a sealing process for downstream drill holes, which not only solved the difficulty of discharging water accumulated in the holes but also solved the problem of water leakage in the holes, which is difficult to drain. The technology not only solves the problem of water accumulation in the holes, which is difficult to discharge but also realises the integration of hole sealing, drainage, and extraction, which in turn improves the effectiveness of coalbed methane extraction.

3.4. Coalbed Methane Extraction Seals

3.4.1. Capsules

One of the sources of sealing capsule-type coalbed methane extraction sealing devices for coalbed methane extraction boreholes is the MWYZ-H I-, II-, III-type active coal seam gas pressure tester developed by the team of the North China Institute of Science and Technology led by academician Zhou Shining [194]. The basic principle of sealing holes in this device is that solid seals liquid and liquid seals gas. These instruments for measuring gas pressure have been widely used in coal mines. The connection of the solid capsules at the front and rear ends of the device and the connection of the external capsule use connecting rods. The main role of the front and rear capsule connecting rods is to connect the front and rear capsules into one and perform traction on the front capsule when recovering the manometer. The main role of the rear capsule orifice end connecting rod is to send the two sections of the capsule to a predetermined drilling depth and in the recovery of the pressure gauge traction rear capsule. The placement status of the existing capsule gas manometer in a coal rock body borehole is shown in Figure 13.

The active sealing of the device on the pressure measurement borehole is mainly reflected in two aspects: Firstly, the solid hole sealing device at both ends will expand and deform under the action of the pressure of the internal expansion fluid to actively support and seal the borehole; secondly, the intermediate guard pipe will support the coal and rock body without large deformation, and the mucus (sealing material used for grouting) will actively seal the fissures around the borehole under pressure.

Bridge packers (Figure 14), i.e., special capsules, are the key components of hydraulic fracturing technology, which is widely used in the field of coalbed methane extraction (Figure 15). It has been found that the common type of failure of bridge packers is capsule rupture, which mostly occurs in sandstone and mudstone formations, with soft rock formations and capsule rupture (Figure 16) being the main factors. Preventing failures requires hard and stable rock formations, proper reaming, and strict quality control, and residual high-pressure water is a key factor leading to pressure relief failures and shot-rod events [203].

3.4.2. Capsular Bag

The capsule bag sealing device (Figure 17) was developed by Professor Sun Yuning’s team at Henan University of Science and Technology, China, and it can be considered as a highly simplified version of the capsule sealing device, and the current “three plugs and two injections” or “new multi-capsule” devices can be considered as an upgraded and modified version of the traditional capsule type by increasing the number of capsules [9,23,204,205]. At present, the hole sealer is mainly researched on the basis of the “two plugs and one injection” sealing process, and researchers have developed hole sealers according to the analysis of the principle of the process, among which the most typical and widely used is the bag sealer. The hole sealer consists of a double-hole extraction pipe, composite bag, bursting valve, one-way valve, etc. The hole sealer has a compact structure and can efficiently seal the holes with pressure-retaining grouting, and the tests in many places have shown that this method has obvious advantages over the conventional polyurethane + cement “two plugs and one injection” hole sealing method.

Other scholars have developed a split hole sealer, which adopts a special pipeline to grout two separate bags and sets another pipeline to grout the intermediate section, which can guarantee the stable grouting of the bag to a certain extent, but due to more pipelines in the hole, the risk of leakage of the intermediate section of the high-pressure grouting between the two bags is increased, and it is not easy to realise the conditions of pressure-retaining grouting. For the problems of “two plugs and one injection” sealing, some scholars and enterprises have designed the “two plugs, one injection, and one row” grouting and sealing process. Compared with the traditional two plugs and one injection process, this technology adds one row of slurry pipe, and the industrial test results show that this technology can improve the concentration of coalbed methane extraction. The industrial test results show that this technology can improve the concentration of coalbed methane extraction. Some scholars have analysed the air leakage mechanism of the drilling holes of the bag sealer, obtained the sealing factors affecting the drilling holes, and thus designed a kind of split-type bag with pressure grouting sealer and the corresponding split-type bag with pressure grouting sealing technology, which improves the sealing effect, and the effect of gas control is more satisfactory. Some scholars have improved the material for filling the bag and developed a new type of bag sealer, which has better compactness and no obvious shrinkage after expansion of the bag, and the sealing effect is significantly improved compared with that before the improvement.

3.4.3. Other Hole Sealers

The principle of airbag sealing with pressure grouting sealing technology developed by Chen Team [206] is shown in Figure 18. In the figure, the two ends of the drilled hole sealing section are two sections of airtight airbags with good airtightness, and the expansion mode of the airbags is a chemical gas-producing reaction, so that the airbags will be expanded and the hole will be sealed by grouting in the airtight space in the middle of the two airbags.

4. Sealing Materials for Coalbed Methane Extraction Boreholes

The coalbed methane extraction drilling sealing material is a material that plays a sealing role in coalbed methane extraction drilling, which can play the role of a single plug or can be used as a grouting material in “two plugs and one injection”. Under the most widely used “two plugs and one injection” sealing process, the slurry injected into the coalbed methane extraction borehole, under the action of reasonable injection pressure, spreads to the cracks of the coal body around the borehole and fills up the pores and concave–convex surfaces, and penetrates deep into the microcracks of the coal body, and the slurry, after solidification, solidifies with the coal particles, thus effectively blocking the air leakage channels around the borehole, thus improving the quality of the drilling process [7,9,113,120,204,205,206,207]. At the same time, as the coalbed methane extraction borehole sealing materials generally need to enter deeper into the borehole, the coalbed methane extraction borehole sealing materials are generally liquids, which need to enter into the borehole by means of a grouting pump or other similar means. Therefore, the coalbed methane extraction borehole sealing materials can be classified into solidifiable and non-solidifiable materials.

4.1. Solidifying Sealing Materials

At present, solidified coalbed methane extraction borehole sealing materials can be broadly classified into two categories, inorganic and organic, according to their chemical compositions, where inorganic materials mainly refer to cement-based sealing materials and organic materials mainly refer to polymer materials [9,120].

4.1.1. Cementitious Sealing Materials

Cement-based hole sealing materials are not only widely used as grouting-type sealing materials but can also be used individually as plugs for drilling sealing, such as cement roll anchors. Cement-based materials are most widely used due to cement being widely available and low cost and cement-based materials exhibit a wide range of characteristics, including compatibility with fly ash, red mud, and other industrial by-products and solid wastes, making them the most widely used materials in construction.

Cementitious materials are the most widely used sealing materials, which cover ordinary cementitious composites and advanced cementitious materials, such as flexible paste materials, high-water materials, PD materials, ultrahigh-water materials, geopolymer materials, and so on.

Ordinary cement-based materials are the most adaptable to a variety of drilling conditions, and slurry viscosity and setting time can be controlled by adjusting the ratio, so that the length of the sealing hole can be guaranteed, and cement can be combined with various industrial by-products, such as fly ash, red mud, other industrial materials, and solid waste, with the formation of a variety of cement-based compliant materials, so as to seal the hole to further reduce the cost of materials. The obvious shortcoming of cement-based materials is that the existence of capillary pores in the late stage of cement hardening can lead to a certain degree of contraction of cement, resulting in a certain degree of crescent-shaped cracks between the cement and the upper part of the drilled hole, which affects the sealing quality of the drilled hole. At present, the main two ways to improve the late contraction of ordinary cementitious materials are to reduce the capillary pores that lead to the dry shrinkage of the hardened body in the late stage of the material and to develop new types of grouting and sealing materials, so that the late stage of the material can still be slowly expanded to compensate for the shrinkage of the hardened body. Some scholars [208,209] showed that when the dosage of fly ash is greater than 50%, the cement hardening body hazard hole capillary pores can be reduced more substantially. Some scholars [210,211] further analysed the various durability of cement slurries doped with fly ash from various perspectives. Lim et al. [212] showed that sand also has an important influence on the performance of cement slurries and that the strength and durability of the cured body of fine sand cement slurries are better than those of coarse sand cement slurries under the condition of high water–cement ratio. Microexpansion is a conventional method to improve cement-based grouting and sealing materials, but how to ensure that the expansion properties are not consumed completely in the early stage and continue to act in the late hardening stage to make up for the shrinkage of the material in the late stage is still a difficult problem, so how to control the expansion agent in the late stage of the hardening of the material to release the force slowly is the direction of future research [213,214,215,216,217,218,219].

Advanced cementitious composites include flexible paste materials, high-water materials, PD materials, ultrahigh-water materials, etc. Xiang et al. [220,221] developed a flexible paste sealing material by using various polymers, admixtures, expansion components, and so on as cement additives. High-water material is a new type of material. A and B are two components in the water–cement ratio of 2.5:1 after mixing with water, the pumpability is good, the reaction time is controllable, and the compressive strength of the hardened body is higher in the later stage [222,223]. Zhai et al. [224] developed a new type of mining sealing material PD by using microencapsulation technology, which can make the expansion force release slowly, but its setting time is long and the cost is high. China University of Mining and Technology developed higher water–cement ratio UHW materials and studied the setting time and compressive strength under various factors, the basic mechanical properties, micromorphology, and stability of the materials [225]. High-water materials and ultrahigh-water materials have low cost due to their high water–cement ratio, but there are certain defects in their strength and stability, which need to further improve. Some scholars have developed a high-strength geopolymer gel material based on fly ash and found that the appropriate amount of CaO in fly ash has a positive promotion effect on the development of the solid body of this material, and some scholars have prepared polymer grouting materials using fly ash, cement, and water glass as raw materials [226]. Although geopolymer materials have the advantages of being green and low raw material cost, geopolymer materials have more components, so it is difficult to control the stability of the materials. In addition, silicate cement, slag, fluorine gypsum, and quicklime were selected as raw materials to prepare grouting and sealing materials for coalbed methane extraction boreholes, and experimental studies have shown that the material has excellent expansion properties, but its compressive strength is only 3–5 MPa. Hao et al. [227] developed a method to prepare an environmentally friendly, mould-resistant, and long-lasting moisturising phase-change gel (EPCG) through a network of modified cellulosic gel polymer. The gel has an adjustable crosslinking time and an initial solidification time of 30–60 min, with excellent water retention and mould resistance to ensure a long-term sealing effect.

4.1.2. Polymer-Based Sealing Materials

Polymer material has the advantages of fast reaction speed, high adhesion, convenient operation, etc. and is not only used for a single plug in the inner wall of the borehole but also widely used as a spraying material for gas management. Common polymer sealing materials are polyurethane, phenolic resin, urea-formaldehyde resin, and so on. Polyurethane is the most commonly used organic polymer material, with the advantages of low density, large expansion coefficient, easy construction, etc. [228,229,230,231,232]. Polymer-based materials can be used to make plugs due to their quick-setting characteristics. Some scholars have studied the expansion and permeability of polyurethane and applied it in the construction process. Polymer-based sealing materials generally have excellent flexibility and abrasion resistance and can adapt to the deformation of the borehole wall. Meanwhile, their fast-curing properties make them excellent in emergency sealing needs, with high mechanical strength and good chemical resistance. However, polymer-based drill hole sealing materials are susceptible to ageing and have low strength, and their long-term stability still requires further research.

4.2. Non-Consolidating Sealing Materials

Non-consolidated sealing mucus material throughout the whole course of the coalbed methane extraction borehole is kept in a non-consolidated state and can effectively achieve the sealing of coalbed methane extraction boreholes [9,195,233]. The development history of mucus sealing materials is relatively short, still relying on the development of the pressure measurement method of Zhou Shining’s rubber ring–mucus. In recent years, there have been mainly bentonite-based mucus materials and polymer mucus materials of two major categories. Cheng Jianwei’s team [197,233] developed non-consolidated inorganic mucus-based materials and studied the grouting course and intelligent applications. Some scholars [198,199] created a new type of sealing slime based on sodium bentonite and studied the swelling, water retention, and sealing properties. Some scholars [200] developed sealing slime with superior performance based on polymer anionic cellulose ether and carried out the development of an adaptive dynamic sealing technology and system for coal seam drilling. Some scholars developed highly absorbent resin-based mucilage sealing materials and carried out performance improvement and field application.

A comparison of the performance of sealing materials for coalbed methane extraction boreholes is shown in

Table 1. Comparison of sealing material properties for coalbed methane extraction boreholes.

Sealing Material	Labour Intensity	Process Complexity	Compressive Strength	Bonding Strength	(Manufacturing, Production, etc.) Costs	Hole Sealing Effect
Cementitious material	High	Moderate	High	Moderate	Relatively low	Good
Polymer	Low	Simple	Low	Good	High	Moderate
Non-consolidating sealing materials	High	Intricate	-	-	Moderate	Good

.

4.3. Study on the Transport Pattern of Sealing Materials in Coalbed Methane Extraction Boreholes

In order to improve the sealing effect of sealing materials in the fissures of the borehole perimeter rock, in the research on sealing materials for coalbed methane extraction boreholes, in addition to the research and development of sealing materials, a large number of researchers will further study and analyse the grouting diffusion law of sealing materials, especially the whole process of sealing slurry in the dynamic sealing of fissures, by means of theoretical analyses, laboratory experiments, and numerical simulations. A large number of slurry transport research results in the field of geotechnics can be lent to the field of sealing hole grouting for coalbed methane extraction, and some researchers will further investigate the slurry transport and diffusion law in the cracks in a situation closer to the actual situation, i.e., considering the roughness of the cracks.

4.3.1. Grout Diffusion Theory without Considering Fracture Roughness

Dupla et al. [234] concluded that the gradual accumulation of cement particles in porous media has a significant effect on the grouting process. Zied Saada et al. [235] presented a theoretical model for the seepage of cement under percolation in porous media. S. Maghous et al. [236] presented a numerical solution for the model equations of the percolation effect. Olivier Chupin et al. [237] investigated the quantitative relationship between grouting pressure and the percolation effect. Jong-Sun Kim et al. [238] concluded that the percolation effect plays a dominant role in the one-dimensional permeation of slurry and the quantitative relationship between grout pressure and the percolation effect. Chupin et al. [237] also studied the quantitative relationship between grouting pressure and the percolation effect and concluded that the percolation effect plays a dominant role in one-dimensional infiltration of slurry. Jong-Sun Kim et al. [238] established a test setup that takes into account the change in porosity and the time-varying viscosity due to the filtration of slurry particles.

B. Amadei and W.Z. Savage [239] obtained the analytical solution for the unsteady flow of Bingham slurry in a parallel planar fracture. Chun-I Chen et al. [240] obtained the pressure and velocity distributions for the unsteady flow. H.K. Moon [241] obtained that the flow of the slurry will be stopped when the slurry’s pressure gradient meets certain conditions. M. Eriksson [242] investigated the flow pattern of slurry in a network of planar “channels”. F. Bouclielaghem et al. [243] also investigated the flow patterns of slurry in a network of planar “channels” and the low-viscosity hydrodynamic properties of slurry during infiltration. These scholars derived theoretical formulas for the flow of different fluids as well as time-varying viscosity Bingham fluids in the fractured rock, determined the grouting pressure in line with the actual situation in the field, used Comsol Multiphysics to derive a reasonable grouting pressure, and investigated the flow of slurry within the rock fissures as well as the correlation between the grouting pressure and the opening of the slit by means of the UDEC. The effect of slurry diffusion under different grouting pressures was analysed, and it was concluded that the permeability of the coal seam was the key factor determining the effect of slurry penetration.

4.3.2. Diffusion Theory under Consideration of Fissure Roughness

The roughness of the tunnel or coal seam boreholes not only significantly affects the damage to the surrounding rock and the distribution and development of fractures but also results in fractures with relatively small widths and a certain degree of roughness. This roughness, in turn, exerts a considerable influence on the flow and diffusion of the grout during the grouting process

Li et al. [244] used the discrete fracture network to establish the fractured rock model, deduced the correspondence between the fracture frequency and Hoek–Brown criterion parameter by calculation, and simulated the anisotropy degree of the rock strength after grouting reinforcement and the stress–strain curves in various directions and under various conditions of grouting cohesion with the help of PFC, which showed that the degree of the anisotropy of the rock body was obviously reduced after grouting, and the mechanical properties gradually changed from ductile to brittle based on the quantitative relationship between JCond and Hoek–Brown criterion parameters. Based on the quantitative relationship between the parameters of JCond and Hoek–Brown criterion, they established the relationship between grouting reinforcement and its impact on rock strength. Some scholars believe that joint roughness significantly affects the damage area and peak shear strength of rock joints. Through theoretical analysis and numerical simulation, it is found that the angle and height of roughness quantify the scope and degree of damage in the shear process, respectively, and through direct shear tests and 3D scanning tests, the morphology parameters for quantifying the damage area on the surface of the joints are proposed, and based on these parameters, a new model for the peak shear strength is constructed. The results show that the model performs well in predicting the shear strength of other nodal samples [245]. Yi et al. [246] investigated the fluid flow in rough microchannels based on the fractal theory, derived an analytical model of the friction factor and Poisson’s number with respect to the relative roughness of an incompressible fluid without empirical constants when passing through the rough microchannels under laminar flow, and derived an analytical model of the relative roughness of the fluid flow in the rough microchannels based on the capillary bundle model of a porous medium. Based on the capillary bundle model, a fractal analytical expression for the permeability of the rough capillary bundle type is derived, and the effect of relative roughness on the relative change in permeability is discussed.

The flow law of fluid in rectangular and circular micron-scale channels was partially investigated by experiments, and the results showed that the shape of the microchannels had no significant effect on the flow resistance characteristics, the inlet effect of the microchannels was more obvious, and the local resistance of water flow in the mutant microchannels was in accordance with the conventional theoretical curves.

Some other studies have shown that, based on fractal geometry, the Weierstrass–Mandelbrot function can be used to describe the surface roughness of multiscale self-affine, a three-dimensional model for laminar flow in the channel was established, numerical simulations were carried out for the effect of surface roughness, and it is pointed out that the Poiseuille number of the rough microchannel increases approximately linearly with the Reynolds number. The higher the relative roughness, the more obvious the flow pressure drop caused by the backflow and separation of the flow. A molecular dynamics model of the coupled process of fluid flow and heat transfer in the rough nanochannel is established, and the influence of the solid–liquid interaction strength and wall stiffness on the velocity slip and temperature step at the interface is analysed, pointing out that under the action of the external force, the velocity distribution in the mainstream region of the nanochannel is parabolic, and the viscous dissipation of the nanochannel caused by the fluid flow makes it more difficult for the fluid to flow through the nanochannel. The viscous dissipation inside the nanochannel leads to a quadratic distribution of the temperature.

5. Future Directions

5.1. Gas Transport Theory

Over the past decades, gas workers have extensively studied the process of gas transport in coal seams, and many theoretical and empirical/semi-empirical models, such as the mathematical model of gas adsorption and desorption and the model of biporous media, have been established. In the theoretical models, most scholars have not reached a consensus on the mechanism of gas flow in coal matrix, and most of them adopt Fick’s law to describe the process of gas diffusion, while some scholars believe that Darcy flow and Fick’s diffusion coexist in the gas flow in coal matrix, and some scholars believe that the gas flow in coal matrix is more in line with Darcy’s law. As for the empirical/semi-empirical models, these models are mostly based on the experimental results or measured results of coal samples from specific mining areas, and it is difficult to prove that they are suitable for coal samples from other mining areas, which makes it difficult to promote their application and has certain limitations. Moreover, some of these models can accurately describe the initial stage of the desorption process but cannot predict the whole desorption process. Some models can accurately predict the final desorption amount but cannot match the desorption trend during the desorption process. Therefore, there is still a lack of models that can be applied to the prediction of the entire desorption process.

In addition, coal is a porous medium composed of matrix and fissures, and scholars have mainly analysed the problem of the apparent phenomenon of desorption when studying the change in gas desorption characteristics during the crushing damage process, used empirical models, or used classical single-porous or double-porous models to fit certain results. When analysing the variation of the desorption curve, the influence of pore changes on the gas desorption curve is mostly analysed qualitatively, and the quantitative characterisation of the diffusion coefficient into a mathematical form containing the parameters of the pore structure is not included. In addition, the assumption of homogenisation of the diffusion coefficient does not fully consider the successive destruction relationship between matrix and fissure. Therefore, there is no clear mathematical model as a bridge between the change in the structural parameters of the pores of the coal body during the crushing process and the morphological change in the desorption curve.

Since the transport of gas in coal seams is affected by many factors, the following outlook is proposed for the research on the mechanism of gas transport:

(1)

Establishment of multi-scale and multi-physical field coupling model: In view of the different views on the gas flow mechanism in the coal matrix in the existing theoretical models, future research should be devoted to the establishment of a multi-scale and multi-physical field coupling model, which will comprehensively describe the theory of gas transport mechanisms in the coal seam by taking into account the microstructure of the coal matrix, the adsorption and desorption, diffusion, and seepage of the gas, and the process of the gas flow.

(2)

Application of advanced experimental techniques: Tracer technology, non-destructive testing technology, high-resolution imaging technology, microscopic observation technology, new sensing technology, and other advanced experimental means should be applied to carry out refined observation and measurement of the transport process of gas in the coal matrix, obtain more accurate experimental data, and provide a reliable basis for the improvement and validation of the theoretical model.

(3)

Unified description of Fickian diffusion and Darcy flow: In-depth study of the mechanism of gas diffusion and seepage in coal matrix, exploring the interrelationship between Fickian diffusion and Darcy flow and their scope of application under different conditions, should be carried out. Attempts will be made to construct a gas transport model that can unify the description of Fickian diffusion and Darcy flow to provide a more accurate theoretical basis.

(4)

Widely applicable empirical/semi-empirical models: In response to the problem that empirical/semi-empirical models are limited to specific mining areas, future research should be devoted to the development of empirical/semi-empirical models with wider applicability. Through large-scale and multi-region experimental data accumulation, combined with machine learning and other data-driven methods, empirical/semi-empirical models that can be widely applied to different mining areas should be established, so as to improve the versatility and applicability of the models, or to respond to China’s “one mine, one policy” and “one side, one policy” for gas management and to put forward proposals that can be adapted to specific mining areas. Or, in response to China’s “one mine, one policy” and “one side, one policy” for gas management, we propose high-precision models adapted to specific coal mines or workings to solve the problem of inaccurate loss of raw gas content measurement.

(5)

Research and development of the whole desorption process prediction model: Currently, there is a lack of models that can accurately predict the whole desorption process. Future research should focus on developing a whole process prediction model that can cover the initial stage of desorption, the intermediate process, and the final amount of desorption to understand the dynamics of gas desorption process in depth and combine with advanced numerical simulation methods to optimise the existing model or propose a new model, which can accurately describe and predict the whole process of desorption dynamics and behaviour of gas desorption.

(6)

Quantitative characterisation of the whole process diffusion coefficient: Quantitative characterisation of the diffusion coefficient into a mathematical form containing the pore structure parameters, and in-depth study of the influence of the change in pore structure on the gas desorption curve, should be carried out. By combining experiments and numerical simulations, a clear mathematical model is established to quantitatively describe the relationship between pore structure parameters and gas diffusion coefficient.

(7)

Relationship between matrix and fissure destruction time effects: In-depth study of the destruction time effects of coal matrix and fissures and their influence on gas transport and construction of a mathematical model that reflects the relationship between matrix and fissure destruction should be carried out. Through experiments and numerical simulations, we will clarify the connection between changes in pore structure parameters of coal and changes in desorption curves and provide a more accurate description of gas transport.

5.2. Coalbed Methane Extraction Hole Sealing Technology

The decay of the coalbed methane extraction concentration is mainly due to the influence of the roadway and the unloading pressure of the borehole. The coal rock borehole under the action of the geostress, with the prolongation of the extraction time, gradually undergoes creep, which leads to the continuous development of the fissure channel between the peripheral rock fissure of the coal rock borehole and the free surface of the roadway, resulting in the inflow of air into the borehole, so that the effect of the coalbed methane extraction is poor [8].

At present, there is no uniform understanding about the reasonable depth of sealing holes, which mainly revolves around whether the depth of sealing holes should exceed the stress peak point in the stress concentration area or reach the original stress area. There is not enough research on the fissure zone under the coupling effect after the excavation of the roadway and the secondary excavation of the coalbed methane extraction borehole, and at present, the fissure zone is only divided into the concepts of loosening circle, unloading circle, plastic zone, crushing zone, the original rock stress zone, and so on. Therefore, there are more debates about the reasonable sealing hole depth determination, especially whether it exceeds the stress peak point of the stress concentration area. As we all know, for hydraulic cutting, hydraulic fracturing, and other means of penetration enhancement in the case of a fissure not connected with the free surface of the roadway, the development of a fissure is instead conducive to coalbed methane extraction, leading to the root cause of drilling leakage which is the coal rock drilling peripheral rock fissure channel communicating with the free surface of the roadway, and the existing research on the fissure zone is more detailed and in line with the requirements of the pumping requirements, that is, regarding whether the fissure is connected to the roadway, there are fewer research studies.

As the main control factor for the quality of coalbed methane extraction drill hole sealing, the depth of the sealing hole directly affects the effect of coalbed methane extraction from the coal seam. The majority opinion is that, when sealing coalbed methane extraction drilling holes in the actual site, due to the excavation and unloading effect of the roadway and drilling holes successively, the fissures in the loose circle of the peripheral rock of the drilling holes have a high degree of development, and it is often difficult to realise that the fissures in the coal body around the sealing holes are completely filled by the sealing materials, which will reduce the concentration of the gas extracted from coalbed methane extraction drilling holes and affect the effect of the extracted gas. The authors of this paper believe that the above statement is debatable. The authors of this paper believe that the quality of fissure sealing is mainly related to the nature of the sealing material, the scale of the fissure, and a series of related factors, but it cannot be stated that the fissures of the loose circle cannot be sealed because of the high degree of fissure development. Otherwise, because there is a small loose circle in the borehole itself in the radial direction, hole sealing material inevitably moves first along the path of least resistance to penetrate into the loose circle in the radial direction of the borehole, causing the formation of a hole sealing material priority transport path. According to the above theory, the cracks in the loose ring cannot be sealed due to the development of cracks, and the sealing material will inevitably flow along the axial direction of the drill hole through the loose ring of the drill hole out of the roadway (upward to the drill hole) or flow into the bottom of the drill hole (downward to the drill hole), and it is impossible for the drill hole to be sealed. On the contrary, the current study shows that the drill hole can achieve a better sealing performance in the case of reliable sealing material, that is, it shows that sealing material with superior performance can complete the sealing of the loose ring to a certain extent.

Based on the research results in the field of coalbed methane extraction sealing technology, the following research outlook is proposed:

(1)

Deepen the research on the mechanism of fissure development: Due to the complex field conditions of mines and the non-homogeneous and anisotropic characteristics of the coal rock body, there is still a lack of comprehensive understanding of the mechanism of fissure development. In the future, we can combine on-site monitoring and laboratory simulation to further reveal the interaction mechanism between fissure development and the mechanical behaviour of coal rock body and gas transport through multi-scale and multi-physical field coupling methods, especially considering the characteristics of the coal rock body surroundings of the drilled hole under the conditions of pressure unloading, fissure roughness, and supporting or sealing section.

(2)

Optimise the sealing depth and length: Introduce advanced monitoring technology, such as 3D scanning, sensor network, etc., to monitor the fissure development and permeability change around the borehole in real time to provide accurate data support for the coalbed methane extraction process, to quantitatively characterise the permeability threshold value of the deep fissure of the borehole through the roadway from the measured data, to take the threshold value with a certain safety coefficient as the reasonable sealing depth, and to optimise the sealing length according to the support function of the sealing section. Then, according to the support function of the sealing section, the sealing length can be optimised.

(3)

Automation and intelligent sealing technology: With the construction of intelligent mines, automation and intelligent sealing technology will become a research hotspot. Through the introduction of sensor technology, artificial intelligence, and robot technology, and drawing on the high degree of automation in equipment such as synthesis excavators and shield machines, we will develop a complete set of multi-functional and integrated sealing technologies such as coalbed methane extraction sealing devices and sealing processes, which integrate sealing, drainage, and extraction and can realise the whole process of sealing, automatic monitoring, real-time adjustments, and remote control of the intelligent sealing process, so as to improve the precision and efficiency of the sealing process.

5.3. Sealing Materials for Coalbed Methane Extraction

Scholars have performed a lot of research on sealing materials for coalbed methane extraction, and the existing widely used sealing methods are mainly “two plugs and one injection” sealing methods, which are characterised by one-time static sealing. The grouting and sealing materials in the middle of the two plugs are mostly solidification-type materials. However, due to the non-homogeneous nature of the rock surrounding the drill hole, the displacement generated by the creep of the drill hole is also non-homogeneous, and the solidified grouting sealing material cannot dynamically seal the cracks in the coal rock drill hole, resulting in the attenuation of coalbed methane extraction from the drill hole over time. Due to the superimposed effect of unloading pressure from roadway and borehole excavation, the coal rock body around the borehole can be regarded as composed of coal rock blocks cut by fissures, and the main channel for the flow of grouting material is the fissures due to the weak permeability of the coal rock blocks. These fissures are important factors affecting the stability of the extraction borehole because of their low strength and deformation parameters and poor physical properties. The geometrical characteristics and distribution form of the fractures in the coal rock body around the drill hole determine the seepage flow with large anisotropy.

In the process of grouting and sealing, the sealing material gradually fills the fissures in the surrounding rock, and after diffusion and solidification, it makes the densification of the coal and rock body around the borehole increase, and the sealing is improved. As the grouting pressure continues to increase, the splitting phenomenon at the tip of the fissure will be induced, and the fissure continues to expand, leading to an increase in the grouting volume. These splitting situations will not only cause a large amount of grouting and sealing materials to be lost but may also rupture part of the intact coal and rock body, going against the purpose of sealing the borehole and exacerbating the degree of air leakage in the borehole. Furthermore, because the seepage process of slurry during the process of grouting and sealing of drill holes is imperceptible, it is currently impossible to monitor the spreading range of slurry and sealing of fissures during the actual construction process. Therefore, how to strike a balance between grouting and sealing materials to seal the fissures and split fissures in the perimeter rock of the borehole becomes a scientific problem that needs to be solved urgently. Meanwhile, studying the impact of non-curing grouting sealing materials on coal seams of different metamorphic grades is essential for improving the sealing effectiveness of boreholes.

The inability to determine the range of slurry diffusion and the degree of sealing fissures not only leads to blindness and uncertainty of the grouting sealing parameters of the borehole but also means that the effect of grouting sealing cannot be judged, especially in complex geological conditions. The spacing of extraction boreholes, the size of the grouting pressure, the length of the grouting time, the ratio of slurry, etc. have a direct impact on the effect of grouting sealing and the stability of the drill hole [88]. Meanwhile, most grouting sealing materials are fluids with time-varying viscosity, but there are few studies that consider the time variation of viscosity when the grouting sealing material seals the fissures of coal and rock bodies around the drill holes, and the viscosity of the grouting sealing material at different locations in the diffusion area of coal and rock body fissures around the drill holes also changes with time.

Therefore, based on the existing research results, the following research outlook is proposed:

(1)

Intelligent sealing materials. The application of intelligent technology in sealing materials for coalbed methane extraction boreholes is promising. In the future, intelligent sealing materials can adapt to the complex and changing conditions of coal seams through self-perception, adjustment, and repair functions. Adopting intelligent control systems and sensor technology, real-time monitoring and dynamic adjustment of sealing materials can be realised to ensure the continuous stability of hole sealing effects. In particular, non-consolidated or liquid sealing materials can facilitate intelligent pressure replenishment and other operations in the whole process of coalbed methane extraction through the grouting system.

(2)

In-depth study of sealing material transport laws. Future research will continue to analyse the grouting diffusion behaviour and sealing effect of sealing materials under different conditions by means of experiments and numerical simulations. In particular, considering the time-varying viscosity of grouting material, the complexity of fissure roughness, and pore structure, a finer transport model will be established to provide a theoretical basis for the optimisation of sealing material.

6. Conclusions

Scholars have conducted a lot of research on the theory of coal seam coalbed methane extraction and hole sealing, coalbed methane extraction and hole sealing devices, coalbed methane extraction drilling sealing material, and supporting coalbed methane extraction and hole sealing technology and have achieved a lot of innovative results and solved a large number of theoretical and technical problems. However, the gas disaster accidents that occur from time to time also show that there is still a great demand for the development of the theoretical technology and materials of coalbed methane extraction hole sealing. With the continuous advancement of coal mine intelligent work, coal mine coalbed methane extraction intelligence, platforms, and systems are also imminent.

(1)

Research on basic theories related to coalbed methane extraction. The theory of gas flow in coal matrix still needs to establish a variety of factors coupled in the porous medium gas transport model, and through gas diffusion and molecular dynamics simulation, lattice Boltzmann simulation and gas transport multi-scale simulation, and other molecular simulation methods, combined with laboratory experiments and field data, a new microscopic scale under the gas flow mechanism can be further put forward and then guide the reasonable selection of gas pumping hole sealing parameters.

(2)

Research on coalbed methane extraction hole sealing technology and devices. The existing coalbed methane extraction sealing technology and device research is relatively mature, but the degree of automation is not high, resulting in the quality of sealing to a large extent being dependent on the level of quality of the construction work, and the current sealing technology is only suitable for sealing shorter holes and cannot be a good match for the widely used and rapidly developing directional drilling technology. Therefore, against the background of intelligent development of coal mines, it is necessary to further develop coalbed methane extraction systems with a high degree of automation or even intelligent systems and to develop the drilling sealing technology for directional drilling.

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Research on coalbed methane extraction drilling leakage detection technology and devices. There are many coalbed methane extraction borehole leakage detection technologies and related patents, mainly focusing on negative pressure detection and tracer gas monitoring, etc. However, due to the complex structure of the surrounding rock of underground boreholes, reference should be made to non-contact measurement methods and non-destructive monitoring methods, and fibre-optic measurements, ultrasonic measurements, magnetic particle detection, ultrasonic detection, liquid penetration detection, etc. should be introduced into the detection of gas leakage of coalbed methane extraction boreholes so as to realise the dynamic monitoring of and early warning for gas leakage. Relevant technologies should be integrated into the coalbed methane extraction system platform.

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Development and application of sealing materials for coalbed methane extraction drilling. At present, there are many kinds of sealing materials for coalbed methane extraction drill holes, most of which are solidified sealing materials, but due to the influence of the roadway and drilling unloading pressure, the creep of coal rock drill holes under the action of geostress is unavoidable with the prolongation of extraction time. Therefore, the development of liquid sealing materials adapted to the development of fissures in the surrounding rock of the drill hole creep has a wide range of prospects for application. At the same time, active support-type sealing materials with high strength, sealing materials with Poisson’s ratio similar to that of the surrounding rock of the borehole, and gel-like sealing materials that can actively adapt to the deformation of the fissures of the surrounding rock of the borehole can all play a better role in specific scenarios.