Experimental Study on High-Energy Gas Fracturing Artificial Coal

Abstract

The low permeability of coal seams has always been the main bottleneck restricting coalbed gas drainage. To improve the permeability of a coal seam, a high-energy gas fracturing technology is proposed. Firstly, based on the high-energy gas fracturing mechanism and gas production principle of fracturing agent, a fracturing agent applicable to coal reservoirs was developed, and its performance and sensitivity were tested. Then, a high-energy gas-fracturing simulated coal sample test was conducted, and the drilling wall pressure and strain of the simulated coal sample were tested. The results show that high-energy gas fracturing technology is a safe and efficient technical means for improving the permeability of coal reservoirs. The pressure–time curve of the borehole wall under the action of high-energy gas can be divided into three stages, namely, the rapid pressure-rising stage, steady pressure stage, and falling stage; the maximum pressure in the borehole is about several hundred MPa, and the pressure distribution in the borehole is not uniform. Compared with explosives blasting, the stress wave intensity in coal caused by the action of high-energy gases is low, the duration is short, and the peak stress attenuates slowly with increasing distance. Under the action of high-energy gas, no crush zone is generated near the borehole; the number of radial cracks produced is small but long. The extent of the fracture zone depends mainly on the quasi-static splitting wedge effect of the high-energy gas.

1. Introduction

Coalbed methane is a typical unconventional energy source that is safe and environmentally friendly. The development of Coalbed methane is conducive to increasing the clean energy supply, reducing coal mine gas accidents and greenhouse gas emissions, and achieving carbon peaking and carbon neutrality [1]. China’s “Fourteenth Five-Year Plan” modern energy system plan in the “enhance the stability and security of the energy supply chain” clearly pointed out: to actively expand the exploration and development of unconventional resources, such as coalbed methane [2]. China is rich in coalbed methane resources, with reserves of about 36.8 trillion cubic meters, ranking third in the world; it has great potential for development and utilization [3,4]. However, the coal seam permeability in China is generally low, generally within (0.001~1.800) × 10⁻³ D [5]. At present, the low permeability of coal seams greatly restricts the safe and green production of coal mines; improving the coal seams’ permeability has become the key to extracting coalbed methane in China [6]. Therefore, it is urgent to study the technology for enhancing the permeability of coal seams. To improve the permeability of coal seams, researchers at home and abroad have made long-term exploration and practice. At present, the techniques to improve coal seam permeability mainly include hydraulic fracturing, hydraulic slitting, mining protective seams, deep hole pre-splitting blasting, plasma pulse, etc. [7,8,9,10,11]. Hydraulic fracturing and hydraulic cuttings consume large amounts of water, and the chemical additives in the fracturing fluid can pollute groundwater resources. In addition, a large amount of fracturing fluid will occupy the channels for gas flow, resulting in lower gas production. Deep hole pre-splitting blasting technology is prone to accidents; mining protective seams are suitable for gas extraction from coal seam groups, but the effect is not ideal for a single thick coal seam. Plasma pulse technology is difficult to guarantee the duration of the coal seam transformation effect, and the cost is too high.

High-energy gas fracturing technology was proposed in the United States in the 1860s [12]. Its essence is to generate a large amount of gas in a non-explosive way; the gas causes the brittle coal rock body to produce longer fractures to communicate more natural fractures, thus forming a complex network of fractures to achieve the purpose of fracturing effect [13,14,15,16]. Chen et al. [17] used fracture mechanics to preliminarily reveal the mechanism of fracture under the action of high-energy gases. Wang et al. [18] conducted field tests in the Enhong Basin and proved that the application of high-energy gas fracturing technology to extract coalbed methane is feasible. Wu et al. [19] established a computational model for high-energy gas fracturing loads using dynamic destruction simulation tests and theoretical studies of rocks under different loading conditions. W.C. Zhu et al. [20] quantified the enhanced gas extraction effect of high-pressure air blasting based on numerical simulation, and the results showed that the destruction caused by high-pressure gas blasting could effectively improve the gas extraction effect. He et al. [21], using numerical simulation and theoretical analysis, explored the mechanism of fracture extension in composite coal seams under the action of high-energy gases. Using chemical fracturing agents to generate high-energy gas is one of the high-energy gas fracturing technologies, which has the following advantages: it does not require large construction equipment, the operation process is similar to that of traditional explosives blasting, the gas production volume and gas production rate can be adjusted by controlling the fracturing agent equivalent and composition. This has been carried out in a great number of applications in the petroleum gas extraction field and is currently a highly recognized fracturing technology. However, its application and research in the coalbed methane mining field are still in the primary stage, especially the lack of systematic research on the pressure distribution law in the borehole and the stress field in the coal body under the action of high-energy gas.

Given this, the principle of gas production by fracturing agent and the mechanism of high-energy gas fracturing was first analyzed and combined with the performance test results of the fracturing agent to determine a fracturing agent formula suitable for improving coal permeability. Then, the high-energy gas fracturing artificial coal experiment was carried out, and the pressure in the borehole wall and the strain in the test block were measured, in addition to analyzing the fracturing effect of high-energy gas. The research results provide a reference for the application of the fracturing agent in the field of coal-seam permeability enhancement.

2. Gas Generation Principle of Fracturing Agent and Fracturing Mechanism of High-Energy Gas

2.1. Gas Generation Principle of Fracturing Agent

The fracturing agent mainly consists of a heating agent, a gas-producing agent, and an oxygen-supplying agent. The oxygen supply agent provides oxygen to the heating agent, causing it to oxidize and emit a large amount of heat, and then the gas production agent is decomposed by heat to produce a large amount of gas. The authors’ team chose aluminum powder as the heating agent, ammonium perchlorate as the oxygen-supplying agent, and the main gas-producing agent to make the fracturing agent. Under high-temperature conditions, ammonium perchlorate decomposes to generate oxygen, and then the aluminum powder is oxidized to give off a lot of heat, heat to promote the thermal decomposition of ammonium perchlorate to produce a large amount of high-temperature and high-pressure gas; the main reaction equation is shown in (1–3).

4Al + 3O₂ = 2Al₂O₃

(1)

2NH₄ClO₄ = N₂ + 4H₂O + Cl₂ + 2O₂

(2)

(NH₄)₂C₂O₄ = 2NH₃ + CO + CO₂ + H₂O

(3)

2.2. High-Energy Gas Fracturing and Desorption Mechanism

The fracturing agent is excited to produce high-temperature and high-pressure gas, which rapidly expands and impacts the borehole wall, generating a shock wave or stress wave inside the coal, thus creating a crushing zone or radial crack around the borehole. The high-temperature and high-pressure gas starts to expand and works based on the stress wave action. First, the gas wedges into the crack in the borehole wall, driving its steady-state expansion, followed by the quasi-brittle fracturing of the crack tip media, leading to continuous crack expansion under the combined effect of the gas quasi-static stress field and the original rock-stress field. Finally, the crack tip is again in small extension due to localized destruction. As a result, a large crack zone was formed around the borehole, which improved the development level of coal seam cracks, unblocked seepage channels, and improved coal seam permeability.

Zhang [22] Hu carried out an experimental study on the vibration desorption characteristics of gas-bearing coal and concluded that the vibration of gas-bearing coal greatly promotes the desorption of gas. Zhang et al. [23] carried out gas desorption tests at a series of temperatures (40 °C, 50 °C, 60 °C) and verified that the gas desorption process is a heat absorption process and that the warming promotes the desorption of gas. Qu et al. [24] tested the adsorption characteristics of single-component CH₄ and CO₂ gases using a high-pressure volumetric gas adsorption device and demonstrated that the adsorption characteristics of CO₂ were much greater than those of CH₄. The above literature illustrates that the vibration of the coal body, the increase in temperature, and the production of CO₂ in the coal body all promote the desorption of gas. Under the action of high-energy gas, the microscopic void structure of coal and the existing state of the gas is disturbed in the elastic zone outside the fracture zone, which breaks the dynamic equilibrium of gas adsorption and desorption in coal. As a result, it converts part of the gas in the adsorbed state into the gas of the free state. This constitutes a perturbative desorption effect. The fracturing agent burns vigorously in the borehole to produce a large amount of high-temperature gas; while the desorption process of gas needs to absorb heat, heat acts on the surrounding coal seam through heat transfer, promoting the desorption of gas in dynamic equilibrium. This constitutes a thermal effect desorption. The fracturing agent contains 95 % organic matter and complete combustion will produce a lot of CO₂. Because the adsorption effect of coal on CO₂ is twice that of CH₄, CO₂ can replace CH₄ to make it desorb into a free state. This constitutes displacement desorption.

3. High-Energy Gas Fracturing Test

3.1. Fracturing Agent Formulation and Parameters

Based on the results of the above analysis, the fracturing agent should meet the following requirements. The fracturing agent does not produce toxic and harmful gases; the energy utilization rate is high when fracturing the coal body; the fracturing agent can produce a large amount of gas that can steadily drive the fracture expansion; the production, transportation, and use process of the fracturing agent should be able to ensure safety. As a result of the analysis, the above requirements were achieved through the following methods. (1) Carefully selecting raw materials and performing oxygen balance calculations to reduce toxic and harmful gases. (2) The gas-production speed of the fracturing agent is controlled by adjusting the ratio so that the intensity of the stress wave generated near the borehole by the high-energy gas impact is between the dynamic tensile strength and dynamic compressive strength of coal. (3) Select ammonium perchlorate as the main gas-producing agent, which can produce a large amount of gas after thermal decomposition. (4) There are no explosive substances such as ammonium nitrate, ammonium hyper phosphate, and nitroamine in the fracturing agent, which will not spontaneously combust under general stimulation such as friction, impact, and static electricity, ensuring the safety of production, storage, transportation, and use.

Through screening and oxygen balance calculation, a fracturing agent suitable for coal mines was developed. The fracturing agent formula is shown in

Table 1. Composition of high-energy gas fracturing agent.

Raw Material	Specification	Ratio/%	Effect
HTPB 1	Type I	13.39	Binder
TDI 2	Industrial Grade		Curing agent
KZ 3	Industrial Grade	3.5	Plasticizer
AP 4	Type III	73.0	Cas generating Agent
AP 4	Type IV	5.0
Al 5	Industrial Grade	5.0	Heating agent
Others		0.11

Notes: ¹ HTPB (HO- [CH₂CH=CHCH₂]n-OH) is Hydroxyl-terminated polybutadiene; ² TDI(C₉H₆N₂O₂) is Toluene diisocyanate; ³ KZ(C₂₆H₅₀O₄) is dioctyl sebacate; ⁴ AP(NH₄ClO₄) is Ammonium perchlorate; ⁵ Al is Aluminum.

. According to the oxygen balance calculation, the oxygen balance value of the fracturing agent is 1.6 %, which means that few toxic and harmful gases, such as CO and CH4, will be produced in the process of fracturing coal with the fracturing agent.

To find the basic performance parameters of the fracturing agent. The rate of ignition, explosion heat, gas production, and the density of the fracturing agent were tested by the target line method, constant temperature method, pressure method, and weighing method, respectively [25]. The results are shown in

Table 2. Fracturing agent (test piece) performance parameters.

Test Piece Name	Formulation Code	Charge Quantity (kg)	Fuel Speed (mm/s)	Gas Production (L/kg)	Explosion Heat (kJ/kg)	Density (g/cm3)
Sy- High	YLD	0.3	14	>1400	≥3500	1.4~1.7

. From the table, it can be seen that the developed fracturing agent produces at least 1400 L/kg of gas, and it can generate a lot of heat.

The impact sensitivity and friction sensitivity of the fracturing agent were tested using the explosion probability method [25], and the test conditions and results are shown in

Table 3. Fracturing agent sensitivity, pressure, and temperature resistance test results.

Test Project	Test Conditions	Test Results
Friction Sensitivity Test/%	Temperature: 20 °C; Humidity: 55% Pendulum angles: 66°; Pressure: 2.5 MPa	8%
Impact Sensitivity Test/%	Temperature: 20 °C; Humidity: 59% Hammer weight: 10.0 kg	10%
Pressure resistance test/(MPa)		120
Temperature resistance test (°C/48 h)		163

. From Table 3, it can be seen that the friction sensitivity and impact sensitivity of the fracturing agent are 8% and 10%, respectively, while the sensitivity of general fire explosives under the same conditions is above 20%, which means the sensitivity of the fracturing agent in this paper is low. Table 3 also shows the test results of the pressure and temperature resistance of the fracturing agent. The results show that the fracturing agent will not affect normal use when the pressure is less than 120 MPa and the temperature is lower than 163 °C.

3.2. Test Method

Based on the experimental results in the literature [26], cement, sand, and water are selected as the basic materials to control the structural strength of the simulated coal; gypsum, perlite, foaming agent, and crushed mica are selected as additives to control the microcracks, microporous, structural surfaces, and gases in the simulated coal. The materials, proportions, and basic physical and mechanical parameters of the simulated coal are shown in

Table 4. Materials, proportions, and basic physical and mechanical parameters of the simulated coal.

Materials and Proportioning	Density ρ (g/cm3)	Poisson’s Ratio, μ	Compressive Strength σe/MPa	Longitudinal Wave Velocity cp/(m/s)	Young’s Modulus E/GPa
Sand:Water:Cement:Gypsum:Crushed Mica:Perlite:Foaming Agent
4.346:0.501:1.581:0.327:0.035:0.023:0.067	1.84	0.18	24.25	2580	4.73

.

To verify the reasonableness of the present simulated material, some basic mechanical parameters of the real coal from Jiaozuo Zhongma Mine are given in

Table 5. Basic mechanical parameters of coal.

Coal Sample Name	Compressive Strength σe/MPa	Young’s Modulus E/GPa	Poisson’s Ratio, μ
Jiaozuo Zhongma Coal Mine No.2	28.735	4.054	0.245
Jiaozuo Zhongma Coal Mine No.2	37.512	3.883	0.184

[27]. Comparing Table 4 and Table 5, it can be seen that the strength, Poisson’s ratio, and Young’s modulus of the present simulated material are less different from those of the real coal, so the simulated coal sample can be used to study the mechanism of high-energy gas fracture coal body.

The test block size is 1000 mm × 1000 mm × 600 mm, mixed manually and compacted with a small vibrating plate in the mold and maintained for 28 days (shown in Figure 1a). In the middle of the test block, a borehole of 50 mm in diameter and 400 mm in depth is left in advance. After putting the fracturing agent charge roll into the borehole, the borehole is sealed with Bar glue. When ready, use the ignition device to excite the fracturing agent. The fracturing agent and ignition device are shown in Figure 1b,c.

The borehole wall pressure test is realized by pasting a pressure sensor on the borehole wall. The pressure sensor is glued to the borehole wall with 502 glue, and then the signal line is connected to the signal acquisition system through the orifice. When the high-energy gas acts on the borehole wall, the sensor is squeezed to generate electrical signals, which are transmitted to the signal acquisition system through the charge amplifier to realize the acquisition of the pressure–time curve. The pressure sensor used in this test was a PVDF piezoelectric sensor (Figure 2a), and the signal acquisition system was the DH5922N dynamic strain gauge. (Figure 2b), which was set at 200 kHz of collection frequency. The arrangement of the measurement points for the borehole wall pressure test is shown in Figure 2c.

The strain brick is shown in Figure 3a and has a size of 20 mm × 20 mm × 20 mm, and its material and proportion are the same as the test block. According to the law of strain wave exponential attenuation, the strain brick is reserved in the middle of the charging section, and the distance between it and the center of the borehole is 125 mm, 225 mm, 350 mm, and 500 mm, respectively, physical and schematic diagrams are shown in Figure 3b,c. One strain gage was attached to each strain brick, and the test was conducted using BF120-3AA strain gages produced by Chengdu Electro-Mechanical Instrument Factory. In addition, the DH5922N dynamic signal acquisition system was used to acquire strain waves. This method was used to analyze the stress wave propagation and attenuation patterns under the action of high-energy gases.

4. Test Results and Analysis

4.1. Borehole Wall Stress Test Results and Analysis

Three simulated coal samples were selected for high-energy gas fracturing tests. In this paper, three specimens with a total of six borehole wall stress measurement points, of which four valid pieces of data were obtained. Two of the data sets were invalid due to destructed PVDF pressure sensors, sticking, etc. To minimize the effect of the PVDF pressure sensor on the borehole wall stress test data, the average of the valid data at the measurement points was used as the final test results, as shown in Table 5. To facilitate the analysis, the distinctive parts of the curve were intercepted for analysis, and some of the test results are shown in Figure 4.

The pressure–time curves of the borehole middle and bottom in the role of high-energy gas are given in Figure 4. It can be seen from the Figure that the pressure–time wave of the borehole middle and bottom in the role of high-energy gas have the same trend. They show a rapidly rising pressure-rising phase, a slowly rising pressure-stabilizing phase, and a non-linearly declining pressure-releasing phase. After the fracturing agent is excited, a large amount of high-temperature and high-pressure gas is generated, which rapidly expands and impacts the borehole wall, causing the pressure of the borehole wall to rise rapidly. At the same time, stress waves inside the coal are induced, and initial cracks appear on the borehole wall under the action of the stress waves. As the borehole wall starts to crack, the high-energy gas wedges into the cracks, causing it to develop steadily. The amount of gas produced by the fracturing agent is greater than the amount of gas entering the coal, so the pressure at the borehole wall continues to rise, but the rate of rise slows. With the development of fracture until penetration, the coal ruptures and gas is rapidly released, and the pressure at the borehole wall drops rapidly to below 1 MPa.

The pressure–time curve has obvious fluctuations in both the rising and falling sections, which are related to the PVDF pressure sensor’s characteristics, the non-uniform flow of gas in the borehole, and the unstable gas production rate of the fracturing agent due to pressure variations in the borehole.

The statistical results of the borehole wall pressure tests are given in

Table 6. Statistical results of borehole wall pressure test.

Statistical Items	Location of Measurement Points	Test Results
Peak pressure/MPa	Borehole middle	118.1
	Borehole bottom	85.3
Pressure increase time/ms	Borehole middle	18.4
	Borehole bottom	18.9

. From the table, it can be seen that the pressure rise time of the borehole wall in the role of high-energy gas is a dozen ms, and the maximum pressure is several tens to hundreds MPa. The borehole wall pressure under the action of explosive blasting can reach thousands or even tens of thousands MPa in a few microseconds [28]; hydraulic fracturing is a static load, and the peak pressure is generally below 100 MPa [29]. This means that the peak pressure and loading rate of the borehole wall in the role of high-energy gas are less than that of explosive blasting and greater than that of hydraulic fracturing. Explosive blasting has a strong load dynamic impact, hydraulic fracturing produces static loads, while the load produced by high-energy gas has both the dynamic action of stress wave and quasi-static action of gas, and the quasi-static effect of the gas is dominant.

In addition, the pressure distribution in the borehole is uneven during the high-energy gas fracturing process, and the peak pressure decreases from top to bottom. Two reasons can explain this phenomenon: (1) The slow response of the fracturing agent and the fracturing agent excitation point is in the upper part, so the high-energy gas acts on the upper part of the borehole wall first, which makes the upper part of the borehole crack first. After the cracks appear in the upper part of the borehole wall, the gas enters the coal continuously. When the gas acts on the bottom, the gas in the borehole decreases, and the pressure drops. (2) The gas has a certain viscosity, so friction occurs between the gas stream and the gas stream, between the gas stream and the borehole wall, and between the gas stream and the fracturing agent pipe wall. Overcoming these frictional forces requires some energy consumption, thus causing gas pressure decay. Since the bottom of the hole is far away from the excitation point, the gas flow length is longer, so the pressure decay is large.

4.2. Strain Waves Test Results and Analysis

The radial strains were tested at four measurement points at 125 mm, 225 mm, 350 mm, and 500 mm from the borehole. To facilitate analysis, the distinctive parts of the waveform were exported from the DHDAS dynamic signal analysis software. Some of the test results are shown in Figure 5.

From Figure 5, it can be seen that each point generally forms two-part strain waves. From its first complete strain wave, it can be seen that the points are first subjected to compressive stress and then to tensile stress. In terms of the time of action, the duration of the strain wave compressive phase is shorter than the tensile phase in the role of high-energy gas, and the overall strain wave duration is much longer than that of explosive blasting. The action time of the strain wave compressive phase produced by the high-energy gas is about 2.3−5.6 ms, and the tensile phase is about 3.1−8.7 ms, while the action time of the strain wave compressive phase produced by the explosive blasting is about 40−50 μs, and the tensile phase is more than 100 μs [30].

Three simulated coal samples were selected for high-energy gas fracturing tests in this paper, three specimens with a total of 12 stress wave measurement points, of which nine valid waveforms were obtained. Three of the waveforms were invalid due to destructed strain sensors, the burial of strain bricks, etc. To reduce the influence of the strain transducer on the test results, the average of the valid data from three sets of tests was taken as the study. The results of the peak strain statistics are given in Table 6.

The results of the peak strain statistics are given in

Table 7. Peak strain of each measuring point.

Points/cm	12.5	22.5	35	50
Radial strain/με	11,812	4510	2517	758

. The peak strain at each measurement point is multiplied by the dynamic Young’s modulus of the coal to obtain their dynamic stress peaks. The calculation formula of dynamic Young’s modulus of coal rock mass is as follows:

$$E_d=\frac{c_p^2\left(1+{\mu }_d\right)\left(1-2{\mu }_d\right)}{(1-{\mu }_d)}$$

(4)

In Formula (4), E_d is the dynamic Young‘s modulus; $c_p$ is the longitudinal wave velocity of coal; ${\mu }_d$ is the dynamic Poisson’s ratio of coal. In the case of high-energy gas loading at high speed, ${\mu }_d$ = 0.8u, u is the static Poisson’s ratio. The calculation yields that ${\mu }_d$ = 0.144, $E_d$ = 6.34 GPa.

To analyze the law of stress wave propagation and attenuation in coal under the action of high-energy gas, the scatter plot of the peak radial stress wave with distance at each measurement point was plotted, and then the scatter plot was fitted nonlinearly (the fitting formula, is $y=\mathrm{a}{\overline{r}}^{\mathsf{\alpha }}$,$\overline{r}$, is the proportional distance, it equal to the ratio of the distance from the measurement point to the borehole and the radius of the borehole). The fitting results are shown in Figure 6. From the fitting results, it can be seen that the stress wave attenuation index α in the coal under the action of high-energy gas is 1.67, which is consistent with $\alpha =2-\mu /(1-\mu )$ ($\mu$ is Poisson’s ratio of coal, 0.14–0.3). However, the stress wave attenuation index in the coal under the action of explosive blasting is basically consistent with $\alpha =3-\mu /(1-\mu )$ [31], which means that the stress wave attenuation is slower in the coal under the action of high-energy gas.

The strain waves duration in coal under the action of high-energy gas is greater than that of explosive blasting, and the peak strain is smaller than that of explosive blasting, which means that the strain rate in coal under the action of high-energy gas is lower than that of explosive blasting. Smaller strain rate results in smaller energy dissipation, so the stress wave in the coal under the action of high-energy gas attenuates more slowly.

4.3. Test Block Destruction Characterization

Figure 7 shows the destruction pattern of the test block under the action of high-energy gas. It can be seen from the Figure that the cracks of the test block under the action of the high-energy gas show a random distribution; all the cracks are penetrated, without secondary cracks, and the number of microcracks around the borehole is small. Both main cracks and microcracks are extended outward approximately along the radial direction of the borehole. The fracture surface of the fragments is mostly irregular and rough, and some of the fragments are spalling off, which is consistent with the characteristics of tensile destruction. The destruction characteristics of the borehole wall show that the surface of the borehole wall is intact, which means that there is no crush zone in the test block under the action of high-energy gas. The fragments produced by the action of high-energy gas were mainly large pieces, and no fragment-popping phenomenon occurred during the test.

Figure 8 shows the SEM scanning results of the specimens before and after the high-energy gas fracturing test. It can be seen from Figure 8a that the pore surface of the specimen is relatively flat, and the closed and semi-closed pores were well developed before the high-energy gas fracturing test. After the high-energy gas fracturing test, the closed and semi-closed pores in the specimen were penetrated, and obvious microcracks appeared. With the development of microcracks, more secondary microcracks appeared. It can be concluded that the test block is not broken to a high degree under the action of high-energy gas, but it has good fracturing and improving permeability effect. The main reasons are as follows:

(1)

High-energy gas fracturing of coal rock is divided into two processes, namely, the stress wave dynamic loading stage and the gas quasi-static loading stage. Firstly, the stress wave causes a dislocation pile-up in the internal medium of the specimen, which promotes the penetration of closed and semi-closed pores and the formation of the initial macro cracks. Then comes the quasi-static loading phase of the high-energy gas, which has a less dynamic shock effect, and its main effect is to wedge into the initial fracture caused by the stress wave to steadily drive the fracture expansion, thus can forming a large fracture zone.

(2)

The stress wave loading rate has an essential effect on the fracture toughness of the material. The fracture toughness of most materials under conditions of shock loading is lower than the static fracture toughness. In the static loading range, the fracture toughness remains relatively stable; in the medium loading range, the fracture toughness decreases with the increasing speed of the applied load. In explosive blasting and high-energy gas fracturing, the loading rate is generally medium loading or impact loading. From Section 3.2, it is known that the strain rate of coal in the role of high-energy gas is low, and the low strain rate means that the stress wave is loaded slowly, which means that the fracture toughness of coal during high-energy gas fracturing is greater than that of explosive blasting, so the test block is not broken to a high degree in the role of high-energy gas.

(3)

The smaller the stress wave loading rate, the fewer the number of cracks near the borehole. Explosive blasting produces stress waves that load at a much greater rate than high-energy gases, so more work can be undertaken in the same amount of time with explosive blasting. This can result in greater synthetic displacement of the media at the opening of the fracture zone and the formation of more cracks near the borehole. In addition, these cracks may divide to expend the energy of the stress wave. Under the action of high-energy gas, the loading rate of stress wave inside the coal is low, and less work is undertaken at the same time; it cannot produce the same amount of cranny as explosive blasting at the same time. The smaller the number of cracks, the less energy is consumed to extend the same distance and the larger the extension range.

(4)

The pressure of the borehole wall is small, and the stress wave intensity is low produced in coal under the role of high-energy gas, which is not enough to produce a crush zone near the borehole. The stress wave intensity near the borehole wall under the action of explosive blasting can reach thousands or even tens of thousands MPa, which is much larger than the dynamic compressive strength of the coal, while the stress wave intensity generated in the coal in the role of high-energy gas is only a few dozen to a few hundred MPa, which is smaller than the dynamic compressive strength of the coal in most cases. Therefore, high-energy gas fracturing techniques generally do not produce crush zones. Explosive explosions can produce high-intensity stress waves, but the majority of the energy is expended in the fragmentation zone, and a smaller percentage of the energy promotes crack expansion. Although the stress wave intensity generated by high-energy gas action is low, it does not produce a crush zone, and most of the energy promotes crack extension.

Figure 8. Comparison of SEM images before and after high-energy gas fracturing test. (a) Before high-energy gas fracturing test; (b) After high-energy gas fracturing test.

Figure 8. Comparison of SEM images before and after high-energy gas fracturing test. (a) Before high-energy gas fracturing test; (b) After high-energy gas fracturing test.

5. Conclusions

In this paper, a high-energy gas fracturing agent was developed, and its performance, sensitivity, pressure, and temperature resistance were tested. High-energy gas fracturing simulated coal tests were carried out, measuring the pressure in the borehole wall and the strain in the test block, in addition to analyzing the fracturing effect of high-energy gas. The following conclusions were drawn:

(1) Analyzed the gas production principle of fracturing agent and the fracturing mechanism of high-energy gas and developed a high-energy gas fracturing agent with high gas production, strong desorption effect, and excellent performance.

(2) The pressure–time curve of the borehole wall under the action of high-energy gas can be divided into three stages, namely, the rapidly rising pressure-rising stage, the slowly rising pressure-stabilizing stage, and the non-linearly declining pressure-releasing stage. The pressure rise time in the borehole is about a dozen us, the maximum pressure is tens to hundreds MPa, and the pressure distribution in the borehole is not uniform.

(3) Under the action of high-energy gases, the stress wave in the simulated coal sample consists of two phases: compressional phase and tensile phase; the intensity of the stress wave is about several tens of MPa, which can only cause tensile destruction to the simulated coal sample but not compressional destruction. In addition, the duration of the stress wave under the action of high-energy gas is long (about 5 ms~15 ms), and the peak value of the stress wave decays slowly with the increase in distance (the decay coefficient α = 1.67).

(4) High-energy gas fracturing technology is primarily based on the quasi-static action of the gas to cause tensile destruction to coal, and the stress wave energy utilization is high, so it has an excellent fracturing effect.

In this work, the influence of ground stress is not considered. Ground stress may affect the fracturing effect of the fracturing agent. In the future, research on the fracturing agent fracturing coal considering ground stress can be carried out.