Promoting Gas Extraction Technology with Screen Pipe for Long Borehole Protection in Soft Seam

Abstract

The inherent properties of soft coal seams and their mechanical environment make long boreholes susceptible to issues like collapse, deformation, and blockages. These problems shorten the service life of the boreholes and hinder extraction efficiency. This paper tackles these challenges by analyzing the deformation and damage patterns of long boreholes in soft coal seams. It examines the stress distribution and deformation characteristics around both protected and unprotected boreholes at different burial depths. Additionally, it recommends using screen pipe protection technology to improve gas extraction and mining operations, as demonstrated in Changping Coal Mine. The results show that screen pipe protection substantially improves the stress distribution and deformation stability of coal seam boreholes. The flow attenuation coefficients of two boreholes equipped with protection technology decreased by 48% and 61%. After 50 days of extraction from boreholes with a protection rate exceeding 90%, gas concentration remained above 50%, which is 2.59 times higher than that of unprotected boreholes. This technology effectively addresses the frequent accidents, poor extraction performance, and inefficiency of long boreholes in soft coal seams, ensuring the mine’s safe and efficient production.

1. Introduction

Coal is a vital source of energy and industrial raw material in China. For decades, it has been a cornerstone of the country’s economic growth. Coalbed methane (CBM), which is closely associated with coal, is a clean energy source with a calorific value comparable to natural gas. Utilizing this resource can enhance China’s energy mix and support the sustainable development of the coal industry [1,2]. In light of this, on 24 October 2021, General Secretary Xi Jinping called for the rapid development of unconventional energy resources like shale gas, coalbed methane, and tight oil and gas.

Extracting mine gas is crucial for preventing underground gas accidents and harnessing coal-related energy resources [3,4,5]. Soft coal seams are particularly challenging for gas extraction due to their weak structure, low permeability, high gas content, and elevated gas pressure, making it difficult to maintain borehole stability. Ground stress, gas pressure, and drilling disturbances further compromise the long-term stability of unprotected boreholes in soft coal seams. After drilling, boreholes often collapse, deform, or experience blockages, resulting in early failure [6,7]. Ensuring the long-term stability of boreholes for continuous gas extraction in soft coal seams has become a critical challenge in gas control.

Applying protective measures in soft coal seams is an effective way to prevent borehole collapse [8]. Sieve pipe protection supports the borehole walls in soft coal seams. Even if the walls collapse, the screen pipe maintains a clear passage for gas extraction, ensuring efficiency and stability [9]. Qi et al. used simulation software to study the creep model around the wellbore and divided it into three processes of wellbore collapse [10]. Geng et al. used a self-designed monitoring experimental system to study the dynamic evolution characteristics of coal drilling during construction and found that the deformation of drilling is mainly affected by the redistribution of coal stress, rather than the influence of drilling parameters [11]. Zhang et al. believe that the pressure relief zone is prone to collapse; the post-peak stress concentration area is prone to collapse and spray holes. These findings offer a strong theoretical foundation for preventing abnormal events like hole collapse and drill sticking during the drilling process, ultimately improving drilling depth in soft coal seams [12,13,14]. However, due to the characteristics of the soft coal seam itself, the friction impact of drilling tools on the borehole wall during drilling withdrawal means that it is very easy to cause borehole collapse failure, and the phenomenon of “drilling withdrawal collapsed immediately” [15].

To address the issues of borehole collapse, deformation, and plugging, a test promoting the use of screen pipe protection for gas extraction in soft coal seams was conducted at Changping Coal Mine. The test shows that after the screen pipe protection technology is applied to long boreholes in soft coal seams, the technology reduces the unprotected borehole time of long boreholes to a minimum. The screen pipe not only improves the extraction effect as a gas extraction channel but also solves the problem of borehole accidents in soft coal seams and ensures the extraction effect.

2. Analysis and Methods

2.1. Deformation Failure Characteristics of the Long Borehole in Soft Coal Seams

The factors that cause the collapse failure of the borehole can be roughly divided into the following two factors [16,17,18,19]: (1) Mechanical characteristics of the coal body and its mechanical environment: the mechanical environment of a coal body buried at a certain depth is relatively complex, and is affected by the vertical stress and horizontal ground stress of the overlying strata and the gas pressure stored in pores and fissures in the coal. After the coal seam is drilled, the stress of the coal body around the borehole is redistributed. When the stress of the coal body exceeds its strength, the hole wall will collapse, which will lead to the serious failure of the drill hole. (2) Drilling engineering factors: a series of engineering factors, such as the increase of water content in a coal seam caused by hydraulic measures and antireflection technology, the borehole inclination, the borehole unprotected extraction time, and the friction and collision of drilling tools against the borehole wall, will all cause the collapse failure of a borehole to a certain extent.

Mohr–Coulomb strength theory is the most widely used failure criterion of coal at present. This theory believes that the failure of coal is mainly shear failure [20,21,22]. According to the Mohr–Coulomb strength theory, when the shear stress of any stress surface of the coal body reaches a certain limit, the coal body will be destroyed. The specific expression is as follows:

$$\tau =C+\sigma tan\phi$$

(1)

where τ is the shear strength; $C$ is coal cohesion, kPa; $\sigma$ is the normal stress of the coal body at the failure position, MPa; φ is the friction angle in coal body.

The stress state of a micro-unit in coal can be described by Mohr’s stress circle. The stress on any plane in the micro-unit can be decomposed into normal stress $\sigma$, and shear stress $\tau$: when Mohr’s stress circle is tangent to the shear strength envelope [23,24,25], the coal is in the stress limit equilibrium state, as shown in Figure 1.

For the coal in the limit equilibrium state, if the stress at the stress point continues to increase, the coal will be damaged. The limit equilibrium condition of the coal body can be expressed by the following formula:

$${\sigma }_1={\sigma }_3{\mathit{tan}}^2\left({45}^{°}+{\displaystyle \frac{\phi }{2}}\right)+2C\mathit{tan}\left({45}^{°}+{\displaystyle \frac{\phi }{2}}\right)$$

(2)

In the formula, ${\sigma }_1$ and ${\sigma }_3$ are the maximum and minimum principal stresses, respectively. As shown in Equation (2), the greater the difference between ${\sigma }_1$ and ${\sigma }_3$, the easier it is for the coal around the borehole to collapse and fail. As can be seen from the Mohr’s stress circle in Figure 2, when $\alpha$ is taken at 90° and 270°, the difference between ${\sigma }_1$ and ${\sigma }_3$ is the largest; this location is the most prone to collapse in the borehole [26]. When considering the pore pressure $p_p$ and stress correction coefficient $\eta$ of the coal body, the radial and circumferential effective stress state of the coal body can be expressed by the following formula:

$${\sigma }_r=p_f-\beta p_p-f\left(p_f-p_p\right)$$

(3)

$${\sigma }_{\theta }=\eta \left(3{\sigma }_V-{\sigma }_H-p_f\right)+\left({\displaystyle \frac{\beta \left(1-2\mu \right)}{\left(1-\mu \right)}}-f\right)\times \left(p_f-p_p\right)-\beta p_p$$

(4)

where $\beta$ is the effective stress coefficient.

Substituting Formula (2) into the above formula, we can obtain the coal body collapse and failure condition around the borehole when considering the pore pressure $p_p$ inside the coal body and the support force $p_f$ on the hole wall [18,19]:

$$p_f={\displaystyle \frac{\eta \left[3{\sigma }_V-{\sigma }_H-\left(\lambda -f\right)\right]+K^2{p}_{p}f-2CK}{\left(f-\beta +1\right)K^2-\eta \left(\lambda -f-1-\beta \right)}}$$

(5)

where $K=\mathit{tan}\left({45}^{°}+{\displaystyle \frac{\phi }{2}}\right)$;$\lambda ={\displaystyle \frac{\beta \left(1-2\mu \right)}{\left(1-\mathsf{\mu }\right)}}$; $f$ is porosity; ${\sigma }_H$ is the vertical geostress; ${\sigma }_V$ is the maximum horizontal geostress; $\eta$ is the nonlinear stress correction factor, with a value of 0.9–0.95.

It can be seen from the above formula that the risk of borehole collapse and failure increases with the decrease of cohesion $C$, which indicates that the softer the coal seam, the more serious the borehole collapse will be [27]. At the same time, the greater the original gas pressure $p_p$ of the coal seam, the greater the risk of borehole collapse failure.

For soft coal seams, the drilling operation will cause changes in the stress environment of the coal around the borehole, and the original stress balance conditions will be broken. It is difficult to maintain the stability of an unprotected borehole for a long time, and it is very easy to cause deformation and failure of the drilling [28].

2.2. Geometric Model Establishment and Simulation Scheme

FLAC^3D numerical simulation software was used to model the structures of the coal seam, borehole, and screen pipe, and to study the stress distribution and deformation characteristics of the coal body around the unprotected borehole and the protected borehole under the conditions of 380, 580, and 780 m burial depth, respectively.

The coal seam model is 3 × 3 × 1 m; the diameter of the borehole is 113 mm, the length is 1 m, and it is located in the center of the coal seam; the borehole protection screen pipe is fitted with the wall of the borehole, and the pipe’s wall thickness is 2 mm. Sliding boundaries are used around the model and the base plate, with free boundary conditions applied to the upper surface and vertical stress applied. The physical model is shown in Figure 2, and the physical parameters of the medium are set as shown in

Table 1. Physical and mechanical parameters of the medium.

Medium	Density (kg/m3)	Bulk Modulus (GPa)	Shear Modulus (GPa)	Cohesion (MPa)	Internal Friction Angle ($°$)	Tensile Strength (MPa)
Coal seam roof	2650	8.05	5.98	15.3	45.4	11.9
Coal seam	1400	0.50	0.16	1.00	28.0	0.50
Coal seam floor	2800	1.60	1.20	3.47	43.0	4.96
PVC screen pipe	1380	3.00	1.20	0.08	34.0	65.0

.

3. Numerical Simulation Results and Discussions

Borehole operations in coal seams inevitably redistribute the stress field in the original rock, and the presence of the boreholes changes the initial stress distribution pattern [29]. The stress distribution and displacement changes in the borehole and the surrounding coal rock are analyzed and sectioned.

3.1. Characteristics of Displacement and Stress Distribution around the Unprotected Borehole

The stress distribution and displacement changes of the coal body around the unprotected borehole at different burial depths are shown in Figure 3.

As regards coal rock displacement, the presence of the borehole disrupted the original stress state of the coal body and created a new stress state equilibrium, and the top and bottom ends of the borehole created significant displacement changes due to stress. The bottom deformation range is larger than the top deformation, but the deformation degree is smaller than the top deformation. The increase in burial depth increases the displacement range of the top and bottom of the borehole, and the deformation degree of the surrounding coal body also increases gradually. Under the condition of unsupported boreholes, collapses, plugs, and other borehole accidents are very likely to occur, and due to the location of stress concentration, borehole accidents are more likely to occur on both sides of the borehole.

The presence of the borehole caused the formation of a stress unloading zone in the surrounding coal rock, and the pressure unloading area at the top and bottom was slightly larger than the pressure unloading area on the left and right sides. After a certain range of pressure, an unloading zone formed on the left and right sides and a certain range of stress concentration phenomenon and stress peaks appeared, forming a pressurized state again. With the increase in burial depth, the stress concentration phenomenon of the coal body around the borehole becomes more obvious, and the location of the unloading and stress concentration area appears to change slightly.

3.2. Characteristics of Displacement and Stress Distribution around the Protected Borehole

The displacement and stress distribution of the coal body around the protected borehole at different burial depths are shown in Figure 4.

For the protected boreholes, the displacement and stress distribution pattern of the coal body is similar to that of the unprotected boreholes, but the range and location of the displacement and stress show a big difference due to the supporting effect of the borehole screen pipe. The borehole screen pipe provides protective support for the borehole; the top and bottom displacements are smaller, and the surrounding rock stress is controlled to a certain extent, which indicates that the internal support structure improves the bearing capacity of the surrounding rock of the borehole. However, with the increase in burial depth, the deformation displacement of the borehole becomes larger, the surrounding rock squeezes the screen pipe, and the screen pipe has a support limit. The screen pipe makes the stress concentration range on both sides of the borehole smaller and the location of the stress peak is changed, which is closer to the outer wall of the screen pipe. When the depth of burial increases to 780 m, the screen pipe cannot fully bear the increase in ground stress, and the stress concentration occurs on both sides of the borehole. However, compared with the unprotected borehole, the stress distribution and displacement deformation are still improved to a certain extent.

3.3. Comprehensive Analysis

To quantitatively evaluate the effect of screen pipe protection on the displacement and stress distribution of the surrounding coal rock, the displacement and stress data of the two boreholes are presented, as shown in Figure 5.

As shown in Figure 5a, the top and bottom displacements of the boreholes increased with the increase in the burial depth, and the trend of the increase was roughly the same. For the unprotected boreholes, the bottom displacements at the three burial depths were 7.08 mm, 9.09 mm, and 12.36 mm, and the top displacements were 9.44 mm, 12.19 mm, and 16.48 mm, respectively, which increased the top and bottom displacements by about 15.0% for every 100 m of burial depth. Due to the supporting effect of the screen pipe, the deformation of the protected borehole is weakened, and the top and bottom displacements are reduced. For example, at a burial depth of 580 m, the top and bottom displacements are 9.14 mm and 6.82 mm respectively, which are reduced by 25.0% compared with the unprotected borehole. The sieve pipe borehole protection technology can better protect the borehole form and ensure the long-term service of gas extraction boreholes.

The stress distribution curve around the borehole is shown in Figure 5b. For the unprotected borehole, the coal body around the borehole is divided into a stress unloading zone, stress concentration zone, and raw rock stress zone from near to far, respectively. The increase in burial depth increases the stress peaks and takes the stress concentration gradually away from the borehole wall. The stress peaks at the three burial depths are 19.62 MPa, 26.21 MPa, and 36.49 MPa, which increase by 33.7% and 39.2%, respectively, and are located at 0.11, 0.15, and 0.17 m from the borehole wall. When the coal wall is in contact with the screen pipe, the screen pipe has a supporting effect on the borehole; the support of the screen pipe decreases the stress peak, and the stress concentration moves towards the borehole wall. When the burial depth is lower than 580 m, the stress concentration occurs in the outer wall of the screen pipe; the screen pipe shares part of the stress extrusion effect for the borehole wall, and the support of the screen pipe provides effective protection for the borehole. The peak stresses at burial depths of 380 m and 580 m are 18.12 MPa and 24.32 MPa, respectively, which are reduced by about 7.5% compared with the unprotected boreholes. When the burial depth reaches 780 m, the screen pipe cannot fully bear the extrusion of the ground stress; this is close to the maximum ultimate strength of the screen pipe, and it is prone to deformation or even destruction. Due to the limited support, the stress concentration is far away from the outer wall of the screen pipe. At 0.98 m from the borehole wall, the stress rises rapidly to the peak value of 32.93 MPa, which is 0.128 m different from the peak position of the unprotected borehole, and the peak stress drops by 9.7%.

With the increase in burial depth, the ground stress rises, the stress peak increases greatly, and the stress peak point is transferred to the deep part of the coal body. The displacement and deformation damage of the hole wall increases, and the damage range of the coal body around the hole increases. Boreholes redistribute the stress in the coal seam, and when the stress on the coal body exceeds its yield strength, the coal rock body in the borehole wall undergoes plastic damage and deformation. The plastic zone is an important index to evaluate the damage range of the coal body around the borehole. Combined with the actual site, the distribution of the plastic zone of two kinds of boreholes under the condition of 580 m burial depth is shown in Figure 6.

From the figure, it can be seen that the existence of the borehole makes plastic damage appear in the surrounding coal rock. The plastic damage at the surrounding sharp corners is serious and shows a tendency to extend to the far side. The supporting effect of the screen pipe on the borehole is very obvious. The existence of the screen pipe only changes the scope of the distribution of the plastic zone; it does not change the shape of the plastic zone of the borehole, which shows the X-shaped distribution pattern. The plastic zone in the surrounding rock of the boreholes was significantly reduced after the protection of the boreholes, and the screen pipes were deformed at the sub-burial depth without plastic damage. The plastic zone area of the two boreholes is 0.087 m² and 0.034 m², respectively, and the plastic zone area is reduced by 60.9%.

4. Field Testing on Gas Drainage with the Borehole Protection Technology

Changping Coal Mine is located in the northwest of Gaoping City, Shanxi Province. Its main mining coal seam 3 # has high original gas pressure and high original gas content. The gas grade identifies it as a high gas mine. The firmness coefficient of the coal seam is between 0.2 and 0.8, with large regional distribution and obvious differences in different positions. The stability of the borehole wall after drilling is poor, and borehole accidents such as collapse, deformation, and plugging failure often occur. Coal seam 3 # is a difficult coal seam to extract. Its permeability coefficient is between 0.0021~0.0324 m²/(MPa²·d), and the attenuation coefficient of borehole gas flow is 0.01~0.1064 d⁻¹. The permeability coefficient is low and the borehole flow attenuation is fast. In recent years, the phenomenon of a slow mining speed and large gas emissions in the process of mining has seriously restricted the safe and efficient production of coal mines. Improving the gas extraction effect of soft coal seams has become an urgent technical problem in Changping Coal Mine.

Experimental boreholes for promoting gas extraction technology using screen pipes for long borehole protection in soft seams are arranged along the transport waste rock roadway and the drilling yard of a sixth panel of the Changping Coal Mine to solve the problems of collapse and deformation of long boreholes in soft coal seams, borehole plugging failure, a short pumping service cycle, and poor pumping effects in the mine.

4.1. Technical Process and Construction Design

The drilling tool can be opened and closed, Φ 113 mm PDC openable drill bit, Φ 89 mm hollow triangular spiral drill pipe, to realize screen pipe pushed in without lifting the drill. Selection of borehole protection equipment; Φ 75 mm PVC screen pipe is used for borehole protection in full hole section, with holes of 8 mm and spacing of 20 cm. Use of borehole protection equipment; Φ 75 mm PVC screen pipe is used for borehole protection in full hole segment. The borehole bottom suspension device is used to ensure the reliability of screen pipe positioning to improve the drilling bit lift efficiency. The borehole protection equipment is shown in the Figure 7.

After borehole cleaning, the borehole bottom suspension device is connected to the screen pipe and is lowered from the tail of the drill pipe through the large diameter hollow three-prism spiral drill pipe. When the screen pipe is pushed into the shallow hole segment, it is sent in manually. When it reaches the deep hole segment, the screen pipe is pushed in with the help of the hydraulic motor of the screen pipe booster. After passing through the drill bit, the wing cutters on both sides of the hole bottom suspension device will open and wedge into the borehole wall under the action of the spring, and the whole set of borehole protection devices will be fixed in the borehole. After the screen pipe is pushed in, the whole set of drilling tools shall be withdrawn from the borehole, and the whole set of borehole protection devices shall be left in the borehole to complete the borehole protection process of the whole hole segment. The technical process diagram is shown in Figure 8.

The technology test was carried out in the transport waste rock roadway, and the drilling yard of the sixth panel of Changping Coal Mine. A total of 12 test boreholes were arranged, including 4 boreholes without protection technology. The construction parameters of the test boreholes are shown in

Table 2. Construction parameters of experimental drilling.

Borehole Number	Main Borehole Length/m	Protective Borehole Length/m	Protective Borehole Rate/%
1 #	248	0	0
2 #	241	0	0
3 #	206	0	0
4 #	348	0	0
5 #	372	241	64.78
6 #	351	261	74.36
7 #	315	227	72.06
8 #	312	226	72.44
9 #	261	243	93.10
10 #	273	256	93.77
11 #	319	304	95.30
12 #	259	250	96.53

.

4.2. Investigation of Technical Pumping Effect

A total of 12 test boreholes were divided into 3 groups, 1 #~4 #, 5 #~8 #, and 9 #~12 #, according to the borehole protection rate. The pure gas extraction data and gas extraction concentration of each borehole were inspected in groups, and the 65-day inspection results were plotted as a comparison curve with time.

The statistics of the net amount of gas extraction from boreholes for 65 days are shown in Figure 9. After 65 days of gas extraction, the average net amount of gas extraction in each group of boreholes had decreased, but the net amount of gas extraction in boreholes 9 #~12 # was higher than that in boreholes 1 #~4 # and 5 #~8 #. The net amount of gas extraction of 1 #~4 # boreholes remained above 0.20 m³/min in the first 9 days and decreased to below 0.10 m³/min after 17 days. The service period of unprotected boreholes was short and the net amount declined rapidly.

After 65 days, the average net amount of gas extraction of the three groups was 0.108, 0.415, and 0.593 m³/min respectively. The net amount of gas extraction of 9 #~12 # was 5.4 times that of 1 #~4 #, and 1.4 times that of 5 #~8 #. The improvement of the borehole protection rate had significantly increased the net amount of gas extraction from gas extraction boreholes. The technology promoting gas extraction technology using screen pipe for long borehole protection in soft seams can effectively improve the net amount of gas extraction and maintain the high level of gas extraction for a longer time.

The attenuation coefficient of gas flow is used to evaluate the difficulty of gas extraction in coal seams. The smaller the attenuation coefficient is, the longer the high-purity extraction time of the borehole is maintained. The attenuation coefficient of gas flow can be calculated according to the following formula:

$$q_t=q_0{e}^{-ad}$$

(6)

where $q_t$ is gas extraction flow of the day, m³; $q_0$ is gas extraction flow of the first day, m³; $a$ is the flow attenuation coefficient, d⁻¹; $d$ is extraction days, d.

The net amount of gas extraction data of three groups of test boreholes were fitted, and the fitting results are shown in Figure 10. The attenuation coefficients of gas flow in three groups of boreholes are 0.031, 0.016, and 0.012 respectively.

The average gas concentration of 1 #~4 # boreholes in 65 days was 20.27%, that of 5 #~8 # boreholes was 36.86%, and that of 9 #~12 # boreholes was 52.45%. The technology of promoting gas extraction technology with screen pipe for long borehole protection in soft seams increased the average pumping concentration by more than 1.8 times. When the borehole protection rate of a long borehole reached more than 90%, the average extraction concentration increased by 2.59 times. After 20 days of extraction in 1 #~4 # unprotected boreholes, the gas extraction concentration decreased to below 20%, and the extraction concentration decreased significantly. The gas concentration in 9 #~12 # boreholes was still 50% after 50 days of extraction, which indicates that the effect of a long borehole screen pipe in a soft coal seam is obvious, and the gas extraction efficiency is significantly improved.

5. Conclusions

• This study analyzed the factors and characteristics contributing to the deformation and failure of long boreholes in soft coal seams, highlighting the impact of coal seam properties, engineering factors, stress conditions, and gas pressure.
• FLAC^3D simulations were conducted to evaluate the stress, displacement, and plastic zones of protected and unprotected boreholes at various burial depths. The findings reveal that screen pipe support alters the stress distribution around the coal rock, significantly reducing deformation and displacement in soft coal seams, thereby prolonging the borehole service life and improving pumping efficiency.
• Field tests at Changping Coal Mine demonstrated that increasing borehole protection significantly boosts gas extraction efficiency. The flow attenuation coefficients of the protected boreholes were reduced by 48% and 61%, respectively, while gas extraction concentrations increased by over 1.8 times.
• This technology effectively mitigates the issue of immediate collapse upon drill withdrawal in soft coal seams. It prevents accidents such as borehole collapse, deformation, and plugging, ensuring longer service cycles and improving the efficiency of gas extraction.