Hydraulic fracturing method for relieving stress concentration in remaining coal pillar in overlying goaf

The stress concentration caused by the presence of the remaining coal pillar in the overlying goaf can easily cause large deformation of the roadway in the lower seam and other problems. To solve this problem, the reasons for stress concentration caused by the remaining coal pillar in the overlying goaf were analyzed. A hydraulic fracturing method for relieving stress concentration has been proposed. By using hydraulic fracturing technology to remove the hanging roof on both sides of the coal pillar and weaken the coal pillar system, high stress is transferred to the goaf. The degree of stress concentration in coal pillars and surrounding rock of roadway can be reduced by 47% and 26%, respectively. This method can also solve the stress concentration problem of the remaining ore pillar in the overlying goaf of the metal ore. This method has been applied in the coal mine. The deformation of the surrounding rock of the test roadway after hydraulic fracturing has been reduced by more than 60%.


| INTRODUCTION
To ensure the stability of the surrounding rock of headentry and tailentry and isolate harmful gases in the goaf, it is generally necessary to leave a sectional protection coal pillar between two working faces.In multi-coal seam mining, because the cantilever structure is formed on both sides of the remaining coal pillar in the overlying goaf, the stress in the goaf on both sides is transferred to the roof of the remaining coal pillar, which will lead to the stress concentration of the remaining coal pillar and its roof and floor, which will easily lead to large deformation of the roadway in the lower seam, a severe manifestation of rock pressure when the lower coal seam working face passes through the coal pillars, rock burst, coal and gas outburst, and other problems.
To address the above issues, a more passive approach is generally adopted.To solve the problem of large deformation of the roadway in the lower seam, the method of increasing the width of the sectional protection coal pillar is generally adopted, and the roadway in the lower level coal seam is arranged below the overlying goaf. 1 Strengthening roadway support can also be used to reduce the deformation of the roadway, 2,3 such as using a combined support process of the bolt, anchor, steel shed, and single hydraulic props.However, increasing the width of the sectional protection coal pillar will result in a waste of coal resources; once the width of the coal pillar is left unfavorable, it can also cause instability of the coal pillar group, posing a great safety hazard. 4Adopting the method of strengthening roadway support will significantly increase the cost of roadway support.To solve the problem of severe manifestation of rock pressure when the lower coal seam working face passes through the coal pillar, it is generally adopted to strengthen roof management, including ensuring that the hydraulic support is tightly connected to the roof, ensuring that the support strength meets the regulations, and strictly prohibiting the occurrence of the empty roof.However, adopting measures of strengthen roof management has not truly relieved the stress concentration of the remaining coal pillars in the overlying goaf.Due to the impact of mining, safety accidents are prone to occur in the working face.Coal seam water injection can be used to solve the problems of rock burst and coal and gas outburst during coal seam mining.Coal seam water injection can change the physical and mechanical properties of the coal seam, soften the mechanical properties of the coal seam, and reduce the outburst proneness of the coal seam. 5,6It is also possible to construct large-diameter pressure relief boreholes in coal seam in the delineated hazardous area to reduce bearing capacity and transfer stress. 7,8However, the method of using coal seam water injection to solve the rock burst problem is ineffective, and the water injection time is longer.The measure of constructing large-diameter pressure relief drilling is usually to arrange dense boreholes, with a large number of boreholes, high labor intensity, and cost. 9,102][13] In recent years, the mining industry has also widely used hydraulic fracturing technology.Using hydraulic fracturing technology to control hard roof, weaken hard bottom coal, improve coal seam permeability, prevent coal and gas outburst, and prevent rock burst has achieved remarkable results. 14,15To control the direction of crack propagation and directionally cut off coal and rock layers, directional hydraulic fracturing has been proposed.Pre-slit and porous simultaneous hydraulic fracturing are widely used directional hydraulic fracturing techniques.To form more cracks in the coal rock mass, which can fully break the coal rock mass or improve the permeability of the coal seam, pulse hydraulic fracturing has been proposed.Compared with the traditional constant displacement hydraulic fracturing, the pump displacement fluctuates periodically with high frequency in the form of pulse wave in the pulse hydraulic fracturing process, resulting in periodic changes of water pressure, which can form a more complex fracture network in the coal and rock layers.
To overcome the shortcomings of the passive methods mentioned above, based on the advantages of hydraulic fracturing technology, and taking the stress concentration in the remaining coal pillar in overlying goaf in Baidong Coal Mine in Datong Mining Area as the research background, this article analyzes the reasons for the stress concentration of the remaining coal pillar, and proposes a hydraulic fracturing method for relieving stress concentration in remaining coal pillar in overlying goaf.It has been applied in the Baidong coal mine and has achieved a good application result.

| ANALYSIS OF STRESS CONCENTRATION OF REMAINING COAL PILLAR IN OVERLYING GOAF
The stress concentration of the remaining coal pillar in the overlying goaf is analyzed based on the condition of the Baidong coal mine in the Datong mining area.

| Geological condition
The Baidong coal mine in the Datong mining area mainly mines the 3 # and 5 # coal seams.The 301 panel area of the overlying 3 # coal seam has been fully mined (2002-2004), and the first working face (8107 working face) of the 5 # coal seam is currently being prepared for mining.The average embedding depth of the area where the 8107 working face is located is about 450 m.According to the stratigraphy of 8107 working face (Figure 1), it can be seen that the thickness of the main roof of 3 # coal seam is about 8.8 m, the thickness of the immediate roof of 3 # coal seam is about 3 m, the thickness of 3 # coal seam is about 4.1 m, the thickness of the floor of 3 # coal seam is 5.1 m, and the thickness of 5 # coal seam is about 11.6 m.The physical and mechanical parameters of coal and rock seam are shown in Table 1.

| Production technical condition
The recoverable strike length of the 8107 working face of the 5 # coal seam is 1354 m and the inclined length is 135.5 m.The working face is arranged along the strike, using the full caving method to manage the roof.The excavation methods used in the Baidong coal mine are retreating longwall mining on the strike and the fully mechanized low position top coal caving.The headentry and tailentry are both arranged along the floor, with a rectangular cross-section.The support method is a combination of bolt, anchor, and wire mesh.The width and height of the headentry are 5.

| Reasons for stress concentration caused by remaining coal pillar in overlying goaf
After the upper coal seam of the closed distance seam group is mined out, the roof of the goaf gradually bends and descends from the edges of both sides of the coal pillar toward the direction of the goaf.After hanging the roof for a certain distance, the roof begins to collapse, and the cantilever beam is formed on both sides of the coal pillar (Figure 2).
The weight of the overlying strata on the cantilever beam in the goaf and coal pillar first acts on the hard main roof above the cantilever beam and coal pillar, and then acts on the stable coal pillar through the hard roof.Because the area of the coal pillar is smaller than the overall area of the hanging roof area and the hard main roof above the coal pillar, stress concentration occurs inside the stable coal pillar.The high stress propagates downwards through the coal pillar, affecting the mining and digging activities of the nearby coal seam in the lower part.The overlapping layout of the two coal pillars in the upper and lower coal seams also poses a risk of instability of the coal pillar group under the long-term rheology effect.
After the mining of the 8107 working face in the 5 # coal seam exceeded the original setup entry of the 8107 working face in the 3 # coal seam, under the combined effect of high stress generated by the remaining coal pillar in the overlying goaf and the dynamic mining pressure of this working face, serious deformation began to occur in the tailentry at 180 m ahead of the working face.The maximum roof-to-floor convergence was 0.81 m and the maximum two-sided displacement was 0.62 m, which seriously affected the normal use of the roadway and posed great safety hazards (Figure 3).

| The idea of using hydraulic fracturing to control stress concentration
After the upper coal seam of the close-range coal seam group is mined out, the weight of the overlying strata first acts on the hard main roof of the goaf and stable coal pillar roof, and then propagates downward stress through the coal pillar, thereby affecting the mining and digging activities of the lower coal seam.This part of the stress can not be removed and can only be transferred, so it is necessary to transfer the high stress to the goaf to reduce the stress concentration of the remaining coal pillar (Figure 4). 16The specific ideas are as follows: (1) Using pulse hydraulic fracturing technology, [17][18][19] a dense network of cracks is formed in the hard roof directly above the coal pillar, fully crushing this area.By using directional hydraulic fracturing technology, 20,21 the hanging roof of the goaf on both sides of the coal pillar is cut off, effectively optimizing the stress above the coal pillar and reducing the source of stress.
(2) By using pulse hydraulic fracturing technology, [17][18][19] a dense network of cracks is formed within the coal pillar, fully crushing this area, reducing the stiffness of the coal pillar, and thereby reducing the bearing capacity of the coal pillar.(3) By using pulse hydraulic fracturing technology, [17][18][19] a dense network of cracks is formed in the hard floor directly below the coal pillar, fully crushing this area, thereby weakening the ability to transmit stress concentration.(4) At the same time, the fractured area formed in the coal pillar and its roof and floor using pulse hydraulic fracturing technology can absorb the dynamic pressure generated by the collapse of the roof on both sides of the coal pillar, preventing the transmission of dynamic pressure to the lower part of the coal pillar.
3.2 | Mechanism of using hydraulic fracturing to control stress concentration

| Hanging roof subsidence conditions and coal pillar stress
Hanging roof subsidence conditions Based on the above analysis, after the mining of the working faces on both sides of the coal pillar is completed and the strata stabilize, a cantilever beam will be formed on both sides of the coal pillar (Figure 4).
To block the transfer of high stress from the goaf to the coal pillar system, a conceptual model was established to achieve stress transfer by cutting off the hanging roof on both sides of the coal pillar (Figure 5).At the borehole section, positive pressure occurs due to the mutual compression of the two roofs, resulting in significant frictional resistance.When the sliding force on the section is less than the frictional resistance, the roof can not collapse promptly and will compress the roof above the roadway, forming a largely concentrated stress and increasing the difficulty of roadway support.When the sliding force on the section is greater than the frictional resistance, the roof of the goaf can smoothly sink at the section.The force analysis shows that: (1) In the formula, q 1 is the vertical load; q 2 is the horizontal load; α and β are the cutting angle; L 1 and L 2 are the length of the hanging roof; d 1 and d 2 are the roof cutting thickness; μ is the coefficient of friction.

Coal pillar stress
The forces acting on the coal pillar through the cantilever beams on both sides of the coal pillar are (3) Above the coal pillar, there is a uniformly distributed load, and its stress is It can be seen that the combined force on the coal pillar is After using hydraulic fracturing to remove both sides of the hanging roof, the collapsed gangue in the goaf fills the goaf.After the strata stabilize, the coal pillar is only subjected to the uniformly distributed load exerted by the roof directly above.At this point, the force on the coal pillar is By comparing and analyzing formulas ( 6) and ( 7), it can be seen that after hydraulic fracturing is performed to remove the hanging roof on both sides of the coal pillar, the stress of the coal pillar is reduced by q 1 L 1 + q 1 L 2 .The phenomenon of stress concentration has been significantly improved, with high stress transferring to the goaf side on both sides of the coal pillar.

| Stress environment of the surrounding rock of the roadway below the remaining coal pillar
Numerical simulation scheme Using UDEC numerical simulation software, Two rectangular numerical models with a length of 385.9 m and a width of 72.48 m were established (Figure 6).The thickness of coal and rock seam is shown in Figure 1, and the physical and mechanical properties of coal and rock seam are shown in Table 1.The middle is a coal pillar, and both sides of the pillar are goaf of two working faces.The left, right, and lower boundaries are fixed boundaries, with an upper boundary subjected to a compressive stress of 11.25 MPa.The overlying remaining coal pillar and its roof and floor in Model 1 are intact coal and rock seam, while the overlying remaining coal pillar and its roof and floor in Model 2 are prefabricated with directional cracks and pulse hydraulic fracturing cracks.The survey line is arranged on the roadway roof to observe the stress environment of the roadway roof before and after hydraulic fracturing.| 1523

Numerical simulation results
The vertical compressive stress cloud diagram of the roadway roof before and after hydraulic fracturing is shown in Figure 7, and the vertical compressive stress curve of the roadway roof before and after hydraulic fracturing is shown in Figure 8.Before hydraulic fracturing, a hanging roof of about 40 m long appeared on both sides of the coal pillar (Figure 7).After hydraulic fracturing, the hanging roof collapses and fills the goaf (Figure 7).Before and after hydraulic fracturing, the peak stress in the coal pillar between the headentry and the tailentry of the 8107 working face were 33.92 and 17.96 MPa, respectively.After hydraulic fracturing, the peak stress decreased by 47% (Figure 8).Before and after hydraulic fracturing, the peak stress of the roof on the side of the working face of the tailentry of the 8107 working face were 16.11 and 11.85 MPa, respectively.After hydraulic fracturing, the stress decreased by 26% (Figure 8).Before and after fracturing, the peak stress of the roof on the side of the working face of the headentry of the 8105 working face were 17.85 and 12.91 MPa, respectively.After hydraulic fracturing, the stress decreased by 28% (Figure 8).By cutting off the hanging roof on both sides of the coal pillar and weakening the hard roof directly above the coal pillar, the source of force is reduced.By weakening the coal pillar, the bearing capacity of the coal pillar is reduced.By weakening the hard floor directly below the coal pillar, the ability to transmit stress concentration is weakened.Under the above comprehensive effects, the phenomenon of stress concentration in the surrounding rock of the roadway below the coal pillar has been significantly alleviated, with high stress transferring to the goaf.

| FIELD TEST SCHEME 4.1 | Borehole layout
As shown in Figure 9, the specific layout of roof cutting borehole and weakening borehole is as follows: (1) At a distance of 400 m ahead of the working face in the tailentry of the 8107 working face in the 5 # coal seam, an A 1 borehole is drilled at a distance of 2 m from the floor on the side wall of the coal pillar.A 1 borehole angle is 44°, borehole diameter is 50 mm, and borehole length is 37 m.At a distance of 5 m from the A 1 borehole toward the stopping line, a B 1 borehole is drilled at a distance of 1.1 m from the side wall of the coal pillar on the roof.B 1 borehole angle is 80°, borehole diameter is 50 mm, and borehole length is 26.5 m.By analogy, 20 class A and B boreholes were drilled each.(2) In the same way, boreholes (numbered C 1 -C 20 , D 1 -D 20 ) were arranged along the headentry of the 8105 working face of the 5# coal seam, and the same type of borehole was staggered with each other.

| Hydraulic fracturing equipment and system layout
In the tailentry of the 8107 working face on the 5 # coal seam, a hydraulic fracturing pump, monitoring system, and control system are arranged at a distance of 500 m from the working face (Figure 10).The rated pressure of the fracturing pump is 70 MPa and the rated flow rate is 120 L/min.The sealing device used for sealing boreholes adopts a special rubber packer.
According to the depth of the hydraulic fracturing borehole, multiple high-pressure sealing installation rods are used to connect with the packer to achieve deep borehole sealing.After the completion of the hydraulic fracturing of the tailentry of the 8107 working face of the 5 # coal seam, the hydraulic fracturing system will be arranged in the same way as the headentry of the 8105 working face of the 5 # coal seam.

| Implementation process of hydraulic fracturing
First, according to the boreholes layout plan, a total of 40 boreholes were drilled in the tailentry of 8107 Vertical compressive stress distribution of roadway roof before and after fracturing.
working face and the headentry of 8105 working face, respectively.Then, in the tailentry of the 8107 working face, we conducted hydraulic fracturing on class A boreholes in sequence from the inside out.Then, hydraulic fracturing was performed on class B boreholes from the inside out in sequence.Class A boreholes adopt pulse hydraulic fracturing method and backward segmented hydraulic fracturing method.Specifically, first, pulse Fracking is carried out on the roof by using Class A boreholes.Then, after retreating the sealing device and reconnecting the equipment, pulse hydraulic fracturing is performed on the coal pillar.Finally, retreating the sealing device and reconnecting the equipment, pulse hydraulic fracturing is performed on the floor.Class B borehole is a directional hydraulic fracturing roof-cutting borehole.
Three boreholes are simultaneously subjected to hydraulic fracking to induce fractures along the borehole line.
Finally, the hydraulic fracturing equipment was transported to the corresponding position in the headentry of the 8105 working face.After installation and debugging, hydraulic fracturing was carried out on all boreholes in the headentry of the 8105 working face using the same hydraulic fracturing method as the tailentry of the 8107 working face.

| Hydraulic fracturing water pressure and water flow curve
The hydraulic fracturing measurement and control instrument was used on-site to collect the water pressure As shown in Figure 11, during the hydraulic fracturing process, the peak water pressure of the A 3 borehole in the tailentry of the 8107 working face was 39 MPa, which means the fracture pressure of the borehole was 39 MPa.The water flow rate near the peak water pressure was small and the pressure value was constantly changing, indicating that the crack was gradually initiating at this time.During the hydraulic fracturing period of the A 3 borehole, the following phenomena were observed: water flow was observed in the A 2 and A 4 boreholes, while no water flow was observed in A 1 and A 5 boreholes (Figure 12).The flow of water from the A 2 and A 4 boreholes indicates that the cracks generated by the A 3 borehole have expanded to the A 2 and A 4 boreholes, with a radius of more than 10 m for the crack propagation.There was no water flow in the A 1 and A 5 boreholes, indicating that the radius of the crack propagation did not reach 20 m.
As shown in Figure 11, during the hydraulic fracturing process, the peak water pressure of the A 7 borehole in the headentry of the 8107 working face was 37 MPa, which means the fracture pressure of the borehole hole was 37 MPa.The water flow rate near the peak water pressure was small and the pressure value was constantly changing, indicating that the crack was gradually initiating at this time.During the hydraulic fracturing period of the A 7 borehole, the following phenomena were observed: water flow was observed in the A 6 and A 8 boreholes, while no water flow was observed in the A 5 and A 9 boreholes.The flow of water from the A 6 and A 8 boreholes indicates that the cracks generated by the A 7 borehole have expanded to the A 6 and A 8 boreholes, with a radius of more than 10 m for crack propagation.There was no water flow in the A 5 and A 9 boreholes, indicating that the radius of the crack propagation did not reach 20 m.

| Roadway deformation after hydraulic fracturing
After hydraulic fracturing, as the 8107 working face of the 5 # coal seam advanced to the hydraulic fracturing area, the deformation of the surrounding rock in the tailentry of the working face is relatively small within 300 m of the advanced working face (Figure 13).The maximum displacement of the roof and floor is 31 cm, which is 62% lower than the displacement of the roof and floor within a range of 300 m ahead of the working face when no hydraulic fracturing measures are taken (Figures 3 and 13).The maximum displacement of the two sides is 23 cm, which is 63% lower than the displacement of the two sides within a range of 300 m ahead of the working face when no hydraulic fracturing measures are taken (Figures 3 and 13).After hydraulic fracturing, a large amount of roadway repair work was reduced while eliminating safety hazards.

| CONCLUSION
(1) Due to the supporting effect of the coal pillar, it is easy to form hanging roof on both sides of the coal pillar, transferring the stress in the goaf through the hanging roof and stable coal pillar system to the coal and rock seam below the coal pillar.(2) A hydraulic fracturing method was proposed to relieve the stress concentration of the remaining coal pillars in the overlying goaf.Hydraulic fracturing technology was used to directionally cut off the hanging roof of the coal pillars and weaken the coal pillar system, transferring high stress to the goaf and reducing the stress concentration of the overlying remaining coal pillars.It can reduce the stress concentration degree of coal pillars by about 47%, and the stress concentration degree of roadway surrounding rock can be reduced by more than 26%.This method can also solve the stress concentration problem of the remaining ore pillar in the overlying goaf of the metal ore.(3) The hydraulic fracturing method for relieving stress concentration in the remaining coal pillars in the overlying goaf has been successfully applied in the Baidong coal mine of the Datong mining area.A borehole spacing of 10 m can ensure that the roof is cut off and the coal pillar system is fully weakened.Compared with the roadway within 300 m ahead of the working face when no fracturing measures were taken, the deformation of the surrounding rock of the test roadway after hydraulic fracturing was reduced by more than 60%, reducing a large amount of roadway repair work and eliminating safety hazards.
3 and 3.5 m, respectively.The width and height of the tailentry are 4.6 and 3.5 m, respectively.The width of the coal pillar between the 8107 and 8105 working faces of the 5 # coal seam is 35 m.The coal pillars of the 3 # coal seam and 5 # coal seam are arranged in overlapping positions, as shown in Figure 1.

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I G U R E 1 Geological and mining conditions of the working face.

F I G U R E 2
Reasons for stress concentration caused by remaining coal pillar in overlying goaf.F I G U R E 3 Serious deformation occurred in the surrounding rock of the tailentry of the 8107 working face in 5 # coal.(A) Roof subsidence.(B) Floor heave.

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I G U R E 4 Principle of using hydraulic fracturing to control stress concentration.F I G U R E 5 Conceptual model of stress transfer.SHAO ET AL.

F I G U R E 6
Numerical calculation model.(A) Grid division before hydraulic fracturing (Model 1).(B) Grid division after hydraulic fracturing (Model 2).(C) Survey line layout.F I G U R E 7 Vertical compressive stress cloud diagram before and after hydraulic fracturing.(A) Before hydraulic fracturing.(B) After hydraulic fracturing.

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I G U R E 9 Borehole layout.(A) Plan view.(B) Section A-A. and flow rate of the roof-cutting borehole, with a data collection frequency of 1 time/s.The water pressure for hydraulic fracturing is between 35 and 40 MPa.During the hydraulic fracturing period, at least water flowed out of the previous fracturing borehole, indicating that the new hydraulic fracture is interconnected with the old hydraulic fracture, with a crack propagation radius of 10-20 m.This is illustrated by two typical hydraulic fracturing tests of roof cutting boreholes in the tailentry of 8107 working face.

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I G U R E 10 Hydraulic fracturing equipment and system layout.F I G U R E 11 Water pressure and flow curve of A 3 and A 7 boreholes in the tailentry of 8107 working face.(A) A 3 borehole.(A) A 7 borehole.SHAO ET AL.| 1527

F I G U R E 12
Water outflow from A 2 and A 4 boreholes during hydraulic fracturing of A 3 boreholes.F I G U R E 13 Comparison of deformation of surrounding rock in the roadway before and after hydraulic fracturing.(A) Before hydraulic fracturing.(B) After hydraulic fracturing.
Physical and mechanical parameters of coal and rock seam.
T A B L E 1