Numerical investigation of the in situ gas explosion fracturing and the enhancement of the penetration in coal seam boreholes

Permeability enhancement of low permeability coal seams is a key tool in coal mine gas extraction and utilization. Numerical investigations are used to analyze the geometric parameters of the drilled gas cavity and the effect of initial pressure on explosion parameters to explore the potential of original gas blasting in enhancing crack penetration in coal seams. On the basis of these analyses, the changes in characteristic parameters of the coal body under blasting pressure are examined. The results show that the maximum explosion pressure tends to be close to the theoretical explosion maximum when the L/Φ is 1.67–2.5, and the mechanical effect of the explosion pressure and surge velocity on the wall reaches the optimum in the experimental group when the L/Φ is 1.67. The initial pressure and the maximum explosion pressure demonstrate a linear positive correlation, and increasing the former can effectively enhance the force applied to the hole wall. During the gas explosion pressurization stage, the cracking range of coal under pressure is 0.05 m, but the duration of peak pressure can extend the range of cracking. The combined effect of the stress and pressure fields causes elastic energy storage in the coal body to fail in the radial region of the hole wall, but regains elastic energy storage as the pressure increases. The effect of fracturing on the radial coal seam permeability of the borehole wall before and after the fracturing effect is expanded by 19.6 times, proving that in situ gas explosion in boreholes can effectively improve the gas seepage characteristics of coal seams and increase the gas recovery rate. The simulations indicate that the application of in situ gas explosion fracturing and permeation technology is limited by the increase in maximum explosion pressure and the cumulative effect time of the peak pressure. This provides a theoretical basis for understanding the constraints of gas explosion fracturing and permeation technology.

analyses, the changes in characteristic parameters of the coal body under blasting pressure are examined.The results show that the maximum explosion pressure tends to be close to the theoretical explosion maximum when the L/Φ is 1.67-2.5, and the mechanical effect of the explosion pressure and surge velocity on the wall reaches the optimum in the experimental group when the L/Φ is 1.67.The initial pressure and the maximum explosion pressure demonstrate a linear positive correlation, and increasing the former can effectively enhance the force applied to the hole wall.During the gas explosion pressurization stage, the cracking range of coal under pressure is 0.05 m, but the duration of peak pressure can extend the range of cracking.The combined effect of the stress and pressure fields causes elastic energy storage in the coal body to fail in the radial region of the hole wall, but regains elastic energy storage as the pressure increases.The effect of fracturing on the radial coal seam permeability of the borehole wall before and after the fracturing effect is expanded by 19.6 times, proving that in situ gas explosion in boreholes can effectively improve the gas seepage characteristics of coal seams and increase the gas recovery rate.The simulations indicate that the application of in situ gas explosion fracturing and permeation technology is limited by the increase in maximum explosion pressure and the cumulative effect time of the peak pressure.This provides a theoretical basis for understanding the constraints of gas explosion fracturing and permeation technology.

| INTRODUCTION
At present, coal mining is gradually entering the deep shaft development stage, facing high gas, high stress, low permeability, and other outstanding problems, in which gas efficient and clean extraction has become the focus of research. 1,2Gas is mainly composed of high concentrations of methane, highlighting its great strategic importance as a clean energy source for development and reuse, moreover efficient gas extraction can be achieved to essentially eliminate the safety hazards associated with gas accumulation in mining operations.The pores and fractures of the coal seam matrix act as the main conduits for gas transport and directly affect the efficiency of gas extraction.Coal seam fracturing and permeation technology is an effective means of expanding the pore space and fractures to achieve more efficient gas extraction and utilization.At present, scholars have made many research results in fracturing and permeation technology, mainly including liquid carbon dioxide phase-change fracturing, 3,4 high-pressure hydraulic fracturing, 5 hot steam injection fracturing, gas-phase explosion fracturing, 6 and so forth.In the oil and gas industry in particular, a great deal of research [7][8][9] has been carried out into the technology of interlayer explosive fracturing, which has demonstrated the effectiveness of explosive shock waves in inducing rock fracture development.Zhao et al. 10 investigated the difference between gas fracturing and hydraulic fracturing and verified that the gas fracturing process can achieve significant efficiency gains in fracturing, Ni et al. 11 carried out pulsed hydraulic fracturing experiments and showed that the gas concentration in fractured and pilot wells increased before and after fracturing, and the permeability increased 48-217 times, Yang et al. 12 investigated the mechanical damage results of hydraulic fracturing on reservoirs and interbeds using a true triaxial test system, revealing the fracture propagation pattern.The aforementioned outcomes are derived from the utilization of high-energy shockwaves generated by the explosion of solid, liquid, and gaseous explosives to achieve fracture expansion and enhance seepage.Among them, the gas explosion fracturing technology in the oil and gas industry achieves the fracturing effect by applying highenergy pressure to the wellbore wall vertically.Both coal formations and shale gas wells share similar structures and axial pressure-bearing characteristics.Therefore, high-energy gas explosion fracturing technology will offer fresh perspectives for coal seam fracturing.
To explore the feasibility of fracturing coal bodies by in situ gas explosion in coal seams based on the research results of scientists in coal seam fracturing and shale interbed fracturing.In situ gas, which is an inherent component of coal seams, differs from solid, liquid, and gaseous explosives.It possesses attributes, such as safety, cost-effectiveness, and environmental friendliness, making it capable of efficiently reducing the energy transfer cost associated with conventional coal seam fracturing technology.At the same time, the current practice of hydraulic reaming of coal seams to improve pore permeability characteristics is still not obvious after reaming.Therefore, in situ gas fracturing with highenergy explosions is of great value in coal seam gas recovery and coal seam permeability improvement.
By establishing a stable closed pressure chamber within the coal seam, in situ gas fracturing technology involves filling the chamber with a combustible premixed gas by introducing an oxidant.Subsequently, the reaming chamber gas is detonated, resulting in an explosioninduced wave that alters the elastic efficiency of the coal seam.This, compounded with the stress field, generates new fractures while expanding existing ones under the high-energy gas impact.These processes effectively enhance the pore fracture structure of the coal seam, creating a channel for gas transport.This improves the permeability of the coal seam and creates the conditions for efficient gas extraction and utilization.
At present, the application of in situ gas explosion overpressure in the field of coal seam penetration improvement, has not yet made the relevant experimental research, many problems need to be explored in depth.Considering the safety risks involved in conducting in situ gas explosion experiments within coal seam boreholes, this thesis relies on computational fluid dynamics (CFD) software for analysis.It utilizes field parameters from coal mine borehole construction in Shanxi Province to construct a gas chamber model based on hydraulic reaming.By establishing models with varying borehole diameters, chamber lengths, and pressure states, comparisons are made, gas explosion data for combustible gas mixtures is monitored, and a study of gas explosion phenomena at different spatial scales is conducted.This leads to the optimization of the gas chamber design, selection of an efficient borehole QIAO ET AL.
| 4129 reaming solution, and blast overpressure data.Coupling the coal mechanics model to analyze the improvement of coal seam properties by blast pressure.Verify the feasibility of the in situ gas explosion coal fracturing scheme.

| FLACS explosion overpressure parameter acquisition
4][15] Currently, the boreholes commonly used in mines have diameters ranging from 200 to 600 mm. 14,16To improve gas extraction efficiency, some mines employ bedding drilling technology, which extends the depth of coal seam drilling to 10 m-20 m, and subsequently conducts hydraulic expansion in the deep area of the borehole.This process of construction is illustrated in Figure 1.According to engineering practice, hydrodynamic reaming forms cylindrical gas chambers are 600-1000 mm in diameter.
In this study, FLACS was used to simulate the development of gas explosions in cylindrical gas chambers with different bore diameters to investigate the pattern of gas explosion overpressure rise in confined spaces.FLACS is a proven CFD software with high acceptance for full-scale explosions and diffusion. 17,18s the diameter and length of the cylindrical gas chamber after reaming of the borehole were variable in this experiment, the geometric reconstruction of the space created by the borehole and the reaming of the coal seam was performed in the FLACS preprocessor (Computer Aided Scenario Design).A cylindrical hollow gas chamber was constructed using concentric cylindrical differential sets of Φ 0.2 m × 2.1 m and Φ 0.1 m × 1 m.The geometry of the hollow chamber was designed based on the experimental control group specified in Table 1.The premixed gas cloud for gas explosion was contained within this cylindrical chamber, with the left orifice serving as a closed port.The specific modeling graphics are illustrated in Figure 2.
By conducting the comparative simulation experiments mentioned above, we can examine the variation in overpressure under different gas explosion conditions.The optimal length-to-diameter ratio, which yields the highest efficiency, is selected for further testing to analyze the increase in explosion pressure.Additionally, gas explosion tests are performed under various pressure states to acquire data on the maximum overpressure value and pressure changes in a fixed-sized pressure chamber.According to coal mine production practice in Shanxi, China, the pressure in a well mine coal seam gas extraction borehole is between 0.1 and 0.7 MPa after the hole has been sealed.After the borehole is filled with oxidizer, the initial pressure in the borehole ranges between 0.1 and 1 MPa.0][21][22] Therefore, this paper is based on practice, in the pressure range of 0.1-1 MPa in six groups to carry out experimental studies to obtain pressure, surge variation, and other data.

| Coal seam fracturing parameter acquisition
We conducted a study on the pressure changes during gas explosion in boreholes using FLACS simulation.By analyzing the results, we determined the optimal overpressure parameter for gas explosion pressure.The boundary conditions of the fracturing model were established based on the explosion pressure data obtained from FLACS simulation and relevant coal parameters measured in the laboratory (Table 2).Additionally, we developed the pore elasticity and Biot seep model, and evaluated the fracturing effect under explosion pressure using COMSOL simulation. 23,24Finally, the explosive fracturing effect was quantified using parameters, such as coal seam stress, gas pressure, and coal permeability changes.
According to the Geological Exploration Report of Coal Seam of a mine in Changzhi, Shanxi Province, the third coal seam is subject to an overburden stress of 6 MPa, and the lateral coal body is under pressure close to the overburden.A 3 m × 10 m two-dimensional (2D) coal seam model was developed to verify the effect of gas explosion pressure on the physical properties of the coal body in the borehole reaming space under the above ambient pressure loads.

| Simulation model parameters
This study focuses on CH 4 in gas.The model geometry parameters were selected according to Table 1 and the hydraulic reaming model was reconstructed in 3D according to the example in Figure 2. The left side of the model shows the normal borehole area with an internal diameter of 0.1 m.The diameter of the air chamber section formed by the reamed hole is modeled according to the control variable group.To facilitate observation of pressure and surge development velocity at different locations within the chamber, a parameter monitoring point was placed at 0.47 m (L = 1 m chamber), 0.72 m (L = 1.5 m chamber), and 0.22 m (L = 0.5 m chamber) intervals within the chamber near the wall, making a total of three points, as shown in Figure 3.According to the characteristics of practical borehole sealing, Φ = 0.1 m and L = 0.5 m were reserved as the air chamber connection area.
In cylindrical pipe gas explosion simulations, FLACS recommends no less than four meshes at the wall and no less than four meshes in the explosion space, so the control body is divided into a 0.0125 m × 0.0125 m × 0.0125 m square tetrahedral structured mesh to meet the FLACS solution requirements. 25,26Hole geometry maximum 2.1 m × 1.1 m × 1.1 m, minimum 2.1 m × 0.2 m × 0.2 m.
In line with the ideal state scenario of gas explosion, the gas cloud volumes for the variable experiments in Groups 1-9 are as follows: 7.854, 31.416,125.664,  282.743, 502.655, 785.398, 141.372, 282.743, and 424.115L. On the basis of experimental measurements, it is determined that the chemical reaction is completed within 480 ms for the aforementioned gas cloud volumes. 27,28Therefore, a calculation time of 500 ms is set for the scenario, and data are saved at intervals of 5 ms.The initial conditions for the control simulation were set to follow the standard composition of "normal" air, as defined in the scenario menu, comprising 20.95% oxygen and 71.05% nitrogen by volume.The gravitational Initial porosity φ 0.034 constant was set to 9.8 m/s 2 , the temperature to 297 K, and the initial pressure to 101325 Pa.FLACS standard Reynolds-averaged Navier-Stokes equations for turbulence models. 29,30as and air premixes are assigned values according to the equivalence ratio. 23They obey Equation (1): where ER is the measurement of the fuel concentration in relation to the stoichiometric concentration, (F/L) actual the ratio of gas concentration to oxidant concentration in the actual gas mixture, and (F/L) stoich the theoretical gas/oxidant reaction concentration ratios for complete reaction.The gas fraction was set at 95% methane, 3% ethane, and 2% propane based on experimental measurements.To ensure an adequate oxygen supply, the chemical equivalent ratio is set at ER = 0.95 and the gas mixture cloud fills the cylindrical gas chamber formed by the reaming hole.

| Results of simulating a gas chamber explosion
As shown in Figure 4A, when L/Φ is 10 (Φ = 0.1 m), due to the radial spatial confinement of flame propagation, the radial wall of the explosion chamber experiences lower forces as the pressure increases.At L/Φ = 5 (Φ = 0.2 m), the rate of pressure rise is significantly increased, flame propagation is accelerated and the maximum explosion pressure rises to 0.61 MPa, a 10-fold increase compared with the 0.1 m diameter working condition; at L/Φ = 2.5 (Φ = 0.4 m), the maximum explosion pressures are elevated by 1183% and 26% compared with the scenarios with diameters of 0.1 and 0.2 m, respectively.However, these pressures are on average 5% lower than those observed under conditions with diameters of 0.6, 0.8, and 1 m.The maximum pressure increase rate parameters are similar for L/Φ > 5. Studies 30 have also shown that the larger the space, the higher the gas explosion pressure, but this is limited by the peak gas explosion pressure.The maximum explosion pressure does not increase significantly and the peak pressure stabilizes at 0.783 MPa when the reaming diameter reaches 0.6 m.As shown in the figure, the time to peak pressure for the gas explosion tends to decrease and then increase as the diameter of the gas chamber increases for different experimental conditions.Comparing the trends of the pressure curves for the different experimental conditions, it can be concluded that the mechanical effect of the explosion on the walls of the gas chamber is better for a reaming diameter of 0.6 m.
Figure 4B shows that the peak shock velocity inside the blast chamber increases with increasing diameter, reaching a maximum of 24 m/s at L/Φ = 1.67 (Φ = 0.6 m) and then decreasing.Analysis of the relationship between peak blast pressure and surge rate at the same diameter shows a positive correlation at L/Φ < 1.67 and a negative correlation at L/Φ > 1.67.At the same time, with a reaming diameter of 0.4 m, the time to peak explosion pressure is minimal, corresponding to a peak velocity of F I G U R E 4 Variation of gas explosion parameters for different reaming diameters.(A) Trends in explosion pressure and (B) peak velocity within the explosive gas chamber.
14 m/s.Due to the inadequate chemical reaction of turbulent gas with oxygen, the gas explosion reaction process at this time is slower than the working condition at 0.6 m diameter consistent with the study, in contrast, at 0.6 m diameter, the flame is more axially developed after ignition and the peak pressure arrival time is longer than at 0.4 m, but the reaction process is faster and produces a greater surge rate.A comprehensive analysis of the pressure and velocity change curves shows that the experimental results are more satisfactory for 0.6 m reaming diameter.
Figure 5 shows the variation in gas explosion pressure at different chamber lengths for a 0.6 m borehole diameter.Peak explosion pressure at L = 0.5 m is 0.78 MPa, rising to 0.79 MPa at L = 1.0 m, and 0.79 MPa at L = 1.5 m.Increased gas chamber length from 0.5 to 1 m, increased explosion pressure by 0.8% and delayed peak pressure by 60 ms; there is a 0.25% increase in explosion pressure with a 50-ms delay in peak pressure arrival time when extending from 1 to 1.5 m.The results above indicate that when the reaming diameter is 0.6 m, the change in the length of the explosion chamber has a minimal impact on the peak pressure but significantly affects the arrival time of the peak pressure.Additionally, as the L/Φ approaches 1, the explosion pressure rises at a faster rate, resulting in a shorter peak pressure arrival time.
Figure 6A shows that for borehole reaming diameters of 0.1 and 0.2 m there is only one peak in surge velocity at monitoring point 1; for diameters greater than 0.4 m there are two peaks in surge velocity.As monitoring point 1 is located at a considerable distance from the ignition source, the reaming hole diameter is small, which limits the radial expansion of the explosion's excitation wave.Consequently, the flame development in the cylindrical explosion gas chamber exhibits a tube-like propagation pattern from the ignition source towards the sides.At a distance far from the center of the explosion, the shock wave accumulates energy, resulting in a singlepeak velocity phenomenon.
However, as the diameter increases to 0.4 m, the flame development transitions from tube-like propagation to spherical propagation.Simultaneously, the shock wave propagating from the ignition center to the wall also assumes a spherical shape.Constrained by the wall, the shock wave creates a reflected wave and generates subsequent high-energy shock waves that repeatedly hit the wall.The waveform is superimposed, resulting in a graphic velocity pattern comprising a peak followed by a surge oscillation.In Figure 6B, the maximum excitation velocity at monitoring point 2 is affected by the diameter of the gas chamber reaming, which is 2.12, 2.28, and F I G U R E 5 Φ = 0.6 m explosion pressure for different gas chamber lengths.| 4133 2.39 m/s for 0.1, 0.2, and 0.4 m diameters, respectively, and 5.51, 4.61, and 6.17 m/s for 0.6, 0.8, and 0.1 m diameters, respectively, compared with the maximum growth rate of the surge velocity is greater than 200% for the 0.1-0.4m diameter of the reaming hole.The peak explosion velocity decreases with increasing diameter for the 0.6, 0.8, and 0.1 m diameters and increases again for the 1.0 m diameter, with a variation of ±16.3% in peak explosion velocity.
As the radius of the explosion chamber increases, the volume of the gas cloud also increases, leading to a longer time required for the chemical reaction to take place.Consequently, the arrival time of the peak velocity in Figure 6B lags behind.However, it is worth noting that observation point 2, being directly above the ignition source, experiences weak influence from the aspect ratio, thus resulting in velocity profiles displaying two peaks.Within the range of Φ = 0.1 m and t = 0-50 ms, the second peak appears higher than the first peak due to the reflection of the shock wave against the wall, followed by superposition with subsequent shock waves; Φ = 0.2 m, the reflected wave from the wall effect lags behind the subsequent high-energy shock wave, so the second peak velocity is significantly lower than the first peak velocity.The frequency and pattern of peak velocities are similar for the remaining conditions, suggesting that the effect of wall reflections on the induction of shock velocities diminishes as the chamber diameter increases beyond 0.2 m and that the peak velocity is dependent on the explosion chemistry itself.Observation point 1 is symmetrical to observation point 3, with similar trends in the variation of the excitation velocity.In the experiment by Gu et al. 31 In the study of methane explosion pressure and flame propagation characteristics in a pipe, we observed two peaks in the evolution of the excitation wave, which showed some resemblance to the results obtained from the simulation experiment.
Figure 7A shows the effect of the initial pressure within the reamed gas chamber on the change in gas explosion pressure for a reamed hole diameter of Φ = 0.6 m.The results showed that the peak pressure arrival times were 185, 285, 340, 398, 436, and 475 ms for initial pressures of 0.1, 0.2, 0.4, 0.6, 0.8, and 1.0 MPa, in that order.As the initial pressure increases, the peak explosion pressure arrival time gradually increases, with a strong nonlinear power-exponential relationship between the two.Figure 7B shows that as the initial pressure increases, the peak gas explosion pressure gradually increases, the initial pressure and the peak gas explosion pressure obey a strong linear relationship, the fitted squared difference index is 1, the growth rate of the peak pressure k = 8.06441 ± 0.00513 MPa/s.The results also suggest that the gas explosion fracturing effect will continue to be optimized as the initial pressure is increased by continually optimizing the technical means of sealing the borehole.In the actual mine production process, the pressure after sealing the coal seam borehole ranges from 0.1 to 1 MPa, and ideally up to 8.05 MPa at the wall inside the reamed gas chamber under a pressure of 1 MPa.
Figure 8 shows the effect of the initial pressure of the flared gas chamber on the maximum rate of pressure rise, which increases linearly with increasing initial pressure.Related research 10,24 has shown that high-energy impact pressure can effectively penetrate and fracture the coal seam.When the initial pressure reaches 1 MPa, the maximum pressure rise rate of the explosion reaches 31.69MPa/s.The effect of the pressure on the wall of the reamed gas chamber causes a change in the structure of the coal body, thus achieving the purpose of increasing the permeability of the coal body.Figure 9 illustrates that the maximum surge velocity of the gas explosion in the gas chamber exhibits a decreasing trend as the diameter of the hole expands.By applying Bernoulli's equation, it can be observed that in a constant volume space, there is a conservation of total energy between the pressure of the mixed gas, its kinetic energy, and potential energy.As the initial pressure increases, the potential energy in the gas chamber remains constant, leading to the superimposition of the explosive surge and reflected surge on the peak velocity.Consequently, the gas chamber's peak velocity demonstrates a downward trend as the initial pressure increases. 10The rapid pressure change phase can significantly expand the range of coal fracture damage, the shock wave propagation speed in porous media attenuation coefficient is larger, and sudden change in pressure on the coal fracture utility is preferable. 32n summary, FLACS-based simulation of gas explosions in coal seam boreholes (reaming gas chambers): (1) By analyzing the effect of different reaming diameters on gas explosion pressure and surge velocity, the peak gas explosion pressure is 0.783 MPa, and the maximum surge velocity is 24 m/s for a length-to-diameter ratio of L/Φ = 1.67 (Φ = 0.6 m), which is better than other experimental conditions for gas chamber wall pressure.
(2) When the reaming diameter is Φ = 0.6 m, the peak blast pressure tends to stabilize as the length of the blast chamber increases, but the peak surge velocity and maximum pressure rise rate show a decreasing trend.(3)  In gas explosion tests under various pressure conditions, the gas explosion pressure and the maximum explosion pressure increase continuously with the initial pressure in the gas chamber, 1 MPa can be used as the ideal initial pressure for the gas explosion fracture test in the reamed gas chamber.

| Simulation model parameters
On the basis of the pressure change curve of gas explosion under the initial pressure of 1 MPa, the effect of gas explosion on coal fracture damage under pressure is studied.The pressure resulting from the gas explosion within the reamed gas chamber applies uniformly to the walls.As a result, a 2D borehole area map was created, as depicted in Figure 10.The coal seam has a thickness of W = 3 m and a length of L = 10 m.The reamed gas chamber is positioned at the geometric center of the 2D layout with a diameter of Φ = 0.6 m.
The coal seam is influenced by the surrounding rock pressure and gas pressure, and the model uses a two-way coupling of porous plasticity and Darcy's law, assuming that the coal seam is an isotropic linear elastic material whose stress-strain relationship obeys the generalized Hooke's law, [33][34][35][36] The mathematical equation is (1) Equations of stress field where E is Young's modulus of the mine body, Pa; υ is the coefficient of Poisson; ε ij is the control body strain; tr (ε ij ) is the trace of the strain tensor matrix; I is the unit vector matrix; ε p is the strain due to gas pressure; σ ij is the stress; α is the biot factor for the deformation of the coal body; p is the coal seam gas pressure, subscript 0 is the initial state; F i is the overall stress in the control body; k s is the skeletal modulus of coal solids.
(2) Seepage field equations where ρ g is the gas density, kg/m 3 ; φ is the porosity of porous media; t is the time, s; k is the permeability, m 2 ; Q is the quality source term, kg/(m 3 s); μ is the gas dynamic viscosity, Pa s; ε V is the volume strain.
The fluid-solid coupling equation is obtained by combining Equations ( 2) and (3).
The relevant physical and boundary condition parameters are given in Table 2.
In Figure 7A, the gas explosion pressure curve at Φ = 0.6 m and a pressure of 1 MPa is utilized as the coupled field pressure boundary to examine the effects of pressure loading on the physical properties and structure of the coal mass.

| Results of simulation
To study the effect of pressure loading on the coal body structure, the stress distribution, pressure state, permeability, and other parameters of the coal body during the 0-5 s period are analyzed to quantitatively visualize the fracturing effect.
Figure 11 shows that at the moment t = 0 s, the coal body in the reaming area is affected by the boundary load, the hole wall is located at the transition boundary between load and nonload, the stress shows a concentrated state, the hole wall points to the inner 1 m area of the coal body, the stress shows a negative exponential decreasing trend, from 4.83 to 0.41 MPa.At the moment of t = 0.2 s, the explosion pressure rises to 1.615 MPa, and in the range of 0-1 m in the x-and y-directions, the explosion pressure affects the coal body stress range up to 0.05 m; in the range of 0.05-0.4m, the stress is slightly lower than at the moment of t = 0 s due to the combined effect of the coal body stress load and the explosion pressure.At the moment of t = 0.3 s, the gas explosion pressure accelerated to 4.78 MPa, at this time the explosion pressure rise rate reached a peak, there is 0.05 m location of the stress inflection point, from the hole wall x-and y-directions 0-0.05 m stress changes F I G U R E 11 Distribution of stress during the fracture initiating action.concentrated, the cumulative effect of the area of integration compared with t = 0.2 s average increase of 43%.At the moment of t = 0.5 s, the explosion pressure reached a maximum of 8.04 MPa, the stress concentration area and t = 0.3 s time the same, located in the x-and y-directions of the hole wall 0-0.05 m area; compared with t = 0.3 s time stress curve, the cumulative effect of stress area increased, while 0.05-1 m area stress shows a decreasing trend.At t = 1.0 s, the effective stress area of the seam is extended to 0.08 m from the wall of the hole.At t = 5.0 s, the stress area continues to move into the coal body, while the stress accumulation zone increases by 73.7% compared with the initial state at 0 s.
The stress change curve at each instant shows that the cumulative zone of action is expanding with time, while the stress in the coal body outside the cumulative zone of action is decreasing with time in the x-and ydirections from the hole wall, and the stress cloud contour also shows a contraction trend.In Figure 11B, the stress in the y-direction of the borehole area of the coal body shows an "inverted drop type," resulting in the difference of stress in the x-and y-directions, resulting in the stress in the y-direction being greater than the stress in the x-direction, and at 5 s, the peak stress in the ydirection wall reaches 11.82 MPa and the peak stress in the x-direction wall reaches 11.62 MPa, with F y expanding by 0.02% relative to F x expanding by 0.02%.
When the hole wall is exposed to the pressure from the gas explosion, the fracturing effect on the coal body within the reamed hole area seems to propagate outward from the center.The stress curves at t = 0.3 and 0.5 s show that the stress changes are concentrated in the range of 0-0.05 m from the hole wall, and the critical value of the gas explosion on the stress field of the coal body is 0.05 m from the hole wall, while the stress curves at t = 1.0 and 5.0 s show that the critical value can be broken by increasing the peak explosion pressure and the peak pressure action time, thus achieving a more efficient fracturing effect.
Figure 12 illustrates that the coal body experiences load stress, and the gas stored within the coal body maintains a pressure of 1.69 MPa.At t = 0.0 s, at a distance of 0.05 m from the hole wall, the gas is influenced by both the stress exerted by the coal body and the explosion pressure from the hole wall.This leads to a pressure increase at that particular location, creating a high-pressure area within the ring depicted in the cloud diagram and shown by the curve.① High-Pressure Area.At t = 0.2 s, after the explosion pressure rises, the stress bearing increases in the range of 0-0.05 m, resulting in the failure of the elastic storage efficiency of the porous coal body, the gas pressure is released in the crushing medium, the pressure release point appears at x = 0.03 m position (diagram ② area), while the elastic storage failure point moves from the hole wall to the inside of the coal body.At the moment t = 0.3 s, the fractured area of the coal body is slowly compacted, the elastic storage efficiency is gradually restored, and the pressure at x = 0.03 m increases relative to the moment t = 0.2 s, while the influence extends to 0.05 m.At t = 0.5 s, the explosion pressure reaches its peak and the elastic storage failure point of the coal body extends to 0.08 m, while the pressure in the area closer to the hole wall gradually increases.At t = 1.0 s, the explosion pressure continues to act and the effect on the pressure field of the coal body steadily extends to the inner part of the coal body, extending the influence to the 0.1 m position.At t = 5.0 s, the x-direction pressure influence area expands to 0.26 m position; Figure 11  | 4137 changes in the x-and y-directions from the hole wall, at the moment of t = 0 s, P y expands 11.5% relative to P x at 0.03 m position from the hole wall, and the coal body crushing failure recovery time in the y-direction relative to the x-direction is expanded, so at the moment of t = 5 s, the y-direction pressure action area is 0.31 m position, expanding 0.02 m relative to the x-direction.
The pressure curve and cloud plot data consistently demonstrate that as the explosion pressure escalates, the elastic storage efficiency of the coal body initially diminishes.The intensity of failure decreases as the distance from the hole wall increases, leading to the formation of a lowpressure ring-shaped area on the pressure cloud plot.Simultaneously, the failure area extends from the hole wall deeper into the interior of the coal body as the explosion pressure transmitted by the hole wall amplifies.][37] Figure 13 depicts the following observations: at t = 0 s, the coal seam experiences stress, leading to a 7% decrease in coal permeability within the range of 0-0.05 m from the hole wall compared with the initial state.At t = 0.2 s, the rising explosion pressure inside the reamed gas chamber elevates both the coal stress and coal matrix gas pressure within the 0-0.05 m range from the hole wall, resulting in increased permeability expansion within this interval.The weighted contribution of the explosion pressure fracturing effect is 0.98 times greater than the initial state.At t = 0.3 and 0.5 s, the improved permeability still occurs within the 0-0.05 m range from the borehole wall, with the weighted contribution of fracturing being 2.6 and 5.6 times larger than the initial state, respectively.At t = 1.0 and 5.0 s, the peak permeability at the borehole wall location stabilizes, and the continuous pressure effect mainly influences the radial permeability deeper within the borehole.The weighted contribution in these instances is 8.6 and 19.6 times greater than the initial state.The weighted contribution remains 8.6 and 19.6 times greater than the initial state throughout.
By examining the correlation between the x-direction pressure and permeability curves obtained from the borehole wall (Figure 13), it can be observed that at the location x = 0.03 m from the borehole wall in the pressure diagram, the coal body enters the zone where elastic energy storage failure occurs.This leads to the impact on the coal body at x = 0.05 m due to the fractured coal body at 0.03 m and the x-direction pressure.As a result, the permeability of the coal body at x = 0.05 m experiences a sharp decrease, falling below the initial permeability of the coal base.This sudden change point can be observed at 0.05 m in the permeability diagram.
By comparing the permeability change curves at each moment, the blast pressure rise phase (t = 0-0.3s), the radius of influence on the permeability of the coal body is controlled in the range of 0-0.05 m; the maximum permeability value stabilizes at 2.57e − 17 after the peak blast pressure is reached, indicating that the peak blast pressure determines the upper limit of the permeability increase in the coal body.The duration of the blast pressure has a significant effect on the magnitude of the permeability gain and the fracture weighting contribution, with the magnitude of the permeability effect extending beyond 0.05 m to the 0.1 m position at t = 1 s and to the 0.38 m position at t = 5 s, indicating that the duration of the pressure can significantly improve the radial permeability of the borehole.The permeability trends in the x-and y-directions are consistent, and since both stress F and pressure P are greater in the y-direction than in the x-direction, the peak permeability φ ymax = 2.61e − 17 > φ xmax = 2.57e − 17.
In summary, based on the established equations of coal mechanics and seepage mechanics, the explosion pressure was substituted as the pressure field boundary to solve for a quantitative analysis of the effect of gas explosion on coal body fracture, and the results showed that: (1) the blast pressure increase stage, the permeability of 0-0.05 m from the hole wall area efficiency is large, in 0.05 m from the hole wall location of the permeability of the sudden change point.During the blast pressure application, the coal body stresses contract radially towards the hole wall, in accordance with the characteristics of coal physics deformation. 38,39(2) Coal body porous elastic energy storage by the superposition of explosion pressure and crustal stress, first in the radial 0.03 m location of the elastic failure zone, gas pressure transient reduction, the emergence of low-pressure annular zone, from the hole wall 0 to 0.05 m range first coal body elastic energy storage failure, after the explosion pressure along the hole wall to the coal body internal radial propagation process, the coal body elastic efficiency gradually restored.(3) Gas explosion pressure rise stage, the impact on the coal body permeability range from 0 to 0.05 m from the hole wall, the explosion pressure duration of action can effectively enhance the permeability efficiency range and the coal body fracture weighted contribution.

| CONCLUSION
This study investigated the utility of in situ gas explosions in coal seams for fracturing coal bodies, analyzed the effect of borehole design on gas explosions in reamed gas chambers through simulation experiments, and recorded changes in coal body properties under the action of explosion pressure, leading to the following conclusions.
(1) Investigation of gas explosions within reamed gas cylinders show that the length to the diameter ratio has a large effect on gas explosion pressure, with a chamber length of L = 1.0 m and a chamber diameter of 0.2 m compared with 0.1 m, the maximum explosion pressure expands by a factor of 10, and for diameters Φ > 0.4 m the maximum explosion pressure is close to the theoretical explosion limit; As the L/Φ ratio decreases, the surge velocity increases and then decreases, and the maximum pressure rise rate tends to increase; the mechanical effects of explosion pressure and surge velocity on the wall are optimal for the experimental group when the L/Φ is 1.67.
(2) The initial pressure of the reamed gas chamber can significantly improve the mechanical efficiency of the explosion on the wall.The maximum explosion pressure and pressure rise rate is linear and positively correlated with the initial pressure.The initial pressure of 1 MPa, ideally the maximum explosion pressure can reach 8.05 MPa.Fracturing efficiency can be effectively improved by increasing the initial pressure, but the current drilling and sealing process is more demanding.
(3) The effect of gas explosion pressure on the fracture of the coal body shows that the explosion pressure rise phase (0-0.5 s) on the coal body effective impact range of 0.05 m.Peak explosion pressure duration can expand the impact on the coal body, continuous action for 5 s, the gas pressure field impact range to 0.3 m, the coal body permeability impact range to 0.38 m. explosion pressure will cause the coal body to undergo elastic the maximum explosion pressure can expand the upper limit of coal permeability efficiency.

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Diagram of drilling and reaming.T A B L E 1 Control experimental group.

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Surge velocity diagram at monitoring point.(A) Monitoring point 1 and (B) monitoring point 2. QIAO ET AL.

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Explosive pressure diagram for different pressure for Φ = 0.6 m. (A) Pressure variation graphs and (B) peak pressure in relation to initial pressure.COD, parameter in origin software; RSS, residual sum of squares.
Graph of pressure rise rate at different pressure carrying conditions for Φ = 0.6 m.F I G U R E 9 Peak velocity of the gas chamber at different pressure carrying conditions for Φ = 0.6 m.F I G U R E 10 Geometry of the coal body model.
stress field F y expands 0.02% relative to F x , analyzing the difference of pressure field F I G U R E 12 Pressure distribution during rupture action.QIAO ET AL.