Analysis of Influence of Ultra-High Pressure Water Jet Cutting Pressure Sequence on Pressure Relief and Reflection Improvement of Coal Seam

In order to explore the pressure relief efect of the two combined pressure relief and antirefection technologies of ultra-high pressure water jet cutting before pressure and pressure before cutting, theoretical analysis, numerical simulation, and feld test were used to study the main control factors of the combined high-pressure water jet slit cutting and fracturing antirefection technology. Tis paper introduces the combined technology of ultra-high pressure hydraulic fracturing, and analyses the mechanism of pressure relief and transparency enhancement of the combination of cutting before pressure and pressure relief after cutting. Te results show that the starting pressure of the coal seam with ultra-high pressure water jet cutting and pressure relief is 13MPa, the infuence radius of hydraulic fracturing is 45–55 m, the starting pressure of the coal seam with pressure cutting and pressure relief is 16MPa, and the infuence radius of hydraulic fracturing is 35–45m. Compared with pressure cutting combined pressure relief and permeability enhancement technology, cutting pressure relief and permeability enhancement technology can improve the permeability of coal seams more evenly and efectively, and reduce the stress of coal seams near the hole. Te ultra-high pressure cutting and pressure combined technology can make the pressure relief of coal body uniform and sufcient, and the overall permeability coefcient of the coal body is greatly improved. Te drilling purity is 2.3 times of the extraction purity of the ordinary single hole drilling, and the extraction infuence range is increased, and the extraction efect is signifcantly improved. At the same time, the stress of coal body is reduced after slitting, and the starting pressure of hydraulic fracturing is reduced. Te research results provide a scientifc basis for the coal seam pressure relief and permeability enhancement under similar conditions in the mining area and have a broad application prospect.


Introduction
At present, there are more than 2000 coal and gas outburst, rock burst, and high gas mines in China, accounting for 30% of the total number of mines.After entering deep mining, the problems of high gas and high ground stress are prominent, the permeability of coal seam is reduced, and the difculty of disaster management is increased [1][2][3].Under the condition of deep mining, the coal seam is upgraded to the protruding coal seam.For the protruding coal seam without protective layer mining or frst mining, the antiprotruding measures in the prepumping area of dense conventional borehole are still the main measures.In order to achieve efcient control of gas disasters in deep mines, the conventional pressure relief and refection improvement technologies in coal mines in China at the present stage mainly include hydraulic punching, hydraulic cutting, hydraulic fracturing, and deep-hole presplitting blasting [4][5][6].
Although the deep-hole presplit blasting technology can signifcantly improve the pressure relief and antirefection efect of coal seam, it is relatively less applied in the pressure relief and antirefection improvement of coal seam, because the long borehole coal seam is prone to collapse, resulting in charging difculties, and it is difcult to eliminate the risk of misfring and explosion refusal in blasting operation.In recent years, the refection improvement technology of hydraulic coal seam in China has entered a stage of rapid development.Te individual technology is constantly improved, and the overall development is in the direction of integration and diversifcation.Te hydraulic measures such as low-pressure hydraulic punching, hydraulic cavitation, hydraulic fracturing, and hydraulic slotting have become the hot spots of research in scientifc research institutions, which provide support for the control of coal mine gas disasters and have achieved results under certain conditions [7][8][9][10].Te hydraulic technology is often used in the feld, and the hydraulic punching pressure is generally 5-20 MPa, which has a certain efect on the pressure relief and refection improvement of soft coal seam, while the hydraulic punching efciency of medium and hard coal seam is low.However, it is difcult to control the shape of punching holes in soft coal seam, and the amount of slag is not uniform, so there may be hole collapse, hole injection, or roadway gas overlimit during operation.Te hydraulic fracturing has a large infuence range and good antirefection efect, and is mostly used in medium and hard coal seams.However, it is difcult to control the fracture propagation direction in the coal body [11][12][13].Hydraulic cutting seam technology is based on high-pressure water cutting technology of coal seam, improve coal seam gas fow state, reduce the stress, can efectively prevent coal, and gas outburst and the impact of ground pressure disasters happen, suitable for high ground stress, gas and low permeability coal seam (seam hardness f > 0.4) bedding face drilling, wear layer drilling, and shimen uncovering coal unloading antirefection and so on [14][15][16].
Te ultra-high pressure water jet cuts the coal body, and the coal body around the slot produces deformation space so that the coal body around the slot can be fully depressurized.At the same time, because a part of the coal body around the drill hole is transported out of the drill hole by the water jet, a large number of cracks are generated in the expansion and deformation of the coal rock, changing the permeability conditions of the coal rock.Ge et al. established a fuid-solid coupling gas drainage model of slotted borehole.Trough numerical simulation analysis, it is believed that the infuence radius of the borehole drainage after hydraulic slotting has a power function relationship with slot disc, permeability, drainage time, gas pressure, and other factors.Trough model research, it is determined that the infuence signifcance of each factor from large to small is permeability, drainage time, gas pressure, and slotting depth [17].Li et al. used a dynamic damage model to study the cutting process of soft coal rock by water jet.During the cutting process, with the gradual release of stress around the slot, the crack continued to expand in some directions.Te tension and shear fractures in coal and rock continue to develop during the damage accumulation process, in which short pulses with high peak stress can form relatively short fractures, and long pulses with low peak stress can form relatively long fractures.Under the continuous action, the cracks around the slot cut by the jet gradually develop and then connect to each other to form a breakthrough failure [18].Tang et al. conducted numerical simulation on the infuence of diferent hydraulic slotting arrangements on the coal seam pressure relief and outburst prevention, and analyzed the infuence of parallel, diamond, and staggered pressure relief.Te results show that the coal rock pressure relief efect above the fracture groove is most obvious.Te results showed that the pressure relief efect of coal and rock above the slot was the most obvious [19].Trough indirect measurement of gas fow through similar material test, it is confrmed that the pressure relief of coal seam has a signifcant impact on permeability, and the permeability coefcient of coal and rock increases synchronously with the degree of pressure relief.By studying the displacement and stress changes of coal body under diferent slit widths after high-pressure water jet slotting, the infuence of slit depth on coal rock disturbance is analyzed.Numerical simulation shows that the pressure relief range of 1.0 m, 1.5 m, and 2.0 m slotting on coal body reaches 2.6 m, 3.8 m, and 5.0 m, and the infuence range of slit on coal body increases with the increase of slit width.Te larger the slit depth is, the more conducive to coal seam pressure relief.
Te antirefection technology of hydraulic fracturing was frst used in the exploitation of oil and gas felds, as a main measure of oil and gas well stimulation.In the 1960s, the hydraulic fracturing technology began to be used in coal mines to increase in coal seam permeability, mainly by drilling deep into the coal body through injection of highpressure water, and fracturing the coal body with water as the energy transmission medium.After high-pressure water fracturing, the stress of surrounding coal body was reduced, and the stress concentration was transferred to the depth of the coal body, thus improving the permeability of coal body around the borehole, providing a good condition for drilling gas extraction [12,[20][21][22].Te research on the hydraulic fracturing technology in coal mine mainly focuses on the theoretical research on the initiation and extension laws of the hydraulic fractures, or the establishment of hydraulic fracturing mathematical model for numerical calculation research, or the physical experiment research of hydraulic fracturing in the laboratory.Hubbert and Willis described the stress distribution law of hydraulic fracturing wall and surrounding coal and rock mass based on classical elastic mechanics, and thus obtained the theoretical calculation model of tensile failure fracture pressure of coal and rock mass: P b � 3σ min − σ max + f t (σ min , σ max , and f t are the minimum horizontal stress, the maximum horizontal stress, and the tensile strength of coal rock, respectively); the comprehensive efects of the tangential principal stress σ θ , straight principal stress σ v , and radial principal stress σ r on the wall of the borehole are not fully considered in this theory [23].Ma et al. conducted an experimental study on the infuence of water pressure with diferent water fow rates on the fracture initiation characteristics of coal.Te results show that the increase in water fow rate makes the fracture morphology more complex, and the research results have important theoretical signifcance for revealing the fracture initiation behavior of boreholes [24].According to the frst strength criterion, Lv deduced the critical value calculation formula of pressure crack initiation pressure and successfully tried it in Pingmei Ten Mine [25].Bouteca [32].Surrounding rock control and support stability of super high mining face was studied by Wang Sheng.Te results of this study can provide guidance for the selection of scafolds and the adoption of measures to improve the stability of scafolds when they are used in ultra-high height conditions [33].
Taking Linyi mining area as the research object, Li Xuelong studied the distribution law of ground stress in deep mines.It is found that the relationship between principal stresses is σ(H) > σ(v) > σ(h), which belongs to the strike-slip stress system.Under this stress condition, the soil lateral pressure coefcients are all greater than 1, and the magnitude of the three principal stresses increases with the increase in depth.Te research results have certain reference signifcance for mine disaster prevention and safety production [34].Liu Haiyan studied the failure mechanism and control technology of cave-side stoping roadway in close distance coal seam.It is proposed that U-shaped steel telescopic support erection and backwall flling are used to control the surrounding rock of goaf mining in the process of roadway excavation, and the on-site monitoring results also meet the engineering requirements.Te research results can provide guidance for roadway design of goaf under similar mining geological conditions [35][36][37][38][39][40][41][42][43][44][45][46][47][48][49][50][51].
Te application of ultra-high pressure hydraulic slotting technology can realize the precise seam cutting, rapid pressure relief, and efcient permeability increase, and drilling engineering quantity is reduced on the basis of the extraction standard time to shorten 30% of the efect.With the application of hydraulic fracturing technology, the impact area of hydraulic fracturing is more than 50 m, with a large impact area and obvious antirefection efect in the region.After the application of ultra-high pressure hydraulic slit technology and hydraulic fracturing technology, the antirefection efect of coal seam is remarkable.Terefore, in the hydraulic cutting seam technology and hydraulic fracturing technology in the application process, there exist the following problems: the super high-pressure hydraulic cutting seam to improve the unloading antirefection efect at the same time, greatly reduce drilling of quantities, as local antirefection measures that it is obvious, but for large area still needs to undertake a large number of slotted drilling construction slot unloading antirefection.Although the antirefection efect of hydraulic fracturing technology has a large infuence range, it is difcult to control the fracture propagation direction in the coal body, and the pressure relief and antirefection improvement are not uniform, and there is stress concentration phenomenon.
To sum up, to better carry out uniform permeability improvement in low permeability coal seam, accurately control the pressure relief and permeability improvement area, greatly reduce the drilling engineering quantity, and solve the technical problems of gas extraction and control in low permeability coal seam.Te combined technology of ultra-high pressure hydraulic slit cutting and hydraulic fracturing is explored, and the mechanism of pressure relief and refection improvement by cutting and pressure relief and cutting is analyzed.Te hydraulic cutting pressure scientifc model is established, and the infuence range of high pressure hydraulic cutting pressure is solved through theoretical analysis.Te PFC software based on the theory of discontinuous media mechanics was used to simulate the initiation and expansion characteristics of seam cutting and fracturing fractures in coal seam, and the distribution rules of fractures and stresses in coal seam were compared and analyzed by hydraulic cutting before pressure and pressure relief after cutting.Combined with the feld research on ultra-high pressure hydraulic cutting before pressure and pressure before cutting and combined pressure before cutting and permeability improvement, the infuence law of ultra-high pressure water jet cutting pressure sequence on pressure relief and permeability improvement of coal seam is further revealed.Te research results are of great signifcance to enrich the comprehensive gas control technology of low permeability coal seam.

Ultra-High Pressure Hydraulics First Cut and Then Pressure Combined with the Principle of Permeation
Te concept of hydraulics frst cut and then pressurized joint mode is to use ultra-high pressure hydraulic cuts to cut the coal seam in the coal seam frst, and then use hydraulic fracturing to fracturing the coal seam after the gap is generated.Te groove generated by the hydraulic cutting in the early stage can guide the hydraulic fracturing so that the extension direction of the fracture in the plastic zone is basically the same as the direction of cracking.Te crack expansion is more uniform.By forming a slot by hydraulic cutting in the coal body in advance, the efective infuence range of the single hole can be expanded to a certain extent.Te original stress balance of the coal body can be destroyed.Te coal body around the cut hole is transported to the space of the slot space, and the pressure relief, deformation, and expansion of the coal seam can occur, further generating more cracks and expanding the plastic area of the coal body near the cut hole.Combined with the empirical formula of plastic theory, it can be seen that the radius of the plastic zone is about 3 to 5 times the radius of the cut groove, and the radius of the high-pressure water groove is determined to be about 2.5 m through feld tests.It is inferred that the radial plastic zone range outside the hydraulic groove is about 7.5 to 12.5 m.After the hydraulic cut is formed into a crevice, a weak surface is generated in the drilling hole, and after the fracturing water enters the crack, it promotes the cracking, expansion, and extension of the weak side crack, resulting in the full and uniform development of the coal body fracture near the borehole.Trough the rational arrangement of the cutpressure joint hole, a three-dimensional fracture network of interpenetration is formed between the drilled holes, which efectively solve the problems of disorderly expansion of the fracture in the coal body during the ordinary hydraulic fracturing, local stress concentration, and pressure relief blind zone after fracturing.Tis joint mode not only solves the problems that the direction of hydraulic fracturing crack is not easy to control, but also the crack propagation in the fracturing area is uneven.It is easy to form a high stress concentration area, and there is a "blind zone" of fracturing, but also increases the scope of impact of fracturing, which saves a lot of drilling engineering compared with ordinary drilling holes and improves the efciency of pressure relief and antiextrusion.At the same time, the problems of uneven pressure relief and stress concentration in individual areas are supplemented by fxed-point hydraulic cutting to achieve uniform and efcient antipenetration purposes.A schematic diagram of the frst cut and then press joint is shown in Figure 1.

Ultra-High Pressure Hydraulics First Press and Then Cut and Increase the Principle of Penetration
Hydraulics frst press and then cut joint mode, that is, hydraulic fracturing is used to supplement the fracture within the infuence range of hydraulic fracturing.Te gap fracture is formed in the blank zone of the hydraulic fracturing afected area, and the fracture formed by hydraulic fracturing is conducted, and more fractures are formed.
Trough hydraulic fracturing operations to rapidly improve the permeability of the coal seam in the area and the gas extraction efect, after the completion of the hydraulic fracturing construction, the fracturing crack is uncontrollable.Although the cracks in the coal seam are generated in a large range, the permeability of the coal seam increases, and the coal body plays a decompression and permeability efect within the scope of the crack.However, the inhomogeneity of the physical and mechanical properties of the coal leads to the uncontrollable weak surface in the coal seam, resulting in uncontrollable hydraulic fracturing cracks.Tere is a blank zone afected by hydraulic fracturing within the scope of infuence of the hydraulic fracturing, and the area with poor pressure relief efect is used as a "blind spot" of hydraulic fracturing, and there is a stress concentration in the uncontrolled area of the crack.Te use of hydraulic cutting joints to accurately increase penetration and strengthen extraction, under the action of ground stress, and the fracturing cracks are connected with the fractured area of the joints, forming an overall pressure relief area, reducing the stress concentration, and efectively improving the gas permeability of the coal seam.Tis mode efectively combines the advantages of fracturing and fracture, solves the problem of stress concentration and uneven fracturing in the fracturing area, and realizes the accuracy of antiprotrusion.Te precise antiprotrusion mode of pressing frst and cutting is shown in Figure 2.

Ultra-High Pressure Hydraulics First Press and Then Cut and Increase the Principle of Penetration
Under the action of ultra-high pressure hydraulic force, when the coal body around the borehole exceeds its own strength, the hole wall is the plastic softening zone and the elastic zone are from inside to outside.Te drilling mechanical model is shown in Figure 3. Te model assumes the following: ① Te borehole is subject to the stress P 0 of the original rock, and the side pressure coefcient λ � 1 is treated according to the axial symmetry problem, which is simplifed to planar strain

Plastic softening zone
The elastic zone

Advances in Civil Engineering
② Te coal around the borehole is homogeneous and isotropic, and the infuence of borehole pressure relief on the borehole is not considered ③ R 0 is the drilling radius; σ p is the peak intensity; σ c is the residual strength; the hydraulic fracturing pressure p i acts evenly on the wall of the drilled hole Assuming that the compressive stress is positive and the tensile stress is negative, the deep borehole is subjected to ground stress, at this time:σ 1 � σ θ , σ 3 � σ r , and σ 2 � σ z � μ(σ θ + σ r ).Te strength characteristics of the elastoplastic state of the borehole wall are described by the unifed strength theory, and the expression is as follows: where σ θ , σ r are the tangential stress and radial stress of the borehole wall, respectively.Since A j , B j are characterizing the parameters of the coal body, representing the relationship between the maximum principal stress and the minimum principal stress.Te μ is for Poisson's ratio; j is the symbolic parameter; j � e represents the initial internal friction angle φ e and cohesion force c e of the coal body; j � p represents the friction angle φ p and cohesion c p of the plastic softening region; b is the median principal stress coefcient, 0 ≤ b ≤ 1.
In the stress-strain curve, failure occurs when the strength of the coal body exceeds its ultimate strength, and this paper assumes that the residual friction angle φ c and the residual cohesion c c are unchanged.Plastic softening occurs when the strength of the coal body exceeds its peak strength, and the values of the friction angle φ p and cohesion c p in the plastic region gradually decrease with the increase in plastic strain, assuming that φ p and c p are linearly softened with the initial internal friction angle φ e and cohesion force c e .Te softening coefcients k φ and k c are introduced, which are as follows: where k φ , k c are the internal friction angle and the cohesive softening coefcient.φ e , φ p , and φ s are the initial internal friction angle, the friction angle of the plastic softening zone, and the residual internal friction angle of the coal body, respectively.c e , c p ,c s are the initial cohesion of the coal body, the cohesion, and residual cohesion in the plastic softening area, MPa.R p is the radius of the plastic zone, m.
Te drilled coal body is in the linear elastic state, p y is set as the radial stress at the junction of the elastic zone of the coal body and the plastic softening zone, and the elastic zone of the coal body is regarded as a thick-walled cylinder under the joint action of p y and ground stress p 0 .It can be seen that the elastic zone stress is as follows: where σ re is radial stress in the elastic zone, MPa.σ θe is the tangential stress of the elastic region, MPa.r is the distance from any point in the coal body to the center of the circle, m. p 0 is the ground stress, MPa.p y is the stress at the elastoplastic junction, MPa.At the elastoplastic junction r � R p , formula (5) satisfes formula (1) and the radial stress is continuous, and the fnishing can be obtained as follows: Any of the study unit points in the coal body satisfy the equilibrium diferential equation: Substituting equation ( 1) into equation ( 6) and integrating, take σ r |r � R 0 � p i as the boundary condition, the radial and tangential stresses of the plastic region can be obtained as follows: Te radial stress σ r is continuous at the elastoplastic junction, which is σ rp |r � R p � σ re |r � R p .Te radius of the plasticity zone of the frst type (7) and the frst type of (4) of the frst type of plasticity zone is as follows: 6 Advances in Civil Engineering

Numerical Simulation Analysis of Ultra-High Pressure Hydraulics Combined with Permeation PFC
Under the action of ultra-high pressure hydraulic force, when the coal body around the borehole exceeds its own strength, the hole wall is the plastic softening zone and the elastic zone from the inside to the outside.Te drilling mechanical model is shown in Figure 3. Te model assumes the following: Te particle fow discrete element method (PFC) is based on the mechanics of discontinuous media to study the germination, expansion, and penetration of fractures, which can truly express the geometric characteristics of jointed rock masses, facilitate the handling of nonlinear deformation and destruction, and refect the diferent physical relationships between multiphase media through a variety of connection methods between cells, which can efectively study noncontinuous phenomena such as cracking and separation.Tere are countless mesoscopic cracks in coal rocks, especially soft coal bodies, showing obvious inelastic deformation characteristics, and these mesoscopic cracks develop into macroscopic cracks or until they break down under increased loads.In the particle fow discrete element, when the contact point node is destroyed, the corresponding particles will produce cracks, and new cracks will be generated at the initial crack tip as a sign of hydraulic fracturing.Te nonlinear deformation failure process of the fracture can be analyzed by direct and indirect methods.Te indirect method uses the constitutive relationship to analyze the failure process, generally assumes the fractured coal body as an ideal uniform material, refects the weakening the overall strength of the fractured coal body through a certain constitutive relationship, and expresses the microstructure failure process in the coal body in this way.Te direct method is a mesoscopic simulation method, which assumes that the fracture coal material is a collection of various microstructures, or some particle combinations connected at the contact point.Te failure process of the fractured coal body can be directly simulated by the microstructure and particle rupture, and the fractured coal body can be studied mescologically without simulation through complex constitutive models.
Te PFC numerical calculation software is a kind of software based on particle fow theory, which links the microstructure of materials with macroscopic mechanical reactions, and directly simulates material failure from a mesoscopic perspective, which is suitable for materials that are difcult to accurately describe their properties through constitutive relationships based on uniform media, such as fractured rock masses.Te bonding parameters of the particles determine the location and number of initial microcracks, so microcracks can only be formed in the connection contact model.Te position and size of the two particles determine the location and geometry of the cracks, which can be simplifed to a cylindrical surface represented by the center point position, normal direction, thickness, and radius parameters.

First Cut and Ten Press Combined with Unloading Coal
Seam Fracture and Stress Distribution.Te model adopts a two-dimensional plane model.Te direction length is 200 m, the height is 200 m, the drilling diameter is 113 mm, the cutting pressure is 100 MPa, the test site elevation is about 550 m, the vertical stress reaches 17.7 MPa, the pressure measurement coefcient is 1, and the horizontal stress is 17.7 MPa.Te model schematic diagram is shown in Figure 4.
First, hydraulic cutting is used to form a slot, and a pressure of 25 MPa is applied to the periphery of the groove for fracturing, and the distribution of fracture, main stress, and permeability of the coal seam after fracturing is analyzed.Te fssure distribution of coal seam in the process of drilling construction, hydraulic cutting, and hydraulic fracturing is shown in Figures 5-7, respectively, and the maximum main stress distribution curve on the midline of the test borehole level is monitored in the simulation.
Figure 5 shows the distribution of coal cracks after drilling construction.It can be seen from the fgure that the fssures in the coal body around the drilling hole are not obvious after the construction of the borehole, and basically maintain the original state.Te stress of the coal body around the borehole is evenly distributed and is basically in a state of stress equilibrium.Only the drilling is excavated, creating plastic deformation zones and elastic zones around the borehole, resulting in reduced seam stress.Figure 6 shows the distribution of coal cracks after drilling and cutting.It can be seen that after drilling and cutting, a circular gap is formed around the drilling hole, and the coal body around the gap groove is damaged or plastically deformed, forming a crack.Te stress of the coal body around the trough is concentrated and transferred to the deep part of the coal body, and a stress-reduced pressure relief zone is formed around the slot area.Figure 7 shows the distribution of fractures in coal with a fracturing pressure of 25 MPa, as the water injection pressure increases, when the fssure expands to a certain extent.Te expansion rate begins to slow down, and the secondary fssures are gradually interconnected to form a highly complex fracture network, but with the continuous development of the fssures in the coal seam, the degree of stress concentration is becoming more and more serious.
In the afected area of fracturing, the fractures are mainly elliptical in distribution.In terms of the degree of damage of the coal body, the coal body in the area near the fracturing hole is better than the coal body far from the fracturing hole area.From the change of water injection pressure during the fracturing process, it can be seen that the cracking pressure is about 13 MPa, and the radius of infuence of hydraulic fracturing can reach 45-55 m.
After hydraulic fracturing of the coal seam, the original stress balance state of the coal seam is destroyed, resulting in a decrease in the stress value of the coal seam in the area near the fracturing hole, forming a pressure relief zone.However, the stress value of the coal seam around the pressure discharge zone increases, forming a stress concentration area.Terefore, according to the stress distribution of the coal seam after fracturing, the coal seam around the fracturing Advances in Civil Engineering hole can be divided into stress reduction zone (pressure relief area), stress concentration area, stress transition area, and original stress area from near and far.     Figure 9 shows the distribution of coal cracks after drilling construction.It can be seen from the fgure that after the construction of the borehole, the fracture of the coal body around the borehole is not obvious and basically maintains the original state.Te stress of the coal body around the borehole is evenly distributed and is basically in a state of stress equilibrium.Only due to the excavation of the drilled hole, the plastic deformation zone and the elastic zone are generated around the drilled hole, resulting in a reduction in the stress of the coal seam.

First Press and Ten Cut Combined with Unloading Coal Seam
Figure 10 shows the fracture distribution of coal with a fracturing pressure of 25 MPa.As can be seen from the fgure, when the fssure expands to a certain extent, the number of new fssures decreases, and there is a stressconcentrated area.In the afected area of fracturing, the distribution of fracture fractures is mainly in the direction of main stress, and the other regions have less fracture development and there are blank areas.Te fracture development is uneven.After the use of hydraulic cuts, the number of cracks increases, and the stress concentration area caused by hydraulic fracturing is signifcantly relieved.From the change of water injection pressure during the fracturing process, it can be seen that the cracking pressure is about 16 MPa, and the infuence radius of hydraulic fracturing can reach 35∼45 m.
Figure 11 shows the fracture distribution and stress distribution after hydraulic fracture in the nonformation area of the coal seam after the fracturing pressure is 25 MPa.As can be seen from the fgure, after the hydraulic fracture is carried out in the later stage, the fracture blank zone within the infuence range of the original hydraulic fracturing generates a crack.Te fracture that has been generated can be further increased.Te stress concentration area is redistributed to play a role in depressurization.
Compared with the same model, there is a big diference between the cutting sequence of ultra-high pressure water jet and the fracture and horizontal direction of the discharge coal seam: (1) Te cracking pressure of the coal seam with the combined pressure of cutting frst and then pressing is 13 MPa, and the radius of infuence of hydraulic fracturing can reach 45-55 m.Te cracking pressure of the coal seam of frst pressure and then cutting and unloading coal seam is 16 MPa, and the radius of infuence of hydraulic fracturing can reach 35-45 m. (2) Te frst cut and then the combined pressure discharge coal seam destroys the original stress balance state of the coal seam, causing the stress value of the coal seam in the area near the fracturing hole to decrease, forming a pressure relief area.Te stress distribution is more uniform, at a low value, forming a better pressure relief area, and the stress value of the coal seam around the pressure relief area is increased, forming a stress concentration area.In the area afected by fracturing, the distribution of fracture fractures is mainly along the direction of main stress.Te fracture development in other areas is less, and there are blank areas.Te development of fractures is uneven, after the use of hydraulic fractures, the number of fractures increases, and the stress concentration area caused by hydraulic fracturing is obvious.But there are pressure fuctuations and instability in the pressure relief area.
In summary, after the cutting pressure combination, the pressure relief of the coal body can be uniform and sufcient.Te overall gas permeability coefcient of the coal body can be greatly improved.Te scope of infuence of extraction can be increased, and the extraction efect can be signifcantly improved.At the same time, the stress after the coal body is cut and the cracking pressure during hydraulic fracturing is reduced.(3) Pumping Volume Analysis, Pumping Pure Volume.Te concentration and extraction scalculus of the drilling hole in the application area are shown in Figures 13 and 14.
It can be seen from the fgure that the extraction concentration in the combined cutting pressure area is 10% to 40%.Te concentration fuctuation is relatively stable.Te attenuation is small.Te extraction purity is 5.12-10.13m 3 / min, and the average single-hole extraction purity is 0.0245 m 3

Application of Compression Cutting Combined with Antirefection Technology
(1) Cut-Pressure Combined Drilling Implementation.Te maximum pressure of fracturing is 22-27 MPa, and the number of fracturing times for each borehole is 3 times.Te fracturing time is 21-32 h, and the total water injection of each borehole is 117.8-145.2m 3 .After the completion of fracturing, the drilling hole is drilled according to the construction of 13 m × 13 m spacing in the control area, and the hydraulic cutting measures are constructed according to the 20 m × 20 m spacing.Te maximum pressure during the cutting period is 90-100 MPa.Te cutout spacing is 3 m, and the single knife cutting time is 5-10 min.A total of 91 cut holes were implemented, and the drilling refow water and slag return was smooth during the seam operation.Te coal output of a single knife was 0.31-0.62t.Te average single knife coal output was about 0.41 t, and the equivalent radius of the average cut was 2.52 m.

Figure 1 :Figure 2 :
Figure 1: Schematic diagram of high efciency anti-penetration mode of cutting before pressing.(a) Schematic diagram of fracture propagation profle after cutting.(b) Schematic diagram of the distribution plane of the slit after cutting.

Figure 4 :
Figure 4: Joint numerical model of coal seam cutting pressure.

Figure 5 :
Figure 5: Fracture and stress distribution of coal seam after drilling.(a) Distribution of fractures.(b) Stress distribution condition.

Figure 6 :
Figure 6: Fracture and stress distribution of borehole after slit.(a) Distribution of fractures.(b) Stress distribution condition.

Figure 7 :Figure 8 :
Figure 7: Fracture and stress distribution when fracturing pressure is 25 MPa.(a) Distribution of fractures.(b) Stress distribution condition.

Figure 9 :
Figure 9: Fracture and stress distribution of coal seam after drilling.(a) Distribution of fractures.(b) Stress distribution condition.

Figure 10 :
Figure 10: Fracture and stress distribution of 25 MPa pressure fractured coal seam.(a) Distribution of fractures.(b) Stress distribution condition.
times.Te fracturing time is 19-28 h, and the total amount of water injection per borehole is 109.6-132.4m 3 .(2) Drilling Quantity Analysis.Te spacing between the layout of the extraction drilling holes in the application area is 13 m × 13 m, and a total of 183 extraction drilling holes are constructed, with a drilling volume of about 11800 m.Compared with the conventional drilling arrangement of the mine (the layout spacing is 10 m × 10 m, and 297 extraction drilling holes need to be constructed, about 19200 m), the drilling volume can be saved by about 38%.

Figure 11 :
Figure 11: Fracture and stress distribution of seam after hydraulic fracturing.(a) Distribution of fractures.(b) Stress distribution condition.

( 2 )
Drilling Quantity Analysis.Te spacing between the layout of the extraction drilling holes in the application area is 13 m × 13 m, and a total of 183 extraction drilling holes are constructed, with a drilling volume of about 11800 m.Compared with the conventional drilling arrangement of the mine (the layout spacing is 10 m × 10 m, and 297 extraction drilling holes need to be constructed, about 19200 m), the drilling volume can be saved by about 38%.

( 3 )
Pumping Volume Analysis, Pumping Pure Volume.Te concentration and extraction scalarity of drilling in the application area are shown in Figures 16 and 17.It can be seen from the fgure that the extraction concentration in the combined cutting pressure area is 30% to 40%.Te concentration fuctuation is relatively stable.Te attenuation is small.Te extraction purity is 3.6-6.8m 3 /min, and the average single-hole extraction purity is 0.026 m 3 /min, which is 2.4 times that of the ordinary drilling single-hole extraction purity of 0.0108 m 3 /min.

( 4 )
Extraction Standard Time.According to the drilling and extraction situation in the application area, when the extraction drilling hole extraction is 43 d, the extraction rate reaches 30%.A total of 5 residual gas content were tested, and the test result was 3.76-4.52m 3 /t, and the extraction standard was achieved.Te extraction time is 35% less than the ordinary drilling and extraction time of 66 d. 6.3.Economic Benefts 6.3.1.Direct Economic Beneft Analysis.After the 220106 working surface adopts the measures of cutting pressure

Figure 13 :
Figure 13: Change curve of extraction concentration.

Figure 14 :
Figure 14: Te net volume change curve of extraction.

6. 3 . 2 .
Indirect Economic Beneft Analysis.220106 working surface drilling and extraction time are shortened by 33 and 25 d.Te working surface is returned to the production in advance, and the early recovery of the working surface produces indirect economic benefts: 25 × 6000 t/d × 600 yuan/t � 90 million yuan Xinji No.2 mine generated a total of 90 million yuan in indirect economic benefts.

Figure 16 :
Figure 16: Change curve of extraction concentration.

Figure 17 :
Figure 17: Te net volume change curve of extraction.

. Engineering Application of Ultra-High Pressure Hydraulic Cutting Combined Process Technology
/min, which is 2.3 times that of the ordinary drilling single-hole extraction purity of 0.0108 m 3 /min.(4) Extraction Standard Time.According to the drilling and extraction situation in the application area, when the extraction drilling hole extraction is 47 days, the extraction rate reaches 30%.A total of 14 residual gas content were tested, and the test result was 3.98-4.75m 3 /t.Te extraction standard was achieved.Te extraction time is 41% lower than the 80 days of ordinary drilling and extraction.
Advances in CivilEngineering  135.12-10.13m 3 /min, and the average single hole extraction purity is 0.0245 m 3 /min, which is 2.3 times of the single hole extraction purity of ordinary drilling.Te time to reach the standard of extraction is 32.8 d shorter than that of ordinary drilling.Te extraction concentration in the combined area of borehole and pressure cutting is 30%-40%, the concentration fuctuation is relatively stable and the attenuation is small.Te extraction pure volume is 3.6-6.8m 3 /min, and the average single hole extraction pure volume is 0.026 m 3 /min, which is 2.4 times of the single hole extraction pure volume is 0.0108 m 3 /min of ordinary drilling.Te time to reach the standard of extraction is 25 d shorter than that of ordinary drilling.(4) Te pressure cutting combined with pressure relief and antirefection technology has been successfully applied in Xinji coal mine, and has created good economic benefts.Tis technology has a wide application prospect.