Experimental Device and Method for Evaluating the Hydraulic Fracturing and Permeability Enhancement Effect of Cross-Measurement Boreholes in Coal Seams

Abstract

The present invention relates to an experimental device and method for evaluating the hydraulic fracturing and permeability enhancement effect of cross-layer boreholes in coal seams, belonging to the technical field of coal mines. The fracturing process of this method is to apply the hydraulic pressure provided by the laboratory hydraulic fracturing system to the coal sample with prefabricated boreholes in advance. By monitoring parameters such as the pump injection pressure and flow rate during the fracturing process, the simulation experiment of the laboratory hydraulic fracturing process can be realized. Different from the conventional hydraulic fracturing similarity simulation experiment, in the experiment, the borehole penetrates the entire coal sample, and during the hydraulic fracturing process, the entire coal sample is under the action of water pressure. Before hydraulic fracturing, it is necessary to saturate the coal sample with CO<subgt;2< / subgt> / CH<subgt;4< / subgt> gas to simulate the gas-containing state of the actual coal seam. The evaluation method for the permeability enhancement effect of hydraulic fracturing is to calculate the corresponding permeability coefficient according to the variation law of the gas flow rate with time measured before and after the hydraulic fracturing of the coal sample, and evaluate the permeability enhancement effect of hydraulic fracturing through the ratio of the permeability coefficients measured before and after hydraulic fracturing.

Description

Technical Field

[0001] The present invention belongs to the technical field of coal mines, and relates to an experimental device and method for evaluating the hydraulic fracturing and permeability enhancement effect of cross-layer boreholes in coal seams. Background Art

[0002] In order to improve the extraction efficiency and effectively eliminate coal seam gas disasters, it is necessary to take permeability enhancement measures for coal seams. Hydraulic fracturing is a permeability enhancement technology that has developed rapidly in recent years and has achieved good permeability enhancement effects in many coal mines in China.

[0003] In order to optimize the hydraulic fracturing parameters, explore the mechanism of hydraulic fracturing and the initiation and propagation laws of fractures, and provide theoretical support for on-site hydraulic fracturing, scholars at home and abroad have carried out a large number of similar simulation experiments of hydraulic fracturing in the laboratory. The previous similar simulation experiment method was to inject high-pressure liquid into a prefabricated coal sample and observe the water pressure, stress and fracture propagation during fracturing. The drilling depth in the coal sample used in the experiment was half of the height of the coal sample. After sealing the borehole, fracturing was carried out, and no adsorbed gas was present in the coal sample. Although this method is conducive to studying the initiation and propagation of fractures during hydraulic fracturing, it has a large difference from the actual situation of cross-layer borehole hydraulic fracturing in coal mines, and cannot fully restore the state of fractures in the coal seam after cross-layer borehole hydraulic fracturing in coal mines, cannot restore the gas-containing state of the coal seam, cannot quantitatively evaluate the gas permeability of the coal sample, and cannot evaluate the permeability enhancement effect of hydraulic fracturing.

[0004] Therefore, establishing a similar simulation experiment method for cross-layer borehole hydraulic fracturing in coal seams and a method for evaluating the permeability enhancement effect of hydraulic fracturing is of great significance for restoring the actual situation of cross-layer borehole hydraulic fracturing in coal mines and evaluating the permeability enhancement effect after fracturing. Summary of the Invention

[0005] In view of this, the purpose of the present invention is to provide an experimental device and method for evaluating the hydraulic fracturing and permeability enhancement effect of cross-layer boreholes in coal seams. By conducting similar simulation experiments of hydraulic fracturing in the laboratory, the actual situation of cross-layer borehole hydraulic fracturing in coal mines is restored, the permeability enhancement effect of hydraulic fracturing is quantitatively evaluated, and it provides a reference for on-site construction.

[0006] To achieve the above object, the present invention provides the following technical solutions:

[0007] An experimental device for evaluating the hydraulic fracturing and permeability enhancement effect of cross-layer boreholes in coal seams, the device includes an upper pressure head 1, an upper backing plate 2, a heat shrinkable tube 3, an air / liquid inlet pipe 4, a specimen 5, an experimental gas 6, a lower backing plate 7, a lower pressure head 8, an inlet valve 9, an outlet valve 10, a gas-water separator 11, a flow meter 12 and a hole protection tube 13.

[0008] The air / liquid inlet pipe 4 is disposed through the hole protection tube 13 inside the heat shrinkable tube 3;

[0009] Upper and lower backing plates 2 and 7 are respectively arranged at both ends of the heat shrinkable tube 3; an upper pressure head 1 is further arranged on one side of the upper backing plate 2 away from the heat shrinkable tube 3; a lower pressure head 8 is further arranged on one side of the lower backing plate 7 away from the heat shrinkable tube 3;

[0010] One end of the air / liquid inlet pipe 4 close to the upper backing plate 2 is provided with an inlet valve 9; one end of the air / liquid inlet pipe 4 close to the lower backing plate 7 is provided with an outlet valve 10; a gas-water separator 11 and a flow meter 12 are successively arranged in the direction away from the lower backing plate 7 of the outlet valve 10;

[0011] A test piece 5 is arranged between the heat shrinkable tube 3 and the hole protection tube 13; the test piece 5 is filled with an experimental gas 6.

[0012] Based on the similar simulation experiment of hydraulic fracturing in coal seam cross-cut boreholes and the evaluation method of permeability enhancement effect of the device, the method includes the following steps:

[0013] Step 1: Determine experimental parameters; [[ID=1s3]]

[0014] According to the actual underground coal seam occurrence conditions, fracturing parameters and coal seam physical and mechanical parameters, determine the experimental axial pressure, confining pressure, pore pressure, borehole size, fracturing flow rate and fracturing time based on the similarity theory;

[0015] Step 2: Manufacture and install experimental test pieces;

[0016] Take on-site samples to make standard test pieces, drill holes in the center of the test pieces according to the borehole size, and put in the hole protection tubes; use a triaxial rock mechanics testing machine as the experimental device, install the coal samples in the sample chamber of the triaxial testing machine, and use a heat shrinkable tube to wrap the coal samples tightly to ensure the airtightness of the coal samples;

[0017] Step 3: Apply stress;

[0018] Apply axial pressure and confining pressure to the coal samples according to the designed experimental parameters;

[0019] Step 4: Inflate and adsorb;

[0020] Close the outlet valve, open the inlet valve, and introduce the experimental gas into the sample chamber through the air / liquid inlet pipe. After the coal samples are saturated with adsorption, close the inlet valve;

[0021] Step 5: First flow rate test;

[0022] After the coal samples are saturated with adsorption, open the outlet valve, and test the gas flow rate through the flow meter. Stop recording when the gas flow rate decays to 0;

[0023] Step 6: Inflate and adsorb again;

[0024] Close the outlet valve, open the inlet valve, introduce the experimental gas into the sample chamber, and close the inlet valve after the coal samples are saturated with adsorption;

[0025] Step 7: Determination of fracturing parameters;

[0026] During the hydraulic fracturing of coal seams, three different injection pressures, namely fixed-flow hydraulic fracturing, medium-pressure water injection, and low-pressure water injection, are selected to simulate the fracturing conditions near the fracturing borehole, at the distal end of the fracturing effect, and at the end of the fracturing effect respectively; the fracturing pressure is P 压 ; The area where the fluid pressure drop is 0 is defined as the vicinity of the fracturing borehole, and the position where the fluid pressure drop is P 压 / 2 is defined as the distal end of the fracturing effect, and the position where the fluid pressure drop is P 压 / n is defined as the end of the fracturing effect; 10 ≥ n ≥ 5;

[0027] (1) Fixed-flow hydraulic fracturing, that is, according to the fracturing flow rate determined in Step 1, inject fracturing fluid into the coal sample until the specimen is fractured, and obtain the fracturing pressure P 压 ;

[0028] (2) Medium-pressure water injection, inject fracturing fluid into the coal sample in a fixed-pressure manner, the injection pressure is P 压 / 2, and the injection time is the fracturing time determined in Step 1;

[0029] (3) Low-pressure water injection, inject fracturing fluid into the coal sample in a fixed-pressure manner, the injection pressure is P 压 / n; the injection time is the fracturing time determined in Step 1;

[0030] Step 8: Hydraulic fracturing and second flow rate test;

[0031] According to the determined hydraulic fracturing parameters; open the inlet valve, inject fracturing fluid into the coal sample, start the experiment, monitor the injection pressure in the specimen until the fracturing time is completed; after the hydraulic fracturing is completed, open the outlet valve, test the liquid flow rate through the change in the mass of the gas-liquid separator, test the gas flow rate through the flow meter, and stop recording when the gas flow rate decays to 0;

[0032] Step 9: Calculation of gas permeability coefficient;

[0033] Based on the measured relationship between the gas flow rate and time, the corresponding gas permeability coefficient is calculated by the following formula;

[0034]

[0035] In the formula: q is the specific flow rate, m 3 / (m 2 ·d); λ is the gas permeability coefficient of the coal sample, m 2 / (MPa 2 ·d); λ = k / 2μp a , k is the permeability of the coal sample, m 2; μ is the hydrodynamic viscosity, MPa·d; P a is the atmospheric pressure under standard conditions, 0.1013 MPa; P is the square of the gas pressure, P = p 2 , MPa 2 ; p is the gas pressure, MPa; r is the distance from any point of the coal sample to the center of the borehole, m; P0 is the square of the original gas pressure of the coal seam, MPa 2 ; r0 is the borehole radius, m; r1 is the sample radius, m; P1 is the borehole gas pressure, MPa;

[0036] For the coal sample after fracturing, the permeability coefficient after fracturing is calculated using the relationship between the gas flow rate after no liquid is discharged and the change with time;

[0037] Step 10: Evaluation of the permeability enhancement effect;

[0038] Based on the calculated permeability coefficients of the coal sample before and after fracturing / water injection, calculate the permeability enhancement coefficient according to the following formula, and evaluate the permeability enhancement effect of hydraulic fracturing based on the value of the permeability enhancement coefficient:

[0039] When the permeability enhancement effect is effective, α1 > 1.1 near the fracturing borehole, α2 ≥ 0.8 at the far end of the fracturing effect, and α3 ≥ 0.6 at the end of the fracturing effect;

[0040] When the permeability enhancement effect is ineffective, α1 ≤ 1.1 near the fracturing borehole, α2 < 0.8 at the far end of the fracturing effect, and α3 < 0.6 at the end of the fracturing effect;

[0041] Calculate the permeability enhancement coefficient:

[0042]

[0043] In the formula: α i is the permeability enhancement coefficient, i = 1, 2, 3, respectively representing the permeability enhancement coefficients near the fracturing borehole, at the far end of the fracturing effect, and at the end of the fracturing effect; λ i1 is the permeability coefficient of the coal sample before hydraulic fracturing, m 2 / (MPa 2 ·d); λ i2 is the permeability coefficient of the coal sample after hydraulic fracturing, m 2 / (MPa 2 ·d); When α1 > 1, it indicates that the hydraulic fracturing is effective; when α1 ≥ 1.5, it indicates that the permeability enhancement effect is good; when α1 ≤ 1, it indicates that the hydraulic fracturing is ineffective.

[0044] Optionally, the hole protection pipe is a metal pipe with a wall thickness of 1 mm and small holes with a diameter of 1 mm.

[0045] The beneficial effects of the present invention are as follows:

[0046] The fracturing process of this method is to apply the hydraulic pressure provided by the laboratory hydraulic fracturing system to the coal sample with prefabricated boreholes. By monitoring parameters such as the pumping pressure and flow rate during the fracturing process, the simulation experiment of the laboratory hydraulic fracturing process can be achieved. Different from the conventional hydraulic fracturing similarity simulation experiment, in this experiment, the borehole penetrates the entire coal sample, and during the hydraulic fracturing process, the entire coal sample is under the action of water pressure. In addition, before hydraulic fracturing, the coal sample needs to be saturated with CO2 / CH4 gas to simulate the gas-containing state of the actual coal seam. The evaluation method for the permeability enhancement effect of hydraulic fracturing is to calculate the corresponding permeability coefficient based on the variation law of the gas flow rate measured before and after the hydraulic fracturing of the coal sample, and evaluate the permeability enhancement effect of hydraulic fracturing through the ratio of the permeability coefficients measured before and after hydraulic fracturing.

[0047] Other advantages, objectives, and features of the present invention will be described in part in the subsequent specification, and to some extent, will be obvious to those skilled in the art based on the study of the following text, or can be taught from the practice of the present invention. The objectives and other advantages of the present invention can be realized and obtained through the following specification. Brief Description of the Drawings

[0048] In order to make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be described in detail with reference to the accompanying drawings, where:

[0049] Figure 1 It is a schematic diagram of a similar simulation experiment and permeability enhancement effect evaluation device for cross-layer borehole hydraulic fracturing in coal seams.

[0050] Figure 2 It is a flowchart of the present invention;

[0051] Reference numerals: 1 - upper pressure head; 2 - upper backing plate; 3 - heat shrinkable tube; 4 - inlet / outlet liquid pipe; 5 - specimen; 6 - experimental gas; 7 - lower backing plate; 8 - lower pressure head; 9 - inlet valve; 10 - outlet valve; 11 - gas-water separator; 12 - flowmeter; 13 - hole protection tube. Detailed Embodiments

[0052] The following illustrates the embodiments of the present invention through specific examples. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through different specific embodiments. The details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that the diagrams provided in the following embodiments only illustrate the basic concept of the present invention schematically. Without conflict, the following embodiments and the features in the embodiments can be combined with each other.

[0053] Among them, the attached drawings are only for illustrative purposes, showing only schematic diagrams rather than physical diagrams, and should not be construed as a limitation to the present invention; for better illustration of the embodiments of the present invention, some components in the attached drawings will be omitted, enlarged or reduced, which do not represent the dimensions of the actual product; for those skilled in the art, it is understandable that some well-known structures and their descriptions in the attached drawings may be omitted.

[0054] In the attached drawings of the embodiments of the present invention, the same or similar reference numerals correspond to the same or similar components; in the description of the present invention, it should be understood that if there are terms such as "upper", "lower", "left", "right", "front", "rear", etc. indicating the orientation or positional relationship, they are based on the orientation or positional relationship shown in the attached drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation. Therefore, the terms describing the positional relationship in the attached drawings are only for illustrative purposes and should not be construed as a limitation to the present invention. For those of ordinary skill in the art, the specific meanings of the above terms can be understood according to specific circumstances.

[0055] As Figure 1 shown, the similar simulation experiment of hydraulic fracturing of cross - seam boreholes and the permeability - enhancement effect evaluation device includes an upper pressure head 1, an upper backing plate 2, a heat - shrinkable tube 3, an air / liquid inlet pipe 4, a specimen 5, an experimental gas 6, a lower backing plate 7, a lower pressure head 8, an inlet valve 9, an outlet valve 10, a gas - water separator 11, a flowmeter 12, and a hole - protecting tube 13.

[0056] The air / liquid inlet pipe 4 is disposed through the hole - protecting tube 13 inside the heat - shrinkable tube 3;

[0057] Both ends of the heat - shrinkable tube 3 are respectively provided with an upper backing plate 2 and a lower backing plate 7; on the side of the upper backing plate 2 away from the heat - shrinkable tube 3, there is also an upper pressure head 1 provided; on the side of the lower backing plate 7 away from the heat - shrinkable tube 3, there is also a lower pressure head 8 provided;

[0058] One end of the air / liquid inlet pipe 4 close to the upper backing plate 2 is provided with an inlet valve 9; one end of the air / liquid inlet pipe 4 close to the lower backing plate 7 is provided with an outlet valve 10; in the direction away from the lower backing plate 7 of the outlet valve 10, there are also a gas - water separator 11 and a flowmeter 12 arranged in sequence;

[0059] A specimen 5 is arranged between the heat - shrinkable tube 3 and the hole - protecting tube 13; the specimen 5 is filled with an experimental gas 6.

[0060] As Figure 2 shown, it is a method for similar simulation experiment of hydraulic fracturing of cross - seam boreholes and permeability - enhancement effect evaluation.

[0061] Step 1: Determine the experimental parameters.

[0062] Based on the actual underground coal seam occurrence conditions, fracturing parameters, and physical and mechanical parameters of the coal seam, the experimental axial pressure, confining pressure, pore pressure, borehole size, fracturing flow rate, and fracturing time are determined based on the similarity theory.

[0063] Step 2: Fabrication and installation of experimental specimens.

[0064] Samples are taken from the field and made into standard specimens. A borehole is drilled in the center of the specimen according to the borehole size, and the hole protection tube 13 is placed. The hole protection tube 13 is a metal tube with a wall thickness of 1 mm and small holes with a diameter of 1 mm. Using a triaxial rock mechanics testing machine as the experimental device, the coal sample is installed in the sample chamber of the triaxial testing machine, and the coal sample is wrapped tightly with the heat shrinkable tube 3 to ensure the airtightness of the coal sample, as Figure 2 shown.

[0065] Step 3: Stress loading.

[0066] Apply axial pressure and confining pressure to the coal sample according to the designed experimental parameters.

[0067] Step 4: Inflation and adsorption.

[0068] Close the outlet valve 10, open the inlet valve 9, and introduce the experimental gas 6 into the sample chamber through the inlet / outlet liquid pipe 4. After the coal sample is saturated with adsorption, close the inlet valve 9.

[0069] Step 5: First flow rate test.

[0070] After the coal sample is saturated with adsorption, open the outlet valve 10, and test the gas flow rate through the flowmeter 12. Stop recording when the gas flow rate decays to 0.

[0071] Step 6: Re-inflation and adsorption.

[0072] Close the outlet valve 10, open the inlet valve 9, introduce the experimental gas into the sample chamber, and close the inlet valve 9 after the coal sample is saturated with adsorption.

[0073] Step 7: Determination of fracturing parameters.

[0074] During the hydraulic fracturing of the coal seam, due to the fluid pressure drop, the effective pressures on the coal bodies at different distances from the borehole are different, and the fracturing degree of the coal seam is also different. Therefore, in the experiment, fixed-flow hydraulic fracturing (fracturing pressure P 压 ), medium-pressure water injection, and low-pressure water injection are selected for three different liquid injection pressures to simulate the fracturing conditions near the fracturing borehole, at the distal end of the fracturing effect, and at the end of the fracturing effect respectively. The area where the fluid pressure drop is 0 is defined as near the fracturing borehole, the position where the fluid pressure drop is P 压 / 2 is defined as the distal end of the fracturing effect, and the position where the fluid pressure drop is P 压 / n (10 ≥ n ≥ 5, judged according to P 压 etc.) is defined as the end of the fracturing effect.

[0075] (1) Fixed flow hydraulic fracturing, that is, according to the fracturing flow determined in step 1, the fracturing fluid is injected into the coal sample until the fracturing of the specimen 5 is completed. At this time, the fracturing pressure P can be obtained. 压 .

[0076] (2) Medium pressure water injection: inject fracturing fluid into the coal sample at a fixed pressure. The injection pressure is P 压 / 2, the injection time is the fracturing time determined in step 1.

[0077] (3) Low-pressure water injection: inject fracturing fluid into the coal sample at a fixed pressure. The injection pressure is P 压 / n. The injection time is the fracturing time determined in step 1.

[0078] Step 8: Hydraulic fracturing and second flow test.

[0079] Based on the determined hydraulic fracturing parameters, the inlet valve 9 is opened to inject fracturing fluid into the coal sample, and the experiment begins. The injection pressure within the test piece is monitored until the fracturing time is complete. After hydraulic fracturing is complete, the outlet valve 10 is opened. The liquid flow rate is measured by the mass change of the gas-water separator 11, and the gas flow rate is measured by the flowmeter 12. Recording stops when the gas flow rate decays to zero.

[0080] Step 9: Calculation of air permeability coefficient.

[0081] Based on the relationship between the measured gas flow rate and its change over time, the corresponding permeability coefficient can be calculated using the following formula.

[0082]

[0083] Where: q is the specific flow rate, m 3 / (m 2 ·d); λ is the permeability coefficient of the coal sample, m 2 / (MPa 2 ·d); λ=k / 2μp a , k is the permeability of the coal sample, m 2 ; μ is the fluid dynamic viscosity, MPa·d; P a is the atmospheric pressure under standard conditions, 0.1013 MPa; P is the square of the gas pressure, P = p 2 , MPa 2 ; p is the gas pressure, MPa; r is the distance from any point of the coal sample to the center of the borehole, m; P0 is the square of the original gas pressure of the coal seam, MPa 2 ; r0 is the borehole radius, m; r1 is the sample radius, m; P1 is the borehole gas pressure, MPa.

[0084] For the coal samples after fracturing, the gas permeability coefficient after fracturing is calculated using the relationship between the gas flow rate without liquid discharge and the change over time.

[0085] Step 10: Evaluation of permeability enhancement effect.

[0086] Based on the calculated gas permeability coefficients of the coal samples before and after fracturing / water injection, the permeability enhancement coefficient is calculated according to the following formula, and the hydraulic fracturing permeability enhancement effect is evaluated by referring to Table 1 based on the value of the permeability enhancement coefficient.

[0087]

[0088] In the formula: α i is the permeability enhancement coefficient, i = 1, 2, 3, representing the permeability enhancement coefficients near the fracturing borehole, at the distal end of the fracturing effect, and at the end of the fracturing effect respectively; λ i1 is the gas permeability coefficient of the coal sample before hydraulic fracturing, m 2 / (MPa 2 ·d); λ i2 is the gas permeability coefficient of the coal sample after hydraulic fracturing, m 2 / (MPa 2 ·d). When α1 > 1, it indicates that the hydraulic fracturing is effective; when α1 ≥ 1.5, it indicates that the permeability enhancement effect is good; when α1 ≤ 1, it indicates that the hydraulic fracturing is ineffective.

[0089] Table 1 Evaluation table of hydraulic fracturing permeability enhancement effect

[0090]

[0091] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not limitations. Although the present invention has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that the technical solutions of the present invention can be modified or equivalently replaced without departing from the purpose and scope of the present technical solution, and they should all be covered within the scope of the claims of the present invention.

Claims

1. Coal seam drilling hydraulic fracturing and permeability enhancement effect evaluation experimental device, characterized by: The device comprises an upper pressure head (1), an upper backing plate (2), a heat shrink tube (3), an air / liquid inlet pipe (4), a test piece (5), an experimental gas (6), a lower backing plate (7), a lower pressure head (8), an inlet valve (9), an outlet valve (10), a gas-water separator (11), a flow meter (12) and a hole protection tube (13); The air / liquid inlet pipe (4) passes through the hole-protecting pipe (13) arranged inside the heat shrink tube (3); An upper pad (2) and a lower pad (7) are respectively provided at both ends of the heat shrink tube (3); an upper pressure head (1) is also provided on the side of the upper pad (2) away from the heat shrink tube (3); and a lower pressure head (8) is also provided on the side of the lower pad (7) away from the heat shrink tube (3); An inlet valve (9) is provided at one end of the air / liquid pipe (4) close to the upper pad (2); an outlet valve (10) is provided at one end of the air / liquid pipe (4) close to the lower pad (7); and an air-water separator (11) and a flow meter (12) are provided in sequence in the direction away from the lower pad (7) of the outlet valve (10). A test piece (5) is arranged between the heat shrink tube (3) and the hole protection tube (13); the test piece (5) is filled with experimental gas (6); The drill hole penetrates the entire coal sample; the hole protection pipe (13) is a metal pipe with a wall thickness of 1 mm and a small hole with a diameter of 1 mm.

2. A similar simulation experiment and permeability enhancement effect evaluation method for coal seam through-drilling hydraulic fracturing based on the device of claim 1, characterized in that: The method comprises the following steps: Step 1: Determine the experimental parameters; According to the actual underground coal seam covering conditions, fracturing parameters and coal seam physical and mechanical parameters, the experimental axial pressure, confining pressure, pore pressure, borehole size, fracturing flow rate and fracturing time are determined based on similarity theory; Step 2: Preparation and installation of experimental specimens; On-site sampling is made into standard test pieces. A hole is drilled in the center of the test piece according to the drilling size, and a hole protection tube is placed in it. A triaxial rock mechanics testing machine is used as the experimental device. The coal sample is installed in the sample chamber of the triaxial testing machine and wrapped tightly with heat shrink tubing to ensure the airtightness of the coal sample. Step 3: Stress loading; Apply axial pressure and confining pressure to the coal sample according to the designed experimental parameters; Step 4: Inflation adsorption; Close the outlet valve, open the inlet valve, and introduce the experimental gas into the sample chamber through the gas / liquid pipe. After the coal sample is saturated with adsorption, close the inlet valve; Step 5: First flow test; After the coal sample is saturated with adsorption, open the outlet valve and test the gas flow rate through the flow meter. When the gas flow rate decays to 0, stop recording. Step 6: Inflate and adsorb again; Close the outlet valve, open the inlet valve, and introduce the experimental gas into the sample chamber. After the coal sample is saturated with adsorption, close the inlet valve; Step 7: Determine the fracturing parameters; During coal seam hydraulic fracturing, three different injection pressures are selected: fixed flow hydraulic fracturing, medium pressure water injection and low pressure water injection, to simulate the fracturing conditions near the fracturing borehole, at the far end of the fracturing action and at the end of the fracturing action respectively; the fracturing pressure is P 压 ; The area where the fluid pressure drop is 0 is defined as the vicinity of the fracturing borehole, and the fluid pressure drop is defined as P 压 The position of / 2 is defined as the far end of the fracturing action, and the fluid pressure is reduced to P 压 The position of / n is defined as the end of the fracturing action; 10≥n≥5; (1) Fixed flow hydraulic fracturing, that is, according to the fracturing flow determined in step 1, the fracturing fluid is injected into the coal sample until the specimen is fracturing and the fracturing pressure P is obtained. 压 ; (2) Medium pressure water injection: inject fracturing fluid into the coal sample at a fixed pressure. The injection pressure is P 压 / 2, the injection time is the fracturing time determined in step 1; (3) Low-pressure water injection: inject fracturing fluid into the coal sample at a fixed pressure. The injection pressure is P 压 / n; injection time is the fracturing time determined in step 1; Step 8: Hydraulic fracturing and second flow test; According to the determined hydraulic fracturing parameters, the inlet valve is opened, and the fracturing fluid is injected into the coal sample to start the experiment. The injection pressure in the test piece is monitored until the fracturing time is completed. After the hydraulic fracturing is completed, the outlet valve is opened, and the liquid flow rate is tested by the change in the mass of the gas-water separator, and the gas flow rate is tested by the flow meter. When the gas flow rate decays to 0, the recording is stopped. Step 9: Calculation of air permeability coefficient; Based on the relationship between the measured gas flow rate and its change over time, the corresponding permeability coefficient is calculated using the following formula: Where: q is the specific flow rate, m 3 / (m 2 ·d); λ is the permeability coefficient of the coal sample, m 2 / (MPa 2 ·d); λ=k / 2μp a , k is the permeability of the coal sample, m 2 ; μ is the fluid dynamic viscosity, MPa·d; P a is the atmospheric pressure under standard conditions, 0.1013 MPa; P is the square of the gas pressure, P = p 2 , MPa 2 ; p is the gas pressure, MPa; r is the distance from any point of the coal sample to the center of the borehole, m; P0 is the square of the original gas pressure of the coal seam, MPa 2 ; r0 is the borehole radius, m; r1 is the sample radius, m; P1 is the borehole gas pressure, MPa; For coal samples after fracturing, the permeability coefficient after fracturing is calculated using the relationship between the gas flow rate after no liquid is discharged and its change over time. Step 10: Antireflection Effect Evaluation; Based on the calculated permeability coefficient of the coal sample before and after fracturing / water injection, the permeability enhancement coefficient is calculated according to the following formula, and the hydraulic fracturing permeability enhancement effect is evaluated based on the permeability enhancement coefficient value: When the permeability enhancement effect is effective, α1>1.1 near the fracturing borehole, α2≥0.8 at the far end of the fracturing effect, and α3≥0.6 at the end of the fracturing effect; When the permeability enhancement effect is invalid, α1 is ≤ 1.1 near the fracturing borehole, α2 is < 0.8 at the far end of the fracturing effect, and α3 is < 0.6 at the end of the fracturing effect; Calculate the antireflection coefficient: Where: α i is the permeability enhancement coefficient, i = 1, 2, 3, representing the permeability enhancement coefficient near the fracturing borehole, the far end of the fracturing action, and the end of the fracturing action, respectively; λ i1 is the permeability coefficient of the coal sample before hydraulic fracturing, m 2 / (MPa 2 ·d);λ i2 is the permeability coefficient of the coal sample after hydraulic fracturing, m 2 / (MPa 2 d); When α1>1, it indicates that hydraulic fracturing is effective; when α1≥1.5, it indicates that the permeability enhancement effect is good; when α1≤1, it indicates that hydraulic fracturing is ineffective.

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