A method and device for testing coal-rock interface stress affected by gas pressure

Abstract

The application discloses a coal-rock interface stress testing method and device influenced by gas pressure, and relates to a device composed of a counterforce frame, a hollow piston rod, two piston cylinders, a gas circuit and a plunger pump. Coal samples and rock samples are respectively placed in the two piston cylinders and assembled with the two ends of the piston rod. The hollow piston rod communicates the inner cavities of the two piston cylinders, simulating the boundary conditions that the displacement amount and the pore pressure of the coal-rock interface are the same. The counterforce frame can apply load to the combination of the coal-containing piston cylinder, the piston rod and the rock-containing piston cylinder and constrain the overall deformation thereof, simulating the constraint condition of the overburden strata on the coal seam. The application can measure the strain values of the piston rod and the counterforce frame in the process of the change of the gas pressure, calculate the pressure change of the end surface of the piston rod, represent the change of the normal stress of the coal-rock interface, and make up the technical blank of the coal seam boundary stress measurement in the process of the coalbed methane extraction or the coal seam carbon dioxide storage.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of coal mine gas extraction and coalbed methane exploitation, and in particular to measuring the normal stress change of coal-rock interface caused by the flow of coalbed gas. BACKGROUND

[0002] Coalbed gas (methane) is a high-quality, clean unconventional geological energy. Developing coalbed (methane) gas resources is of great significance to ensuring the sustainable use of China's coal resources, safe production of coal mines, and achieving the "double carbon" goal. China's coalbed gas industry has undergone nearly four decades of research and development. Although the development technology has changed from borrowing foreign technology to independent innovation, the coalbed gas industry in China still faces the current situation of "low exploration and development degree and low single well production". In view of the geological conditions of coal reservoirs, historical production data and resource reserves recovery degree, a deeper understanding of the seepage mechanism of coalbed gas is needed to develop more efficient coalbed gas development technology.

[0003] The coalbed gas production process is first to drain coalbed water, reduce coalbed pore pressure, and promote the desorption of adsorbed methane in coal pores into free state, and then the free methane flows along the coal fracture network to the wellbore under the driving of the pore pressure gradient. The key technical indicator to measure the gas flow capacity in coal reservoirs is coal permeability. The greater the permeability, the easier the gas flows in the coal pore and fracture, and the more gas flows to the wellbore. Considering the influence of multiple factors, it is difficult to represent the internal mechanism of coalbed gas production by calculating the coalbed permeability according to the well test or production well drainage data. It is also necessary to study the variation law of coal permeability under the influence of various factors in the laboratory. At present, most scholars and technical personnel in the industry design various experimental conditions to study the interaction of effective stress, matrix shrinkage and slippage of coal reservoirs on the whole process of coalbed gas production, including pressure reduction, adsorbed methane desorption, diffusion, seepage and output, resulting in dynamic changes in coal reservoir permeability.

[0004] It is worth noting that most of the existing laboratory test methods focus on how the elastic mechanical behavior of coal under various stress conditions affects the fluid seepage characteristics. In fact, the coalbed and its roof and floor form a sandwich structure, as shown in Figure 1The roof exerts vertical stress on the coal seam, and the floor supports the coal reservoir. Due to the fact that the elastic modulus of coal is smaller than that of rock, when the pressure of the coal reservoir decreases, there is bound to be a difference between the deformation of the coal seam and the deformation of the roof and floor, which will change the lower support of the roof or the upper support of the floor of the coal seam, and finally change the interface stress of the coal seam and the rock. However, most of the current research simplifies the boundary stress of the coal reservoir as a constant, which causes a large error in predicting the permeability of the coal reservoir. Therefore, according to the structural characteristics of the coal measure stratum, the present application provides a device and a testing method for observing the change of the interface stress between coal and rock, which makes up for the technical blank of the measurement of the boundary stress of the coal seam during the process of coalbed methane extraction or carbon dioxide storage, and provides strong measurement technical support for accurately evaluating the gas production of the coal seam. SUMMARY

[0005] The present application provides a coal and rock interface stress testing method and device affected by gas pressure, which simulates the contact conditions of the coal seam and the rock stratum, can measure the change of the interface stress between rock and coal caused by the change of the coal pore pressure, and provides a measurement method for accurately evaluating the stress state of the coal seam.

[0006] To solve the above problems, the technical scheme adopted by the present application is:

[0007] A coal-rock interface stress testing method affected by gas pressure, characterized in that it comprises the following steps:

[0008] Firstly, the coal block and the rock block are cored and processed into cuboid-shaped coal samples and rock samples with the same cross section, the height ratio of the coal sample and the rock sample is consistent with the thickness ratio of the actual coal seam and the roof, then the coal sample and the rock sample are wrapped with lead foil and loaded into respective piston cylinders, and the sample is attached to the wall of the inner cavity of the piston cylinder;

[0009] Secondly, the rock-containing piston cylinder and the coal-containing piston cylinder are assembled with the two end heads of a hollow piston rod respectively to form a rock-containing piston cylinder-piston rod-coal-containing piston cylinder combination, which simulates the interlayer structure form of the coal measure stratum, and then the combination is installed on a counterforce frame;

[0010] Thirdly, the counterforce frame is twisted to apply a certain pre-stress to the combination, and the length of the combination is limited to be unchanged, the coal-containing piston cylinder is injected with gas to balance the adsorption of the coal sample, the original state of the coal seam is simulated, then the gas pressure in the coal-containing piston cylinder is gradually reduced in stages to simulate the gas production process of the coal reservoir under pressure reduction, the strain of the piston rod and the counterforce frame column is measured throughout the process, and the normal stress of the coal-rock interface is calculated according to the force balance relationship of the piston rod; it is required that the strain of the column and the piston rod can be kept stable during each loading and unloading stage.

[0011] Further, the detailed process of step three is:

[0012] 3.1: According to the measured stress value, the combined body is prestressed by the reaction frame, then the coal-containing piston cylinder is injected with methane, and after the methane pressure in the coal-containing piston cylinder is basically stable, it is considered that the coal sample and the rock sample reach the adsorption equilibrium state, at this time the gas pressure in the coal sample is recorded as the initial equilibrium pressure P0;

[0013] 3.2: The gas pressure in the coal sample is gradually reduced until the gas pressure value in the coal-containing piston cylinder approaches atmospheric pressure, the gradient refers to the equal amplitude of each time the gas pressure is reduced, and the gas pressure P in the coal-containing piston cylinder and the piston rod strain ε are recorded throughout the process;

[0014] 3.3: According to the force balance relationship of the piston rod, the coal-rock interface stress σ is calculated by the formula σA1+PA1=βA2=EεA2, wherein E is the elastic modulus of the piston rod, A1 is the cross-sectional area of the piston head, A2 is the cross-sectional area of the piston rod, and β is the cross-sectional stress of the piston rod.

[0015] In order to realize the above method, the present application provides a test device, which comprises a coal-containing piston cylinder, a rock-containing piston cylinder, a hollow piston rod, a reaction frame, a gas injection system, a data acquisition instrument, a pressure sensor and a plurality of resistance strain gauges; wherein:

[0016] The hollow piston rod can be assembled with the coal-containing piston cylinder and the rock-containing piston cylinder at both ends to form a combined body in series of "coal-containing piston cylinder-piston rod-rock piston cylinder";

[0017] The reaction frame can apply load to the combined body of "coal-containing piston cylinder-piston rod-rock-containing piston cylinder";

[0018] The gas injection system is that the gas first flows from the gas cylinder into the intermediate gas container, then the gas pressure in the intermediate gas container is adjusted by the plunger pump, then the gas flows from the intermediate gas container into the upstream gas container, then flows into the coal-containing piston cylinder, and finally flows into the downstream gas container.

[0019] The side wall of the coal-containing piston cylinder has a pair of interfaces connected to the upstream gas container and the downstream gas container respectively, the upstream gas container is connected to the downstream gas container and is provided with a valve, and the gas pressure value in the coal-containing piston cylinder is measured by the pressure sensor;

[0020] The plurality of resistance strain gauges are respectively attached to the middle parts of the four columns of the reaction frame and the middle part of the hollow piston rod, and all the resistance strain gauge leads are connected to the data acquisition instrument.

[0021] Further, the gas injection system comprises a gas cylinder, an intermediate gas container, an upstream gas container and a downstream gas container, wherein the plunger pump controls the gas pressure in the intermediate gas container; valves are arranged on the pipelines between the downstream gas container, the gas cylinder and the intermediate gas container, and the intermediate gas container and the upstream gas container.

[0022] Further, the hollow piston rod has a pipeline along its axis penetrating the piston head at both ends, so that the gas in the coal-containing piston cylinder can flow into the rock-containing piston cylinder through the piston rod.

[0023] Further, a pair of interfaces are reserved on the sidewall of the rock-containing piston cylinder.

[0024] Further, interfaces are also reserved on the base of the two piston cylinders.

[0025] The advantages of the present application are:

[0026] Firstly, the boundary conditions of the coal-rock interface are reconstructed. In the process of coal seam pressure reduction for gas production, the coal seam is mainly characterized by vertical compression. The present application respectively loads coal samples and rock samples into a piston cylinder, and the coal samples and rock samples can only deform in a single direction in the inner cavity of the piston cylinder. Then, a high-rigidity hollow piston rod is assembled with the two piston cylinders. One end of the piston rod contacts the coal sample, and the other end contacts the rock sample. The hollow structure inside the piston rod connects the inner cavities of the two piston cylinders, forming a combination of "coal-containing piston cylinder-piston rod-rock-containing piston cylinder", which simulates the interlayer structure of the formation. In the process of gas pressure change in the coal sample, the gas in the rock-containing piston cylinder also changes. The coal sample and the rock sample will deform to different degrees and push the piston rod until a new mechanical equilibrium state is reached. It meets the characteristics that the coal-rock interface can only deform in one direction, and the displacement and pore pressure are the same.

[0027] Secondly, the boundary conditions of the coal measure stratum are determined. Because the roof and floor rocks have low permeability and the elastic modulus is greater than that of the coal seam, the change of the coal seam pore pressure can only cause the roof to elastically deform in the area contacting the coal seam, and the compression or swelling deformation of the coal seam does not affect the overall size of the interlayer structure. It can be considered that the stratum in the research area is under the condition of limited displacement boundary. The present application uses a high-rigidity counterforce frame to apply load to the combination and constrain the overall deformation of the combination, simulating the constraint conditions of the overburden on the coal seam. According to the strain values of the piston rod and the counterforce frame, the pressure change of the end surface of the piston rod is calculated, the change of the normal stress of the coal-rock interface is represented, the technical gap of measuring the boundary stress of the coal seam in the process of coalbed methane extraction or carbon dioxide storage is filled, and a research method for accurately evaluating the relationship between the coal seam permeability and stress is provided. BRIEF DESCRIPTION OF DRAWINGS

[0028] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the drawings needed in the embodiment description will be briefly introduced as follows. The drawings in the following description are only some embodiments of the present application, and other drawings can also be obtained by those skilled in the art without creative labor on the basis of these drawings.

[0029] Figure 1 It is a schematic diagram of the coal measure stratum structure.

[0030] Figure 2 The force diagram of the two end faces of the piston rod;

[0031] Figure 3 The force diagram of the two end faces of the piston rod;

[0032] Figure 4 The force diagram of the two end faces of the piston rod;

[0033] Figure 5 The micro-strain and load change diagram of the piston rod and the counter-force frame during the step loading and unloading process, wherein Figure 5 (a) the micro-strain of the four columns of the counter-force frame during the step loading and unloading process is shown, Figure 5 (b) the micro-strain of the piston rod during the step loading and unloading process is shown, Figure 5 (c) the load curve of the piston rod and the counter-force frame;

[0034] Figure 6 The structural schematic diagram of the testing device of the present application;

[0035] Figure 7 The stress change diagram of the coal-rock interface and the micro-strain change diagram of the column and the piston rod caused by the decrease of the gas pressure in the piston cylinder containing coal, wherein Figure 7 (a) the micro-strain change of the column and the piston rod during the pressure decrease process is shown, Figure 7 (b) the stress change of the coal-rock interface during the gas pressure decrease process is shown;

[0036] Figure 8 The horizontal effective stress-time and permeability-time change curve diagram of coal caused by the decrease of the pore pressure tested by the testing device of the present application, wherein Figure 8 (a) the pore pressure decay curve of the coal sample is shown, Figure 8 (b) the horizontal effective stress change curve of the coal sample with time is shown, Figure 8 (c) the permeability ratio change curve of the coal sample with time is shown;

[0037] Figure 9 The horizontal effective stress and permeability change curve diagram of coal caused by the decrease of the pore pressure considering the influence of the roof rock and not considering the influence of the roof rock.

[0038] In the figure: 1- push rod, 2- column, 3- pressure plate, 4- first interface, 5- rock sample, 6- piston head, 7- strain gauge, 8- data acquisition instrument, 9- lower base, 10- upper base, 11- piston cylinder base, 12- piston cylinder body, 13- second interface, 14- center channel, 15- piston rod, 16- coal sample, 17- third interface, 18- upstream gas container, 19- intermediate gas container, 20- gas cylinder, 21- plunger pump, 22- computer, 23- pressure sensor, 24- downstream gas container, 25- sealing ring.

[0039] A-first valve, B-second valve, C-third valve, D-fourth valve. DETAILED DESCRIPTION

[0040] The preferred embodiments of the present invention are described in detail below with reference to the accompanying drawings so that the advantages and features of the present invention can be more easily understood by those skilled in the art, thereby making a clearer and more precise definition of the protection scope of the present invention.

[0041] The following combination Figure 2 and 4 Description The components of the test device of the present invention are described in detail.

[0042] like Figure 6 As shown, the test device of the present invention includes two piston cylinders, a piston rod 15, a reaction frame, an air injection system, a pressure sensor 23, a computer 22, a data acquisition device 8 and multiple sets of strain gauges 7; one piston cylinder is used to place a coal sample 16, which is called a coal-containing piston cylinder, and the other piston cylinder is used to place a rock sample 5, which is called a rock-containing piston cylinder. The coal-containing piston cylinder and the rock-containing piston cylinder have the same structure. Figure 6 As can be seen, both are composed of a piston cylinder base 11 and a piston cylinder body 12. The structure of the piston rod 15 is shown in FIG. Figure 2 ,from Figure 2 It can be seen that the two ends of the piston rod 15 are piston heads 6, which are respectively installed in the coal-containing piston cylinder and the rock-containing piston cylinder to form a "rock-containing piston cylinder-piston rod-coal-containing piston cylinder" combination. The two piston heads 6 of the piston rod 15 are both provided with sealing rings 25, which ensure that the two piston heads 6 are tightly attached to the inner wall of the piston cylinder to achieve sealing performance. A central channel 14 is provided in the piston rod 15 to connect the inner cavities of the two piston cylinders. The reaction frame is composed of four columns 2, an upper base 10, a lower base 9, a pressure plate 3 and a push rod 1. Strain gauges 7 are attached to the piston rod 15 and the four columns 2 of the reaction frame. When in use, the push rod 1 of the reaction frame is rotated to push the pressure plate 3, clamping the rock-containing piston cylinder-piston rod-coal-containing piston cylinder combination and applying pressure to the combination. The strain data of the piston rod 15 and the four columns 2 are recorded by the data acquisition instrument 8.

[0043] from Figure 6As can be seen, the gas injection system includes a gas cylinder 20, an intermediate gas container 19, an upstream gas container 18, and a downstream gas container 24. The intermediate gas container 19 is connected to a plunger pump 21. The plunger pump 21 controls the gas pressure within the intermediate gas container 19. A fourth valve D is installed at the end of the downstream gas container 24 to evacuate the gas within the downstream gas container 24. During operation, a pair of ports on the sidewall of the coal-containing piston cylinder, namely third ports 17, connect the upstream gas container 18 and the downstream gas container 24, respectively. The upstream gas container 18 then connects to the downstream gas container 24. To facilitate pressure control in the coal-containing piston cylinder, a first valve A is installed in the connecting pipe between the upstream gas container 18 and the intermediate gas container 19; a second valve B is installed in the connecting pipe between the gas cylinder 20 and the intermediate gas container 19; and a third valve C is installed in the connecting pipe between the upstream gas container 18 and the downstream gas container 24. Both the upstream gas container 18 and the downstream gas container 24 are connected to a pressure sensor 23 to control the pressure in the coal-containing piston cylinder.

[0044] In addition, a pair of interfaces, namely second interfaces 13, are reserved on the side walls of the rock-bearing piston cylinder, and interfaces, namely first interfaces 4, are also reserved on the bases of the two piston cylinders. The first interfaces 4 and second interfaces 13 are not involved in the test method of the present invention as extended interfaces.

[0045] The following describes how the present invention uses the above-mentioned test device to measure the coal-rock interface stress during gas flow based on the accompanying drawings.

[0046] S1: Sample preparation and installation

[0047] First, the coal block and the rock block are processed into rectangular parallelepiped samples with equal cross-sections. The lengths of the coal sample 16 and the rock sample 5 are determined according to the ratio of the thickness of the coal seam to the thickness of the roof. The cross-sectional dimensions of the samples are slightly smaller than the inner cavity of the piston. In order to eliminate the gap between the sample and the inner cavity of the piston cylinder, the coal sample 16 and the rock sample 5 are wrapped with lead foil of a certain thickness. Then, the coal sample 16 and the rock sample 5 are installed in their respective piston cylinders. The piston heads 6 at both ends of the piston rod 15 are installed in the coal-containing piston cylinder and the rock-containing piston cylinder respectively to form a "coal-containing piston cylinder-piston rod-rock piston cylinder" combination (see Figure 2 ),simulation Figure 1 The sandwich structure of the coal-bearing strata shown in the figure has a strain gauge 7 attached to the middle of the piston rod 15 along the axial direction;

[0048] S2: Overall assembly and application of pre-tightening compressive stress

[0049] S2.1: Place the assembly on the reaction frame. Attach strain gauges 7 to the middle of the four columns 2 of the reaction frame. Connect the wires of all strain gauges 7 to the data acquisition device 8. Connect the gas pipeline system of the coal-bearing piston cylinder. Seal the second interface 13 reserved on the rock-bearing piston cylinder and the first interface 4 reserved on the two piston cylinder bases.

[0050] S2.2: Apply pressure to the assembly based on known in-situ stress data;

[0051] S3: Coal-Rock Interface Stress Test

[0052] S3.1: Gas Injection Adsorption Equilibrium

[0053] Open the first valve A and the fourth valve D, close the second valve B and the third valve C, connect the fourth valve D to the vacuum degassing device, and vacuum degassing the gas injection system for more than 6 hours. If the pressure of the vacuum gauge does not rise significantly, it can be judged that the air tightness of the system is good, then close the fourth valve D, open the second valve B and the third valve C, use the plunger pump 21 to control the gas pressure to maintain at P0, then inject gas into the coal-containing piston cylinder, until the pressure values of the upstream and downstream pressure sensors 23 maintain at P0, which is considered as the initial equilibrium pressure, close the third valve C, and wait for the strain of the piston rod 15 ε0 0 , ε0 1 , ε0 2 , ε0 3 , ε0 4 , ε0 Figure 3 ;

[0054] S3.2: Gradually reduce the gas pressure in the coal-containing piston cylinder, measure the corresponding column 2 strain and piston rod 15 micro-strain at each level of gas pressure, calculate the coal-rock interface stress σ, the test results are shown in Figure 7 , the specific process is as follows:

[0055] Close the third valve C and the fourth valve D, open the second valve B and the first valve A. Start from the initial equilibrium pressure P0, control the plunger pump 21 to gradually reduce the gas pressure P in the coal sample 16, about Δp at each level, record the gas pressure P in the coal-containing piston cylinder and the strain ε of the piston rod under each stress condition;

[0056] S3.3: When the strain of the piston rod 15 remains basically unchanged, it is considered that the piston rod 15 is in a mechanical equilibrium state, and the mechanical equilibrium equation is σA1+PA1=βA2=EεA2, and the coal-rock interface stress σ is calculated, where E is the elastic modulus of the piston rod 15, A1 is the cross-sectional area of the piston head 6, A2 is the cross-sectional area of the piston rod 15, and β is the cross-sectional stress of the piston rod 15, as shown in Figure 4 . During the process of gradually reducing the gas pressure in the coal-containing piston cylinder, the micro-strain of the counter-force frame column 2 and the piston rod 15 is shown in Figure 7 (a), and the real-time change of the coal-rock interface stress is shown in Figure 7 (b).

[0057] The following is a set of tests to verify the structural reliability of the test device provided by the present invention. The detailed method is: after the vacuum operation is completed, close all valves, first twist the top rod 1 of the reaction frame clockwise to apply stepped compressive stress to the assembly, and require that the strain value of the piston rod 15 remains unchanged under each level of load, and then apply the next level of pressure until the maximum load value. Then slowly twist the top rod 1 counterclockwise to unload the pressure in steps, wait for the strain value of the piston rod 15 corresponding to each level of load to stabilize, and then reduce the pressure to the next level until the strain of the piston rod 15 approaches zero. At this time, the inspection of the sample and the equipment is completed. From the step-by-step pressurization to the pressure relief process, the micro-strain and load of the reaction frame and the piston rod 15 can be seen in the whole process. Figure 5 Observe and record the strain ε0 of the four columns 2 from loading to unloading 1 、ε0 2 、ε0 3 、ε0 4 and the strain value ε0 of the piston rod 15 0 According to Hooke's law, the total tension of the four columns 2 is calculated as (ε0 1 +ε0 2 +ε0 3 +ε0 4 )×E1×A, the pressure of the piston rod 15 is ε0 0 ×E2×A1, see Figure 3 and Figure 4 , where E1 is the elastic modulus of the column 2, E2 is the elastic modulus of the piston rod 15, A is the cross-sectional area of ​​the column 2, and A1 is the cross-sectional area of ​​the piston cylinder. Figure 5 As shown in (a), during the step-by-step pressurization process, the micro-strain of the piston rod 15 decreases step by step. Figure 5 As shown in (b), the micro strain of the four columns 2 increases step by step, indicating that the piston rod 15 is in a compressed state and the four columns 2 are in a tensile state; during the step-by-step pressure reduction process, the micro strain of the piston rod 15 increases step by step, and the micro strain of the four columns 2 decreases step by step, indicating that the piston rod 15 changes from a compressed state to a tensile state. Figure 5 (c) It can be seen that the tensile load of the four columns 2 is substantially equal to the compressive load of the piston rod 15, which confirms that the forces between the columns and the piston rod of the test device of the present invention are balanced and the mechanical structure is reliable.

[0058] In order to verify the influence of the coal-rock interface stress on the coal permeability measured by the test method of the present invention, the following example is used to illustrate:

[0059] Since the coal sample 16 and the rock sample 5 are both placed in the piston cylinder, and the inner wall of the piston cylinder is in contact with the side of the coal sample 16 or the rock sample 5, the sample is only allowed to deform toward the piston rod 15, that is, it is in the boundary condition of uniaxial strain. [1,2] , according to the boundary condition of no deformation of the sample side wall, that is, the horizontal strain εex = 0, the horizontal effective stress Δσ ex and the vertical effective stress (i.e. the coal-rock interface stress σ measured by the present application) Δσ ez have the relationship shown in equation (1)

[0060]

[0061] where E is the elastic modulus of the sample, μ is the Poisson's ratio, the horizontal effective stress Δσ ex Equation (1) is rewritten as equation (2)

[0062]

[0063] Using the Reiss permeability model, the coal permeability is controlled by the horizontal effective stress, and the ratio (k / k0) of the coal permeability k to its initial value k0 is shown in equation (3)

[0064]

[0065] where C f is the compressibility of the coal, C f = 0.29 MPa -1 .

[0066] The present application reduces the gas pressure in the piston cylinder when measuring the coal-rock interface stress σ, thereby reducing the pore pressure in the assembly, and the coal-rock interface vertical stress Δσ ez decreases during the pore pressure reduction process. As shown in equation (2), the coal-rock interface vertical stress Δσ ez directly affects the horizontal effective stress Δσ ex of the coal seam. As shown in equation (3), the horizontal effective stress Δσ ex of the coal determines the permeability k. Figure 8 (a) is the coal pore pressure decay curve observed using the experimental device of the present application, Figure 8 (b) and 8(c) respectively draw the horizontal effective stress and permeability ratio of the coal sample with time, and it can be seen that, with the decrease of the pore pressure, the horizontal effective stress and the permeability ratio (k / k0) of the coal gradually increase with time. If the influence of the roof rock is not considered, the horizontal effective stress value and the permeability ratio (k / k0) of the coal sample are both greater than the parameter values considering the influence of the roof rock.

[0067] In order to compare the influence degree of the pore pressure change on the permeability, Figure 9 the permeability ratio (k / k0) with the pore pressure ratio (P / P0) change curves are drawn under the conditions of considering the influence of the roof rock and not considering the influence of the roof rock. From Figure 9It can be seen that, considering the influence of the roof rock, with the decrease of the pore pressure ratio, the coal permeability ratio increases; if the influence of the roof rock is not considered, the coal sample permeability ratio is greater than the permeability value considering the influence of the roof rock.

[0068] From Figure 8 With Figure 9 The middle curve can conclude that, with the continuous decrease of the pore pressure, the pressure of the coal seam roof rock on the coal seam decreases, leading to the coal permeability being less than the case without considering the influence of the roof rock. In other words, according to the current most assumptions that the pressure of the roof rock on the coal seam remains unchanged, the coal permeability value will be overestimated. However, by using the test method described in the present application, the coal-rock interface stress can be accurately measured, providing a key index for evaluating the change of the coal seam permeability.

[0069] The above only describes the preferred embodiments of the present application and is not used to limit the present application, and any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A method for testing coal-rock interface stress under the influence of gas pressure, characterized in that: The following steps are involved: In the first step, cores are taken from coal and rock blocks to form rectangular parallelepiped coal and rock samples with equal cross-sections. The height ratio of the coal and rock samples is consistent with the actual thickness ratio of the coal seam to the roof. The coal and rock samples are then wrapped in lead foil and placed in their respective piston cylinders, with the samples fitting closely to the inner wall of the piston cylinder. The second step is to assemble the rock-containing piston cylinder and the coal-containing piston cylinder with the two ends of a hollow piston rod to form a rock-containing piston cylinder-piston rod-coal-containing piston cylinder assembly, simulating the sandwich structure of the coal-bearing strata. The assembly is then installed on the reaction frame. The third step is to apply prestress to the assembly by twisting the reaction frame and limit the length of the assembly to remain unchanged. The coal-containing piston cylinder is injected with gas to balance the gas adsorption of the coal sample to simulate the original state of the coal seam. Then, the gas pressure in the coal-containing piston cylinder is reduced in steps to simulate the decompression and gas production process of the coal reservoir. The strain of the piston rod and the reaction frame column is measured throughout the process. The normal stress of the coal-rock interface is calculated based on the force balance relationship of the piston rod. It is required that the strain of the column and the piston rod can remain stable during each loading and unloading stage.

2. The method for testing coal-rock interface stress affected by gas pressure according to claim 1, wherein: The detailed process of step three is: Section 3.1: Based on the measured ground stress value, a reaction frame is used to apply prestress to the assembly. Then, methane is injected into the coal-containing piston cylinder. After the methane pressure in the coal-containing piston cylinder stabilizes, it is considered that the coal sample and the rock sample have reached an adsorption equilibrium state. At this time, the gas pressure in the coal sample is recorded as the initial equilibrium pressure P0. Step 3.2: Reduce the gas pressure in the coal sample in a stepwise manner until the gas pressure in the coal-containing piston cylinder reaches atmospheric pressure. The stepwise reduction means that the gas pressure amplitude is equal each time. The gas pressure P in the coal-containing piston cylinder and the piston rod strain ε are recorded throughout the whole process. Section 3.3: Based on the force balance relationship of the piston rod, the coal-rock interface stress σ is calculated using the formula σA1+PA1=βA2=EεA2, where: E is the elastic modulus of the piston rod, A1 is the cross-sectional area of ​​the piston head, A2 is the cross-sectional area of ​​the piston rod, and β is the cross-sectional stress of the piston rod.

3. A testing device for the method for testing coal-rock interface stress affected by gas pressure according to claim 1 or 2, characterized in that: The test device includes a coal-containing piston cylinder, a rock-containing piston cylinder, a hollow piston rod, a reaction frame, an air injection system, a data acquisition instrument, a pressure sensor and multiple resistance strain gauges; wherein: The two ends of the hollow piston rod can be assembled with the inner cavities of the coal-containing piston cylinder and the rock-containing piston cylinder to form a series-type combination of the coal-containing piston cylinder-piston rod-rock-containing piston cylinder; The reaction frame is used to apply load to the combination of coal-containing piston cylinder-piston rod-rock-containing piston cylinder; The gas injection system is as follows: gas first flows from the gas cylinder into the intermediate gas container, and then the plunger pump is used to adjust the gas pressure in the intermediate gas container. Then, the gas flows from the intermediate gas container into the upstream gas container, then into the coal-containing piston cylinder, and finally into the downstream gas container. The side wall of the coal-containing piston cylinder has a pair of interfaces, which are respectively connected to an upstream gas container and a downstream gas container. The upstream gas container is connected to the downstream gas container and a valve is provided. The gas pressure value in the coal-containing piston cylinder is measured by a pressure sensor. The multiple resistance strain gauges are respectively attached to the middle of the reaction frame column and the middle of the hollow piston rod, and all the resistance strain gauge wires are connected to the data acquisition instrument.

4. The testing device for the method of testing coal-rock interface stress affected by gas pressure as claimed in claim 3, characterized in that: The gas injection system includes a gas cylinder, an intermediate gas container, an upstream gas container and a downstream gas container, wherein a plunger pump controls the gas pressure in the intermediate gas container; valves are provided on the pipelines at the end of the downstream gas container, between the gas cylinder and the intermediate gas container, and between the intermediate gas container and the upstream gas container.

5. The testing device for the method of testing coal-rock interface stress affected by gas pressure as claimed in claim 3, characterized in that: The hollow piston rod has a pipeline along its axis that passes through the piston heads at both ends, so that the gas in the coal-containing piston cylinder can flow into the rock-containing piston cylinder through the piston rod.

6. The testing device for the method of testing coal-rock interface stress affected by gas pressure as claimed in claim 3, characterized in that: A pair of interfaces are reserved on the side wall of the rock-containing piston cylinder.

7. The testing device for the method of testing coal-rock interface stress affected by gas pressure as claimed in claim 3, characterized in that: Interfaces are reserved on the bases of the two piston cylinders.

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