Method and device for testing coal-rock interface stress influenced by gas pressure

Abstract

The invention discloses a method and a device for testing coal-rock interface stress influenced by gas pressure. The device involved in the method is composed of a counter-force frame, a hollow piston rod, two piston cylinders, a gas circuit and a plunger pump. A coal sample and a rock sample are placed in the two piston cylinders respectively and assembled with the two ends of the piston rod, the hollow piston rod is communicated with inner cavities of the two piston cylinders, and the boundary condition that displacement and pore pressure of the two piston cylinders are the same on a coal-rock interface is simulated. The counter-force frame can apply load to the coal-containing piston cylinder-piston rod-rock-containing piston cylinder assembly and restrain overall deformation of the coal-containing piston cylinder-piston rod-rock-containing piston cylinder assembly, and the constraint condition of an overlying rock layer on a coal seam is simulated. In the gas pressure change process, the strain values of the piston rod and the reaction frame can be measured, the pressure change of the end face of the piston rod can be calculated, the change of the normal stress of the coal-rock interface can be represented, and the technical blank of coal seam boundary stress measurement in the coal seam gas extraction or coal seam carbon dioxide sequestration process is filled up.

Description

Technical Field

[0001] The invention relates to the field of coal mine gas extraction and coal seam gas mining, and in particular is used to measure the change of normal stress on the coal-rock interface caused by coal seam gas flow. Background Art

[0002] Coalbed methane (methane) is a high-quality, clean, unconventional geological energy source. The development of coalbed (methane) gas resources is of great significance to ensure the sustainable utilization of my country's coal resources, safe production of coal mines, and the realization of the "dual carbon" goals. my country's coalbed methane industry has undergone nearly 40 years of research and development. Although the development technology has changed from borrowing foreign technology to independent innovation, my country's coalbed methane industry is still facing the current situation of "low exploration and development level and low single well production". In view of the geological conditions of coal reservoirs, historical production data and the degree of resource reserves recovery, it is urgent to have a deeper understanding of the coalbed methane seepage mechanism in order to develop more efficient coalbed methane development technology.

[0003] The coalbed methane production process is to first pump out the coalbed water, reduce the pore pressure of the coalbed, and promote the desorption of adsorbed methane in the coal pores into a free state. Then, under the drive of the pore pressure gradient, the free methane flows along the coal fracture network to the wellbore. The key technical indicator for measuring the gas flow capacity in the coal reservoir is the coal permeability. The greater the permeability, the easier it is for the gas to flow in the coal pores and fractures, and the more gas flows to the wellbore. Considering the influence of multiple factors, it is difficult to characterize the internal mechanism that controls the production of coalbed methane by calculating the coalbed permeability based on the drainage data of test wells or production wells. It is also necessary to study the change law of coal permeability under the influence of various factors in the laboratory. At present, most scholars and technicians in the industry design various experimental conditions to study the interaction of the three effects of effective stress, matrix shrinkage and slippage in coal reservoirs on the whole process of coal reservoir decompression-adsorbed methane desorption-diffusion-seepage-production, resulting in dynamic changes in the permeability of coal reservoirs.

[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 coal seam and its roof and floor form a sandwich structure, see Figure 1As shown. The roof applies vertical stress to the coal seam, and the floor supports the coal reservoir. Since the elastic modulus of coal is smaller than that of rock, when the pressure of the coal reservoir drops, there must be a difference between the deformation of the coal seam and the deformation of the roof and floor, which will change the lower support conditions of the coal seam roof or the upper support conditions of the floor, and ultimately change the interface stress between the coal seam and the rock formation. However, most current studies simplify the boundary stress of the coal reservoir to a constant, and this simplification causes a large error in predicting the permeability of the coal reservoir. To this end, the present invention proposes a device and a testing method for observing the change of coal-rock interface stress based on the structural characteristics of the coal-bearing strata, which fills the technical gap in the measurement of coal seam boundary stress during coalbed methane extraction or coal seam carbon dioxide storage, and can provide powerful measurement technology support for the accurate evaluation of coal seam gas production. Summary of the invention

[0005] The present invention provides a method and device for testing the interface stress of coal and rock under the influence of gas pressure. The method simulates the contact conditions between coal seams and rock strata, and can measure the changes in the interface stress between rock and coal caused by changes in coal pore pressure, thereby providing a measurement method for accurately evaluating the stress state of coal seams.

[0006] To solve the above problems, the technical solution adopted by the present invention is:

[0007] A method for testing coal-rock interface stress under the influence of gas pressure, characterized in that it comprises the following steps:

[0008] In the first step, the cores of the coal block and the rock block are respectively processed into rectangular coal samples and rock samples with equal cross-sections. 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 respectively wrapped with lead foil and loaded into their respective piston cylinders, and the samples are fitted with the inner wall of the piston cylinder;

[0009] 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 respectively to form a rock-containing piston cylinder-piston rod-coal-containing piston cylinder assembly to simulate the sandwich structure of the coal-bearing strata, and then install the assembly on the reaction frame;

[0010] The third step: The torsion reaction frame applies a certain prestress to the assembly and limits the length of the assembly to remain unchanged. Gas is injected into the coal-containing piston cylinder to balance the adsorbed gas in 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 depressurization and gas production process of the coal reservoir. The strains of the piston rod and the reaction frame column are 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 strains of the column and the piston rod can remain stable during each loading and unloading holding stage.

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

[0012] Section 3.1: According to the measured ground stress value, the reaction frame is used to apply prestress to the assembly, and then methane is injected into the coal-containing piston cylinder. After the methane pressure in the coal-containing piston cylinder is basically stable, 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;

[0013] Section 3.2: The gas pressure in the coal sample is reduced step by step until the gas pressure in the coal-containing piston cylinder is close to the atmospheric pressure. The step means that the gas pressure amplitude is equal each time, and the gas pressure P in the coal-containing piston cylinder and the piston rod strain ε are recorded during the whole process;

[0014] Section 3.3: According to 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.

[0015] In order to implement the above method, the present invention provides a test device, which 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 a plurality of resistance strain gauges; wherein:

[0016] 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-connected "coal-containing piston cylinder-piston rod-rock piston cylinder" combination;

[0017] The reaction frame can apply load to the combination 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, 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.

[0019] 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.

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

[0021] Furthermore, 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.

[0022] Furthermore, 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.

[0023] Furthermore, a pair of interfaces are reserved on the side wall of the rock-containing piston cylinder.

[0024] Furthermore, interfaces are also reserved on the bases of the two piston cylinders.

[0025] The advantages of the present invention are:

[0026] First, the boundary conditions of the coal-rock interface are reconstructed. In the process of coal seam depressurization and gas production, the coal seam is mainly subjected to vertical compression. The present invention loads the coal sample and the rock sample into a piston cylinder respectively. The coal sample and the rock sample 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 to form a combination of "coal-containing piston cylinder-piston rod-rock-containing piston cylinder", simulating the sandwich structure of the formation. During the change of gas pressure in the coal sample, the gas in the rock-containing piston cylinder also changes accordingly. 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. The characteristics that both can only deform in one direction at the coal-rock interface, and the displacement and pore pressure are the same are met.

[0027] Secondly, determine the boundary conditions of the coal-bearing strata. Since the permeability of the roof and floor rocks is low and the elastic modulus is greater than that of the coal seam, changes in the pore pressure of the coal seam can only cause elastic deformation of the roof in the area in contact with the coal seam. The compression or expansion deformation of the coal seam does not affect the overall size of the sandwich structure. The strata in the study area can be considered to be in a limited displacement boundary condition. The present invention uses a high-rigidity reaction frame to apply a load to the assembly and constrain the overall deformation of the assembly to simulate the constraints of the overlying rock strata on the coal seam. By measuring the strain values ​​of the piston rod and the reaction frame, the pressure change on the end face of the piston rod is calculated to characterize the change in the normal stress of the coal-rock interface, which fills the technical gap in the measurement of coal seam boundary stress during coalbed methane extraction or carbon dioxide storage in coal seams, and provides a research method for accurately evaluating the relationship between coal seam permeability and stress. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings required for use in the description of the embodiments will be briefly introduced below. The accompanying drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other accompanying drawings can be obtained based on these accompanying drawings without paying creative work.

[0029] Figure 1 This is a schematic diagram of the coal-bearing stratum structure;

[0030] Figure 2 It is the position relationship diagram of the piston rod and the coal sample and rock sample;

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

[0032] Figure 4 To analyze the piston rod end face force and cross-section force diagram using the cross-section method;

[0033] Figure 5 is the micro-strain and load change diagram of the piston rod and reaction frame during the step loading and unloading process, where Figure 5 (a) shows the micro strain of the four columns of the reaction frame during the step loading and unloading process. Figure 5 (b) shows the microstrain of the piston rod during step loading and unloading. Figure 5 (c) is the load curve of the piston rod and the reaction frame;

[0034] Figure 6 It is a schematic diagram of the structure of the testing device of the present invention;

[0035] Figure 7 The graphs are the stress change diagram of the coal-rock interface caused by the decrease of gas pressure in the coal-containing piston cylinder and the micro-strain change diagram of the column and piston rod. Figure 7 (a) represents the micro-strain changes of the column and piston rod during the depressurization process. Figure 7 (b) represents the stress change of coal-rock interface during the process of gas pressure reduction;

[0036] Figure 8 The test device of the present invention is used to test the horizontal effective stress-time and permeability-time change curve of coal caused by reducing pore pressure, wherein Figure 8 (a) shows the pore pressure decay curve of the coal sample, Figure 8 (b) shows the curve of horizontal effective stress of coal sample changing with time. Figure 8 (c) shows the curve of coal sample permeability ratio changing with time;

[0037] Fig. 9 This is a curve diagram of the horizontal effective stress and permeability change of coal caused by the reduction of pore pressure with and without considering the influence of 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 in conjunction with 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 definite definition of the protection scope of the present invention.

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

[0042] like Figure 6 As shown, the test device of the present invention comprises two piston cylinders, a piston rod 15, a reaction frame, an air injection system, a pressure sensor 23, a computer 22, a data acquisition instrument 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, and at the same time, the two piston heads 6 are ensured to be close to the inner wall of the piston cylinder to achieve sealing performance. A central channel 14 is provided in the piston rod 15 to achieve communication between 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 pasted on 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, clamp the rock-containing piston cylinder-piston rod-coal-containing piston cylinder combination, and apply 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 6It can be seen that 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, and the intermediate gas container 19 and a plunger pump 21. The plunger pump 21 can control the gas pressure in the intermediate gas container 19, and a fourth valve D is installed at the end of the downstream gas container 24 to evacuate the gas in the downstream gas container 24. When in use, a pair of interfaces on the side wall of the coal-containing piston cylinder, that is, the third interface 17, are respectively connected to the upstream gas container 18 and the downstream gas container 24, and the upstream gas container 18 is then connected to the downstream gas container 24. In order to facilitate the control of the pressure of the coal-containing piston cylinder, a first valve A is provided on the connecting pipeline between the upstream gas container 18 and the intermediate gas container 19, a second valve B is provided on the connecting pipeline between the gas cylinder 20 and the intermediate gas container 19, and a third valve C is provided on the connecting pipeline between the upstream gas container 18 and the downstream gas container 24, and the upstream gas container 18 and the downstream gas container 24 are both connected to a pressure sensor 23, so as to control the pressure of the coal-containing piston cylinder.

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

[0045] The following is an explanation of how the present invention utilizes 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 mounting

[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, and the cross-sectional size of the sample is 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 respectively wrapped with lead foil of a certain thickness, and 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 respectively installed in the coal-containing piston cylinder and the rock-containing piston cylinder to assemble into a combination of "coal-containing piston cylinder-piston rod-rock piston cylinder" (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-compression stress

[0049] S2.1: Place the assembly on the reaction frame, and also paste strain gauges 7 in the middle of the four columns 2 of the reaction frame. Connect all the strain gauges 7 wires to the data acquisition device 8, connect the gas pipeline system of the coal-containing piston cylinder, and block the second interface 13 reserved on the rock-containing 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 geostress 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 equipment, and vacuum degas the gas injection system for more than 6 hours. If the vacuum gauge pressure 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 remain at P0, and then inject gas into the coal-containing piston cylinder until the pressure value of the upstream and downstream pressure sensors 23 is maintained at P0, which is considered to be the initial equilibrium pressure, close the third valve C, and wait for the piston rod 15 to strain ε0 0 The strain values ​​of the four reaction frame columns ε0 1 , ε0 2 , ε0 3 , ε0 4 After the change, it can be considered that coal sample 16 and rock sample 5 have reached an adsorption equilibrium state, see Figure 3 ;

[0054] S3.2: Reduce the gas pressure in the coal-containing piston cylinder step by step, measure the strain of the column 2 and the microstrain of the piston rod 15 corresponding to each level of gas pressure, and 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, and open the second valve B and the first valve A. Starting from the initial equilibrium pressure P0, control the plunger pump 21 to gradually reduce the gas pressure P in the coal sample 16, with a pressure drop of about Δp at each level, and record the gas pressure P in the coal-containing piston cylinder and the piston rod strain ε under each stress condition;

[0056] S3.3: When the strain of the piston rod 15 remains basically unchanged, the piston rod 15 is considered to be in a state of mechanical equilibrium, and its mechanical equilibrium equation is σA1+PA1=βA2=EεA2. 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, such as Figure 4 As shown in the figure, during the process of stepwise reduction of the gas pressure in the coal-containing piston cylinder, the micro strains of the column 2 of the reaction frame and the piston rod 15 are shown in the figure. Figure 7 (a), the real-time change of 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 push 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 push rod 1 counterclockwise to relieve 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, as shown in 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 stretched 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 stretched 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 proves 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 measured by the test method of the present invention on the coal permeability, the following example is used to illustrate:

[0059] Since the coal sample 16 and the rock sample 5 are placed in the piston cylinder, and the inner wall of the piston cylinder fits 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 that the side wall of the sample has no deformation, that is, the horizontal strain εex = 0, we can get the horizontal effective stress Δσ ex and the vertical effective stress (i.e. the coal-rock interface stress σ measured by the present invention) Δσ ez The relationship is shown in formula (1):

[0060]

[0061] Where E is the elastic modulus of the sample, μ is Poisson’s ratio, and horizontal effective stress Δσ ex Formula (1) is rewritten as formula (2)

[0062]

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

[0064]

[0065] Among them C f is the compression coefficient of coal, C f Value: 0.29MPa -1 .

[0066] The present invention reduces the gas pressure in the piston cylinder when testing the coal-rock interface stress σ, thereby reducing the pore pressure in the assembly. During the process of pore pressure reduction, the vertical stress Δσ at the coal-rock interface ez From formula (2), we can see that the vertical stress Δσ at the coal-rock interface ez Directly affects the horizontal effective stress Δσ of the coal seam ex From formula (3), we can see that the horizontal effective stress Δσ of coal ex Determines the permeability k. Figure 8 (a) is the coal pore pressure decay curve observed by using the test device of the present invention, Figure 8 (b) and 8(c) respectively plot the curves of the ratio of horizontal effective stress to permeability of coal samples over time. It can be seen that as the pore pressure decreases, the ratio of horizontal effective stress to permeability of coal (k / k0) gradually increases with time. If the influence of roof rock is not considered, the horizontal effective stress value and permeability ratio (k / k0) of the coal sample are greater than the parameter value when the influence of roof rock is considered.

[0067] In order to compare the influence of pore pressure change on permeability, Fig. 9 The permeability ratio (k / k0) and pore pressure ratio (P / P0) curves are plotted with and without considering the influence of roof rock. Fig. 9It can be seen that, considering the influence of roof rock, as the pore pressure ratio decreases, the coal permeability ratio increases instead; if the influence of roof rock is not considered, the permeability ratio of the coal samples is greater than the permeability value when the influence of roof rock is considered.

[0068] from Figure 8 and Fig. 9 The middle curve can be used to conclude that as the pore pressure continues to decrease, the pressure of the coal seam roof rock on the coal seam also decreases, resulting in a lower coal permeability than when the influence of the roof rock is not considered. In other words, according to the current assumption that the pressure of the roof rock on the coal seam remains unchanged, the coal permeability value will be overestimated. The test method described in the present invention can accurately measure the coal-rock interface stress and provide a key indicator for evaluating the change in coal seam permeability.

[0069] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

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, the cores of the coal block and the rock block are respectively processed into rectangular coal samples and rock samples with equal cross-sections. 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 respectively wrapped with lead foil and loaded into their respective piston cylinders, and the samples are fitted with 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 respectively to form a rock-containing piston cylinder-piston rod-coal-containing piston cylinder assembly to simulate the sandwich structure of the coal-bearing strata, and then install the assembly on the reaction frame; The third step: the torsion reaction frame applies prestress to the assembly and limits the length of the assembly to remain unchanged. Gas is injected into the coal-containing piston cylinder to balance the adsorbed gas in 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 depressurization and gas production process of the coal reservoir. The strains of the piston rod and the reaction frame column are 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 strains of the column and the piston rod can remain stable during each loading and unloading holding stage.

2. The method for testing coal-rock interface stress under the influence of gas pressure as claimed in claim 1, characterized in that: The detailed process of step three is: Section 3.1: According to the measured ground stress value, the reaction frame is used to apply prestress to the assembly, and then methane is injected into the coal-containing piston cylinder. After the methane pressure in the coal-containing piston cylinder is stabilized, 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; Section 3.2: The gas pressure in the coal sample is reduced step by step until the gas pressure value in the coal-containing piston cylinder reaches the atmospheric pressure. The step means that the gas pressure amplitude is equal each time, and the gas pressure P in the coal-containing piston cylinder and the piston rod strain ε are recorded during the whole process; Section 3.3: According to 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 as claimed in 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 a plurality of 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 that the 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, and 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 for 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 of the method for testing coal-rock interface stress affected by gas pressure as claimed in claim 3, characterized in that: The hollow piston rod has a pipeline running through the piston heads at both ends along its axis, 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 of the method for 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 for 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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