Method for evaluating stress release modifiability of tectonic coal reservoir

Abstract

The invention discloses a tectonic coal reservoir stress release modifiability evaluation method, and belongs to the technical field of coal bed gas development. The method comprises the four steps of sample testing, evaluation index selection, evaluation index weight calculation and tectonic coal reservoir stress release modifiability evaluation. The method comprises the following steps: quantitatively evaluating the reformability of a tectonic coal reservoir in a stress release manner, designing a coal measure strata structure reconstruction and similar material simulation device to test a tectonic coal sample to obtain sample parameters, and comparing the sample under different strata environment occurrence conditions. According to the change conditions of the gas permeability and the gas desorption amount after the stress release of the tectonic coal reservoir, proper evaluation indexes are selected, and the influence weight of each evaluation index on the gas permeability and the gas desorption amount of a sample is calculated, so that the method for evaluating the stress release modifiability of the tectonic coal reservoir is established and is highly consistent with the actual engineering condition; the method is high in operability, can be quantified, and can truly and effectively evaluate the potential of exploiting the coal bed gas of the tectonic coal reservoir.

Description

Technical Field

[0001] The present invention relates to a method for evaluating the stress release reformability of a tectonic coal reservoir, and in particular to an evaluation method for a coal-bearing reservoir suitable for studying reservoir mechanical characteristics, tectonic coal seam gas seepage and desorption during stress release of a tectonic coal reservoir. Background Art

[0002] The extensive development of tectonic coal and the rich tectonic coal-bed methane resources are the significant characteristics of China's coal and coal-bed methane resources. The amount of tectonic coal-bed methane resources accounts for a large proportion of the total amount of coal-bed methane resources in my country. Tectonic coal has outstanding characteristics such as rich gas, low permeability, and softness. Most of them are coal and gas protruding coal seams. Due to the great harm and difficulty in extraction and utilization, coal mine production often discharges its wind into the atmosphere. Therefore, the energy, safety, and ecological significance of efficient development of tectonic coal-bed methane are very prominent.

[0003] The study of stress release in tectonic coal reservoirs is of great significance to the development of tectonic coalbed methane. Since the permeability of tectonic coal reservoirs is extremely low and the effects of transformation methods such as hydraulic fracturing are very poor, the theory of hydrophobic pressure reduction and desorption gas production is obviously not suitable for tectonic coal reservoirs. Exploration and development practice also shows that the coalbed methane exploration and development technology based on the theory of hydrophobic pressure reduction and desorption gas production cannot achieve efficient development of tectonic coalbed methane. Therefore, by unloading and increasing the permeability of tectonic coal reservoirs to release the stress of coal rocks and expand their volume, not only can the fluid pressure of the coal seam be significantly reduced, but also the permeability of the coal seam can be greatly improved, and a large amount of coalbed methane can be desorbed and seeped into the wellbore of the surface well, thereby realizing the efficient development of depressurized coalbed methane surface wells in coal mining areas.

[0004] At present, there are various types of coal reservoir transformation methods. For the hydraulic fracturing transformation of conventional coal reservoirs, the research on the transformability evaluation method using the length and height expansion of cracks during hydraulic fracturing as measurement indicators is relatively extensive. However, there are relatively few studies on stress release and permeability enhancement of tectonic coal reservoirs, resulting in a shallow development of tectonic coalbed methane. A quantifiable and highly operational stress release transformability evaluation method for tectonic coal reservoirs has not yet been proposed, which cannot effectively guide engineering practice. Summary of the invention

[0005] Technical problem: The purpose of the present invention is to address the deficiencies in the prior art and provide a method for evaluating the stress release transformability of tectonic coal reservoirs. By analyzing the influence of evaluation indicators on reservoir gas permeability and gas desorption amount, the transformability of tectonic coal reservoirs can be quantitatively evaluated, thereby improving the technical level of tectonic coalbed methane development.

[0006] Technical solution: To achieve the above-mentioned purpose, a method for evaluating the stress release transformability of a tectonic coal reservoir of the present invention is characterized by comprising the following steps:

[0007] S1. Screening samples: Collect samples of the types of coal, coal quality characteristics and their changing laws as well as the technological properties of coal in the mineable coal seams, screen the collected samples, and select structural coal samples of different coal rocks and coal qualities;

[0008] S2. Conduct gas adsorption-desorption test on tectonic coal: Use coal-bearing stratum structure reconstruction and similar material simulation device to simulate the occurrence conditions of tectonic coal samples in different reservoir environments, and conduct gas adsorption-desorption test on tectonic coal samples; the formation test temperature is set at three temperature points of 30°C, 35°C, and 40°C respectively. Under stress loading and unloading conditions, the tectonic coal gas adsorption-desorption test needs to be conducted three times at three different temperature points of 30°C, 35°C, and 40°C respectively;

[0009] S3. Conduct gas permeability test of tectonic coal: Use coal-bearing stratum structure reconstruction and similar material simulation device to simulate tectonic coal samples under different reservoir environment conditions, and conduct gas permeability test on tectonic coal samples; the different stratum environment conditions include: reservoir gas content, ground stress and ground temperature; the tectonic coal permeability experiment under stress loading and unloading conditions is set to three cycles of loading and unloading, and the temperature of each cycle of loading and unloading is set to 30°C, 35°C and 40°C respectively;

[0010] S4. Select evaluation index: According to the test results of tectonic coal samples, select the evaluation index that has a greater impact on gas permeability and gas desorption;

[0011] S5, calculating the weight of the evaluation index: performing weight calculation on each evaluation index screened in step S4;

[0012] S6. Establishing an evaluation method: Using regression analysis method, a regression model between each evaluation index and gas permeability and gas desorption amount is established, and then combined with the weight of each evaluation index in step S5, finally a stress release transformability evaluation model of tectonic coal reservoir is established.

[0013] In step S1, the tectonic coal samples of different coal rocks and coal qualities are crushed coal, crushed coal, and mylonitic coal, and the tectonic coal samples of different coal rock types are bright coal, semi-bright coal, semi-dark coal, and dark coal.

[0014] In step S2, the different reservoir environment conditions include: reservoir gas content, ground stress and ground temperature.

[0015] In step S2, the coal-bearing stratum structure reconstruction and similar material simulation device comprises a gas cylinder, a pressure reference cylinder, a stress loading and unloading device, a hot air blower, a vacuum pump, a gas flow meter and a computer that are sealed and connected in sequence; a pressure sensor is provided on the pipeline connecting the gas cylinder and the pressure reference cylinder, a pressure reducing valve is provided on the pipeline between the gas cylinder and the pressure sensor, a first and a second gas valve are provided on the pipeline connecting the pressure sensor and the pressure reference cylinder, a second pressure gauge and a third gas valve are also provided between the stress loading and unloading device and the first gas valve, a fourth gas valve is provided between the stress loading and unloading device and the vacuum pump, a third pressure gauge and a fifth gas valve are provided between the stress loading and unloading device and the gas flow meter, and a first pressure gauge and a thermometer are provided on the pressure reference cylinder respectively; all loading plates can individually control the loading pressure through a computer, so as to more realistically simulate the stress environment of the tectonic coal sample in the natural environment.

[0016] In step S2, the stress loading and unloading device is a true three-axis loading and unloading device, which includes three hydraulic cylinders and six loading plates in the shape of a cube in the front, back, left, right, top and bottom. The six loading plates are respectively connected to the three hydraulic cylinders, among which the first hydraulic cylinder is connected to the fifth loading plate and the sixth loading plate, the second hydraulic cylinder is connected to the first loading plate and the second loading plate, and the third hydraulic cylinder is connected to the third loading plate and the fourth loading plate.

[0017] In step S2, the gas adsorption-desorption test is performed on the tectonic coal sample:

[0018] Different types of structural coal are placed in the stress loading and unloading device according to the proportions to be studied, with the overall length, width and height of the sample being 4:3:3; a vacuum pump is used to evacuate to -0.1MPa and maintained for 30 minutes to ensure the sealing of the system and the discharge of gas in the original coal sample; non-adsorbed helium is injected into the reference cylinder, gas pipeline, and stress loading and unloading device pressure to calculate the void volume and pipeline free space of the structural coal sample;

[0019] After the pressure in the reference cylinder is stable, open the third gas valve between the reference cylinder and the stress loading and unloading device, and the coal sample begins to adsorb. The adsorption time is set to 12 to 15 hours. From 0 to 2 hours after the adsorption process, the pressure change in the reference cylinder should be less than 0.01MPa. After that, close the third gas valve and adjust the stress loading and unloading device to unload the pressure to simulate the reduction of effective stress. At the same time, record the pore gas pressure and strain change values ​​in the coal sample column.

[0020] When the pressure gauge pressure reading changes by less than 0.01MPa, the pressure is considered stable. The fifth outlet valve is opened, and the coal sample column begins to desorb. At this time, the real-time gas flow rate and cumulative flow rate in the gas flow meter at the tail end are recorded; the desorption time is set to 6-8 hours. When the temperature and atmospheric pressure changes are less than 0.1℃ and 0.01kPa respectively, the test data is reliable and data recording is performed. According to the in-situ formation conditions, the tectonic coal gas adsorption-desorption test under stress loading and unloading conditions needs to be tested three times at three different temperature points of 30℃, 35℃, and 40℃ respectively;

[0021] According to the experimental requirements, the adsorbed gas pressure of the reference cylinder is set to 2-4MPa, the effective stress of the tectonic coal sample is set to 2-3MPa, the pressure of the stress loading and unloading device starts from 6MPa, the loading / unloading gradient is 3MPa, and the maximum confining stress is 21MPa.

[0022] In step S3, the coal permeability test is conducted under stress loading and unloading conditions:

[0023] Based on the reconstruction of coal-bearing stratum structure and similar material simulation device, the tectonic coal permeability test under stress change conditions was carried out to analyze the influence of different coal body structures, coal rock types, gas content and temperature on the gas permeability change of tectonic coal reservoirs under stress release conditions;

[0024] Open the first, third and fifth gas valves between the gas cylinder and the stress loading and unloading device, close the second and fourth gas valves, open and adjust the pressure reducing valve, and wait until the gas pressure displayed by the pressure reducing valve reaches 2-6MPa, observe the reading of the gas flow meter, and wait until the reading of the gas flow meter fluctuates less than 0.1cm 3 / min, the gas flow reading is considered stable, and the computer is operated to pressurize the stress loading and unloading device;

[0025] The pressure of the stress loading and unloading device starts from 2MPa and increases to 17MPa at a maximum. Every 3MPa is a pressure gradient. After each loading action is completed, it is necessary to wait for the gas flow meter reading to stabilize, record the current gas flow meter data, and then start the next pressurization test;

[0026] When the pressure of the stress loading and unloading device is increased to 17MPa, after recording the current data, the pressure relief operation begins, with each 3MPa as a pressure gradient until the pressure is unloaded to 2MPa; after each pressure relief operation is completed, it is necessary to wait for the gas flow meter reading to stabilize, record the current gas flow meter data, and then start the next pressure relief test;

[0027] The pressure of the stress loading and unloading device is increased from 2MPa to 17MPa, and then unloaded from 17MPa to 2MPa, which is one cycle of loading and unloading. The permeability experiment of structural coal under stress loading and unloading conditions is set up with three cycles of loading and unloading, and the temperature of each cycle of loading and unloading is set to 30℃, 35℃, and 40℃ respectively.

[0028] In step S4, the evaluation indexes include five evaluation indexes: coal body structure, coal rock type, gas content, loading stress and temperature.

[0029] In step S5, the method used to calculate the evaluation index weight is the hierarchical analysis method:

[0030] Establish a pairwise comparison judgment matrix:

[0031]

[0032]

[0033] In the table: a, b, c, d, e, f, g, h, i, j are judgment matrix elements;

[0034] The weight vector is calculated using the square root method;

[0035] Based on the hierarchical judgment matrix A n×n Get the column vector M = (m1, m2, m3…, m n ),in:

[0036]

[0037] Normalized column vector M = (m1, m2, m3…, m n ), we get the eigenvector W = (w1,w2,w3…,w n );

[0038]

[0039] Calculate the maximum eigenvalue λ of the judgment matrix max :

[0040]

[0041] Calculate the consistency index CI and consistency ratio CR of the judgment matrix

[0042]

[0043] Where: A—hierarchical judgment matrix, a ij —Judgement matrix elements, m i —component of column vector, M—column vector, W—eigenvector, w i —weight value, λ max— Maximum eigenvalue, B i is the i-th row vector of the judgment matrix, CI is the consistency index, RI is the average consistency index, CR is the consistency ratio, and n is the dimension;

[0044] Then the consistency test of the judgment matrix is ​​performed. When the consistency ratio CR is less than 0.1, the relative importance judgment between the indicators is acceptable;

[0045] Gas-Physical Property System Coal rock type Coal body structure Reservoir gas content Ground stress Ground Temperature Weight value <![CDATA[w1]]> <![CDATA[w2]]> <![CDATA[w3]]> <![CDATA[w4]]> <![CDATA[w5]]>

[0046] In step S6, the regression analysis method is used to establish a regression model between each evaluation index and the gas permeability and gas desorption amount, and then combined with the weight of each evaluation index in step S5, finally a stress release reformability evaluation model of the tectonic coal reservoir is established:

[0047] First, data points including evaluation index, gas permeability and gas desorption are collected, and the data points are plotted in a rectangular coordinate system. The type of relationship between and is roughly determined based on the shape of the scatter plot. If the scatter plot is in the shape of a straight line, it is suitable for a linear model. If it is in the shape of a curve, such as a quadratic curve or an exponential curve, a nonlinear model needs to be considered.

[0048] Linear model fitting equation:

[0049] y=β0+β1x+∈

[0050] Polynomial model fitting equation:

[0051] y=β0+β1x+β2x 2 +∈

[0052] Exponential model fitting equation:

[0053] y=αe βx +∈

[0054] In the formula: y is the fitting equation, β0, β1, β2 are the equation coefficients, ∈ is a constant;

[0055] The least squares method is used to minimize the sum of squared differences and obtain the coefficients of each equation; finally, the model is evaluated and the determination coefficient R is used. 2 , mean square error MSE to evaluate the degree of model fit; determination coefficient R 2 The closer it is to 1, the better the model fits, the smaller the MSE, and the higher the model prediction accuracy;

[0056] The equations fitted by the above five evaluation indicators are multiplied by their respective weight values ​​and then added together to obtain the final evaluation model.

[0057] Beneficial effects: Due to the adoption of the above technical scheme, the present invention studies the influence of different factors on reservoir gas permeability and gas desorption in tectonic coal reservoirs through simulation experiments, selects factors with greater influence on reservoir gas permeability and gas desorption as transformability evaluation indicators, uses hierarchical analysis method to determine the weights of each evaluation indicator, and combines regression analysis method to establish a stress release transformability evaluation method for tectonic coal reservoirs, and comprehensively evaluates the transformability of tectonic coal reservoirs. It makes up for the gap that the existing coal reservoir transformability evaluation method is not applicable to tectonic coal reservoirs. The method is flexible and highly operable, and the evaluation indicators are adjusted according to the reservoir characteristics of different research areas. The evaluation method is highly consistent with the actual engineering situation, has strong operability, can be quantified, and can truly and effectively evaluate the potential of tectonic coal reservoirs to develop coalbed methane. By analyzing the influence of evaluation indicators on reservoir gas permeability and gas desorption, the transformability of tectonic coal reservoirs is quantitatively evaluated, thereby improving the technical level of efficient development of tectonic coalbed methane and safe coal mining, which has good engineering application prospects; it is of great significance to improve the technical level of efficient development of tectonic coalbed methane and safe coal mining in my country. BRIEF DESCRIPTION OF THE DRAWINGS

[0058] Figure 1 This is the technical roadmap for this application.

[0059] Figure 2 It is a schematic diagram of the structure of the coal-bearing stratum structure reconstruction and similar material simulation test device of the present invention.

[0060] In the figure: 1-gas cylinder; 2-reference cylinder; 3-stress loading and unloading device; 3-1-first hydraulic cylinder; 3-2-second hydraulic cylinder; 3-3-third hydraulic cylinder; 3-1-first hydraulic cylinder; 3-4-first loading plate; 3-5-second loading plate; 3-6-third loading plate; 3-7 fourth loading plate; 3-8 fifth loading plate; 3-9 sixth loading plate; 4-vacuum pump; 5-pressure reducing valve; 6-pressure sensor; 7-1-first gas valve; 7-2-second gas valve; 7-3-third gas valve; 7-4-fourth gas valve; 7-5-fifth gas valve; 8-1-first pressure gauge; 8-2-second pressure gauge; 8-3-third pressure gauge; 9-thermometer; 10-gas flow meter; 11-hot air blower; 12-computer. DETAILED DESCRIPTION

[0061] The present invention will be further described below with reference to the embodiments in the accompanying drawings:

[0062] like Figure 1 As shown, a method for evaluating the stress release transformability of a tectonic coal reservoir of the present invention comprises the following specific steps:

[0063] S1. Screening samples: Collect samples of coal types, coal quality characteristics and their changing patterns as well as coal processing properties of mineable coal seams, and screen the collected samples to select tectonic coal samples of different coal rocks and coal qualities; the tectonic coal samples of different coal body structures are crushed coal, crushed coal, and mylonitic coal; the tectonic coal samples of different coal rock types are bright coal, semi-bright coal, semi-dark coal, and dark coal.

[0064] S2. Conduct gas adsorption-desorption test of tectonic coal: simulate the occurrence conditions of samples in different stratum environments, and use coal-bearing stratum structure reconstruction and similar material simulation devices to conduct stress release tests on tectonic coal samples; put different types of tectonic coal into the pressure chamber of the stress loading and unloading device 3 according to the proportions to be studied, and the overall length, width and height of the sample should be 100mm*75mm*75mm; use a vacuum pump 4 to evacuate the entire equipment to -0.1MPa and maintain it for 30 minutes to ensure the sealing of the system and the discharge of gas in the original coal sample; use non-adsorbed helium to inject into the reference cylinder 2, gas pipeline, and pressure chamber respectively to calculate the void volume of the coal column and the free space of the pipeline; use nitrogen instead of methane to conduct adsorption and analysis tests, because nitrogen has similar adsorption-desorption characteristics as methane in coal, and its chemical properties are more stable;

[0065] After the pressure in the reference cylinder 2 is stabilized, the third gas valve 7-3 between the reference cylinder 2 and the stress unloading device 3 is opened, and the coal sample begins to adsorb. The adsorption time is set to be more than 15 hours. Within 2 hours after the adsorption process, the pressure change in the reference cylinder 2 should be less than 0.01MPa. After that, the third gas valve 7-3 of the stress unloading device 3 is closed and the device is adjusted to unload pressure to simulate the effective stress change, and the pore gas pressure and strain changes in the coal column are recorded at the same time;

[0066] Once the pressure is stable, the fifth gas valve 7-5 is opened, the coal column begins to desorb, and the real-time gas flow rate and cumulative flow rate in the tail gas flow meter 10 are recorded. The desorption time is set to 7 hours. When the changes in temperature and atmospheric pressure are less than 0.1°C and 0.01kPa respectively, the test data are considered reliable. According to the in-situ formation conditions, the test temperatures are set to 30°C, 35°C, and 40°C;

[0067] The adsorbed gas pressure of reference cylinder 2 is 4 MPa, and the minimum effective stress is set to 2 MPa. Therefore, the minimum pressure starts loading from 6 MPa, the loading / unloading gradient is 3 MPa, and the maximum confining pressure stress is 21 MPa.

[0068] A coal-bearing stratum structure reconstruction and similar material simulation device was used to simulate the occurrence conditions of tectonic coal samples in different reservoir environments, and a gas adsorption-desorption test was carried out on the tectonic coal samples; the formation test temperature was set to three temperature points of 30°C, 35°C, and 40°C, respectively; the tectonic coal gas adsorption-desorption test under stress loading and unloading conditions required three tests at three different temperature points of 30°C, 35°C, and 40°C, respectively; the different reservoir environment occurrence conditions included: reservoir gas content, ground stress, and ground temperature.

[0069] The device for reconstructing the structure of coal-bearing strata and simulating similar materials comprises a gas cylinder 1, a pressure reference cylinder 2, a stress loading and unloading device 3, a hot air blower 11, a vacuum pump 4, a gas flow meter 10 and a computer 12 which are sealed and connected in sequence; a pressure sensor 6 is provided on the pipeline connecting the gas cylinder 1 and the pressure reference cylinder 2, a pressure reducing valve 5 is provided on the pipeline between the gas cylinder 1 and the pressure sensor 6, a first and a second gas valve 7-1 and 7-2 are provided on the pipeline connecting the pressure sensor 6 and the pressure reference cylinder 2, a second pressure gauge 8-2 and a third gas valve 7-3 are also provided between the stress loading and unloading device 3 and the first gas valve 7-1, a fourth gas valve 7-4 is provided between the stress loading and unloading device 3 and the vacuum pump 4, a third pressure gauge 8-3 and a fifth gas valve 7-5 are provided between the stress loading and unloading device 3 and the gas flow meter 10, and a first pressure gauge 8-1 and a thermometer 9 are provided on the pressure reference cylinder 2 respectively; all loading plates can individually control the loading pressure through the computer 12, so as to more realistically simulate the stress environment of the tectonic coal sample in the natural environment.

[0070] The stress loading and unloading device 3 is a true three-axis loading and unloading device, which includes three hydraulic cylinders and six loading plates in the shape of a cube, front, back, left, right, top and bottom. The six loading plates are connected to the three hydraulic cylinders respectively, among which the first hydraulic cylinder 3-1 is connected to the fifth loading plate 3-8 and the sixth loading plate 3-9, the second hydraulic cylinder 3-2 is connected to the first loading plate 3-4 and the second loading plate 3-5, and the third hydraulic cylinder 3-3 is connected to the third loading plate 3-6 and the fourth loading plate 3-7.

[0071] The gas adsorption-desorption test on the tectonic coal sample is as follows:

[0072] Different types of structural coal are placed in the stress loading and unloading device 3 according to the proportion to be studied, and the overall length, width and height of the sample are 4:3:3; for example, the overall length, width and height of the sample are 100mm*75mm*75mm; vacuum pump 4 is used to evacuate to -0.1MPa and maintain for 30 minutes to ensure the sealing of the system and the discharge of gas in the original coal sample; non-adsorbed helium is used to be injected into the reference cylinder 2, the gas pipeline, and the pressure chamber of the stress loading and unloading device 3 respectively, and the void volume and pipeline free space of the structural coal sample are calculated; nitrogen is used instead of methane for adsorption and analysis tests, because nitrogen has similar adsorption-desorption characteristics as methane in coal, and its chemical properties are more stable;

[0073] After the air pressure in the reference cylinder 2 is stabilized, the third gas valve 7-3 between the reference cylinder 2 and the stress loading and unloading device 3 is opened, and the coal sample begins to adsorb. The adsorption time is set to 12 to 15 hours. From 0 to 2 hours after the end of the adsorption process, the air pressure change in the reference cylinder 2 should be less than 0.01MPa. After that, the third gas valve 7-3 is closed and the stress loading and unloading device 3 is adjusted to unload the pressure to simulate the reduction of effective stress, and the pore gas pressure and strain change values ​​in the coal sample column are recorded at the same time.

[0074] When the pressure reading of the pressure gauge 8-3 changes by less than 0.01MPa, the pressure is considered stable, and the fifth gas valve 7-5 is opened, and the coal sample column begins to desorb. At this time, the real-time flow rate and cumulative flow of the gas in the gas flow meter 10 at the tail end are recorded; the desorption time is set to 6-8 hours. When the changes in temperature and atmospheric pressure are less than 0.1°C and 0.01kPa respectively, the test data is reliable and data recording is performed. According to the in-situ formation conditions, the tectonic coal gas adsorption-desorption test under stress loading and unloading conditions needs to be conducted three times at three different temperature points of 30°C, 35°C, and 40°C respectively;

[0075] According to the experimental requirements, the adsorbed gas pressure of the reference cylinder 2 is set to 2-4MPa, the effective stress of the tectonic coal sample is set to 2-3MPa, the pressure of the stress loading and unloading device 3 starts loading from 6MPa, the loading / unloading gradient is 3MPa, and the maximum confining stress is 21MPa.

[0076] S3. Gas permeability test of tectonic coal under stress loading and unloading conditions:

[0077] Open the first and second gas valves 7-1 and 7-2 between the gas cylinder 1 and the reference cylinder 2. When the gas pressure in the reference cylinder 2 reaches the required value, close the first gas valve 7-1 between it and the gas cylinder, and open the third gas valve 7-3 between it and the stress unloading device;

[0078] The airflow changes dynamically in the early stage, and then it will reach stability within 10 minutes. Once the flow changes stabilize, the coal permeability is tested immediately, and then the test conditions are adjusted to the next target value, and then the permeability test is carried out;

[0079] The test confining pressure starts from a minimum of 2MPa and increases to a maximum of 17MPa, with a pressure gradient of 3MPa. The test temperatures are set at 30℃, 35℃, and 40℃.

[0080] A coal-bearing stratum structure reconstruction and similar material simulation device was used to simulate the occurrence conditions of tectonic coal samples in different reservoir environments, and a gas permeability test was conducted on the tectonic coal samples; the different stratum environment occurrence conditions include: reservoir gas content, ground stress and ground temperature; the tectonic coal permeability experiment under stress loading and unloading conditions was set to three cycles of loading and unloading, and the temperature of each cycle of loading and unloading was set to 30°C, 35°C, and 40°C respectively; the tectonic coal permeability experiment under stress loading and unloading conditions:

[0081] Based on the reconstruction of coal-bearing stratum structure and similar material simulation device, the tectonic coal permeability test under stress change conditions was carried out to analyze the influence of different coal body structures, coal rock types, gas content and temperature on the gas permeability change of tectonic coal reservoirs under stress release conditions;

[0082] Open the first, third and fifth gas valves 7-1, 7-3 and 7-5 between the gas cylinder 1 and the stress unloading device 3, close the second and fourth gas valves 7-2 and 7-4, open and adjust the pressure reducing valve 5, and after the gas pressure displayed by the pressure reducing valve 5 reaches 2-6MPa, observe the reading of the gas flow meter 10 and wait until the reading of the gas flow meter 10 fluctuates less than 0.1cm 3 / min, the gas flow reading is considered stable, and the computer 12 starts to pressurize the stress loading and unloading device 3;

[0083] The pressure of the stress loading and unloading device 3 starts from 2MPa and increases to 17MPa at a maximum. Every 3MPa is a pressure gradient. After each loading action is completed, it is necessary to wait for the reading of the gas flow meter 10 to stabilize, record the current data of the gas flow meter 10, and then start the next step of pressure test;

[0084] When the pressure of the stress adding and unloading device 3 is increased to 17MPa, after recording the current data, the pressure relief operation begins, with each 3MPa as a pressure gradient until the pressure is unloaded to 2MPa; after each pressure relief operation is completed, it is necessary to wait for the reading of the gas flow meter 10 to stabilize, record the current data of the gas flow meter 10, and then start the next pressure relief test;

[0085] The pressure of the stress loading and unloading device 3 is increased from 2MPa to 17MPa, and then unloaded from 17MPa to 2MPa, which is one cycle of loading and unloading; the structural coal permeability experiment under stress loading and unloading conditions is set up with three cycles of loading and unloading, and the temperature of each cycle of loading and unloading is set to 30℃, 35℃, and 40℃ respectively.

[0086] S4. Select evaluation indicators: According to the test results of tectonic coal samples, select evaluation indicators that have a greater impact on gas permeability and gas desorption; the evaluation indicators include five evaluation indicators: coal body structure, coal rock type, gas content, loading stress and temperature.

[0087] S5, calculating the weight of the evaluation index: performing weight calculation on each evaluation index screened in step S4; the method used for calculating the weight of the evaluation index is the analytic hierarchy process, and the calculation process of the method is mainly introduced here, and other details are not described in detail;

[0088] Establish a pairwise comparison judgment matrix:

[0089]

[0090]

[0091] In the table: a, b, c, d, e, f, g, h, i, j are judgment matrix elements;

[0092] The weight vector is calculated using the square root method;

[0093] Based on the hierarchical judgment matrix A n×n Get the column vector M = (m1, m2, m3…, m n ),in:

[0094]

[0095] Normalized column vector M = (m1, m2, m3…, m n ), and obtain the eigenvector W = (w1, w2, w3…, w n );

[0096]

[0097] Calculate the maximum eigenvalue λ of the judgment matrix max :

[0098]

[0099] Calculate the consistency index CI and consistency ratio CR of the judgment matrix

[0100]

[0101] Where: A—hierarchical judgment matrix, a ij —Judgement matrix elements, m i —component of column vector, M—column vector, W—eigenvector, w i —weight value, λ max — Maximum eigenvalue, B i is the i-th row vector of the judgment matrix, CI is the consistency index, RI is the average consistency index, CR is the consistency ratio, and n is the dimension;

[0102] Then the consistency test of the judgment matrix is ​​performed. When the consistency ratio CR is less than 0.1, the relative importance judgment between the indicators is acceptable;

[0103] Gas-Physical Property System Coal rock type Coal body structure Reservoir gas content Ground stress Ground Temperature Weight value <![CDATA[w1]]> <![CDATA[w2]]> <![CDATA[w3]]> <![CDATA[w4]]> <![CDATA[w5]]>

[0104] S6. Establishing an evaluation method: Establishing a stress release and transformability evaluation model for tectonic coal reservoirs according to the weight calculation results of each evaluation index in step S5. The establishment of the stress release and transformability evaluation model for tectonic coal reservoirs is: using a regression analysis method to establish a regression model between each evaluation index and gas permeability and gas desorption amount, and then combining the weights of each evaluation index in step S5, finally establishing a stress release and transformability evaluation model for tectonic coal reservoirs:

[0105] First, collect data points including evaluation index, gas permeability and gas desorption amount, plot the data points in a plane rectangular coordinate system, and roughly judge the relationship type between and according to the shape of the scatter plot; if the scatter plot presents a straight line shape, it is suitable for a linear model; if it is a curve shape, such as a quadratic curve (parabola, hyperbola) or an exponential curve, a nonlinear model needs to be considered;

[0106] Linear model fitting equation:

[0107] y=β0+β1x+∈

[0108] Polynomial model fitting equation:

[0109] y=β0+β1x+β2x 2 +∈

[0110] Exponential model fitting equation:

[0111] y=αe βx +∈

[0112] In the formula: y is the fitting equation, β0, β1, β2 are the equation coefficients, ∈ is a constant;

[0113] The least squares method is used to minimize the sum of squared differences and obtain the coefficients of each equation; finally, the model is evaluated and the determination coefficient R is used. 2 , mean square error MSE to evaluate the degree of model fit; determination coefficient R2 The closer it is to 1, the better the model fits, the smaller the MSE, and the higher the model prediction accuracy;

[0114] The equations fitted by the above five evaluation indicators are multiplied by their respective weight values ​​and then added together to obtain the final evaluation model.

[0115] The transformability of tectonic coal reservoirs is quantitatively evaluated by stress release. A coal-bearing stratum structure reconstruction and similar material simulation device are designed to test tectonic coal samples to obtain sample parameters. Based on the changes in gas permeability and gas desorption of tectonic coal reservoirs after stress release under different formation environmental conditions, appropriate evaluation indicators are selected, and the influence weights of each evaluation indicator on the gas permeability and gas desorption of the samples are calculated, thereby establishing an evaluation system for the transformability of tectonic coal reservoirs by stress release.

Claims

1. A method for evaluating the stress release transformability of a tectonic coal reservoir, characterized in that: The following steps are involved: S1. Screening samples: Collect samples of the types of coal, coal quality characteristics and their changing laws as well as the technological properties of coal in the mineable coal seams, screen the collected samples, and select structural coal samples of different coal rocks and coal qualities; S2. Conduct gas adsorption-desorption test on tectonic coal: Use coal-bearing stratum structure reconstruction and similar material simulation device to simulate the occurrence conditions of tectonic coal samples in different reservoir environments, and conduct gas adsorption-desorption test on tectonic coal samples; the formation test temperature is set at three temperature points of 30°C, 35°C, and 40°C respectively. Under stress loading and unloading conditions, the tectonic coal gas adsorption-desorption test needs to be conducted three times at three different temperature points of 30°C, 35°C, and 40°C respectively; S3. Conduct gas permeability test of tectonic coal: Use coal-bearing stratum structure reconstruction and similar material simulation device to simulate the occurrence conditions of tectonic coal samples in different reservoir environments, and conduct gas permeability test on tectonic coal samples; The different formation environment conditions include: reservoir gas content, ground stress and ground temperature; the tectonic coal permeability experiment under stress loading and unloading conditions is set to three cycles of loading and unloading, and the temperature of each cycle of loading and unloading is set to 30°C, 35°C and 40°C respectively; S4. Select evaluation index: According to the test results of tectonic coal samples, select the evaluation index that has a greater impact on gas permeability and gas desorption; S5, calculating the weight of the evaluation index: performing weight calculation on each evaluation index screened in step S4; S6. Establishing an evaluation method: Using regression analysis method, a regression model between each evaluation index and gas permeability and gas desorption amount is established, and then combined with the weight of each evaluation index in step S5, finally a stress release transformability evaluation model of tectonic coal reservoir is established.

2. A method for evaluating the stress release transformability of a tectonic coal reservoir according to claim 1, characterized in that: In step S1, the tectonic coal samples of different coal rocks and coal qualities are crushed coal, crushed coal, and mylonitic coal, and the tectonic coal samples of different coal rock types are bright coal, semi-bright coal, semi-dark coal, and dark coal.

3. A method for evaluating the stress release transformability of a tectonic coal reservoir according to claim 1, characterized in that: In step S2, the different reservoir environment conditions include: reservoir gas content, ground stress and ground temperature.

4. A method for evaluating the stress release transformability of a tectonic coal reservoir according to claim 1, characterized in that: In step S2, the device for reconstructing the structure of coal-bearing strata and simulating similar materials comprises a gas cylinder (1), a pressure reference cylinder (2), a stress loading and unloading device (3), a hot air blower (11), a vacuum pump (4), a gas flow meter (10) and a computer (12) which are sealed and connected in sequence; a pressure sensor (6) is provided on the pipeline connecting the gas cylinder (1) and the pressure reference cylinder (2); a pressure reducing valve (5) is provided on the pipeline between the gas cylinder (1) and the pressure sensor (6); a first and a second gas valve (7-1, 7-2) are provided on the pipeline connecting the pressure sensor (6) and the pressure reference cylinder (2); and the stress loading and unloading device (3) is provided on the pipeline connecting the gas cylinder (1) and the pressure reference cylinder (2). A second pressure gauge (8-2) and a third gas valve (7-3) are also provided between the device (3) and the first gas valve (7-1), a fourth gas valve (7-4) is provided between the stress loading and unloading device (3) and the vacuum pump (4), a third pressure gauge (8-3) and a fifth gas valve (7-5) are provided between the stress loading and unloading device (3) and the gas flow meter (10), and a first pressure gauge (8-1) and a thermometer (9) are provided on the pressure reference cylinder (2); all loading plates can individually control the loading pressure through a computer (12), so as to more realistically simulate the stress environment of the tectonic coal sample in the natural environment.

5. A method for evaluating the stress release reformability of a tectonic coal reservoir according to claim 4, characterized in that: The stress loading and unloading device (3) is a true three-axis loading and unloading device, which comprises three hydraulic cylinders and six loading plates in the shape of a cube, which are arranged in the front, back, left, right, top and bottom directions. The six loading plates are respectively connected to the three hydraulic cylinders, wherein the first hydraulic cylinder (3-1) is connected to the fifth loading plate (3-8) and the sixth loading plate (3-9), the second hydraulic cylinder (3-2) is connected to the first loading plate (3-4) and the second loading plate (3-5), and the third hydraulic cylinder (3-3) is connected to the third loading plate (3-6) and the fourth loading plate (3-7).

6. A method for evaluating the stress release reformability of a tectonic coal reservoir according to claim 1 or 4, characterized in that: In step S2, the gas adsorption-desorption test is performed on the tectonic coal sample: Different types of structural coal are placed in a stress loading and unloading device (3) according to the proportions to be studied, with the overall length, width and height of the sample being 4:3:3; a vacuum pump (4) is used to evacuate the sample to -0.1 MPa and maintain the vacuum for 30 minutes to ensure the sealing of the system and the discharge of gas from the original coal sample; non-adsorbable helium is injected into the reference cylinder (2), the gas pipeline and the stress loading and unloading device (3) respectively to calculate the void volume and pipeline free space of the structural coal sample; After the air pressure in the reference cylinder (2) is stabilized, the third gas valve (7-3) between the reference cylinder (2) and the stress loading and unloading device (3) is opened, and the coal sample begins to be adsorbed. The adsorption time is set to 12 to 15 hours. 0 to 2 hours after the end of the adsorption process, the air pressure change in the reference cylinder (2) should be less than 0.01MPa; then, the third gas valve (7-3) is closed and the stress loading and unloading device (3) is adjusted to unload the pressure to simulate the reduction of effective stress, and the pore gas pressure and strain change values ​​in the coal sample column are recorded at the same time; When the pressure reading of the third pressure gauge (8-3) changes by less than 0.01 MPa, the pressure is considered stable, and the fifth gas valve (7-5) is opened, and the coal sample column begins to desorb. At this time, the real-time flow rate and cumulative flow of the gas in the gas flow meter (10) at the tail end are recorded; the desorption time is set to 6-8 hours, and when the changes in temperature and atmospheric pressure are less than 0.1°C and 0.01 kPa respectively, the test data is reliable and data recording is performed. According to the in-situ formation conditions, the tectonic coal gas adsorption-desorption test under stress loading and unloading conditions needs to be conducted three times at three different temperature points of 30°C, 35°C, and 40°C respectively; According to the experimental requirements, the adsorption gas pressure of the reference cylinder (2) is set to 2-4MPa, the effective stress of the tectonic coal sample is set to 2-3MPa, the pressure of the stress loading and unloading device (3) starts loading from a minimum of 6MPa, the loading / unloading gradient is 3MPa, and the maximum confining stress is 21MPa.

7. A method for evaluating the stress release reformability of a tectonic coal reservoir according to claim 1 or 4, characterized in that: In step S3, the coal permeability test is conducted under stress loading and unloading conditions: Based on the reconstruction of coal-bearing stratum structure and similar material simulation device, the tectonic coal permeability test under stress change conditions was carried out to analyze the influence of different coal body structures, coal rock types, gas content and temperature on the gas permeability change of tectonic coal reservoirs under stress release conditions; Open the first, third and fifth gas valves (7-1, 7-3 and 7-5) between the gas cylinder (1) and the stress loading and unloading device (3), close the second and fourth gas valves (7-2 and 7-4), open and adjust the pressure reducing valve (5), wait for the gas pressure displayed by the pressure reducing valve (5) to reach 2-6 MPa, observe the reading of the gas flow meter (10), and wait for the reading of the gas flow meter (10) to fluctuate less than 0.1 cm 3 / min, the gas flow reading is considered stable, and the computer (12) starts to operate to pressurize the stress loading and unloading device (3); The pressure of the stress loading and unloading device (3) starts from a minimum of 2MPa and increases to a maximum of 17MPa, with each 3MPa being a pressure gradient. After each loading action is completed, it is necessary to wait for the gas flow meter (10) reading to stabilize, record the current gas flow meter (10) data, and then start the next step of pressure test; When the pressure of the stress adding and unloading device (3) is increased to 17 MPa, after recording the current data, the pressure relief operation is started, with each 3 MPa as a pressure gradient until the pressure is unloaded to 2 MPa; after each pressure relief operation is completed, it is necessary to wait for the reading of the gas flow meter (10) to stabilize, record the current data of the gas flow meter (10), and then start the next pressure relief test; The pressure of the stress loading and unloading device (3) is increased from 2MPa to 17MPa, and then unloaded from 17MPa to 2MPa, which is one cycle of loading and unloading. The permeability experiment of structural coal under stress loading and unloading conditions is set to three cycles of loading and unloading, and the temperature of each cycle of loading and unloading is set to 30°C, 35°C, and 40°C respectively.

8. A method for evaluating the stress release transformability of a tectonic coal reservoir according to claim 1, characterized in that: In step S4, the evaluation indexes include five evaluation indexes: coal body structure, coal rock type, gas content, loading stress and temperature.

9. A method for evaluating the stress release transformability of a tectonic coal reservoir according to claim 1, characterized in that: In step S5, the method used to calculate the evaluation index weight is the hierarchical analysis method: Establish a pairwise comparison judgment matrix: In the table: a, b, c, d, e, f, g, h, i, j are judgment matrix elements; The weight vector is calculated using the square root method; Based on the hierarchical judgment matrix A n×n Get the column vector M = (m1, m2, m3…, m n ),in: Normalized column vector M = (m1, m2, m3…, m n ), and obtain the eigenvector W = (w1, w2, w3…, w n ); Calculate the maximum eigenvalue λ of the judgment matrix max : Calculate the consistency index (CI) and consistency ratio (CR) of the judgment matrix Where: A—hierarchical judgment matrix, a ij —Judgement matrix elements, m i —component of column vector, M—column vector, W—eigenvector, w i —weight value, λ max — Maximum eigenvalue, B i is the i-th row vector of the judgment matrix, CI is the consistency index, RI is the average consistency index, CR is the consistency ratio, and n is the dimension; Then the consistency test of the judgment matrix is ​​performed. When the consistency ratio CR is less than 0.1, the relative importance judgment between the indicators is acceptable; 10. A method for evaluating the stress release reformability of a tectonic coal reservoir according to claim 1, characterized in that: In step S6, the regression analysis method is used to establish a regression model between each evaluation index and the gas permeability and gas desorption amount, and then combined with the weight of each evaluation index in step S5, finally a stress release reformability evaluation model of the tectonic coal reservoir is established: First, data points including evaluation index, gas permeability and gas desorption are collected, and the data points are plotted in a rectangular coordinate system. The type of relationship between and is roughly determined based on the shape of the scatter plot. If the scatter plot is in the shape of a straight line, it is suitable for a linear model. If it is in the shape of a curve, such as a quadratic curve or an exponential curve, a nonlinear model needs to be considered. Linear model fitting equation: y=β0+β1x+∈ Polynomial model fitting equation: y=β0+β1x+β2x 2 +∈ Exponential model fitting equation: and=αe βx +∈ In the formula: y is the fitting equation, β0, β1, β2 are the equation coefficients, ∈ is a constant; The least squares method is used to minimize the sum of squared differences and obtain the coefficients of each equation; finally, the model is evaluated and the determination coefficient R is used. 2 , mean square error MSE to evaluate the degree of model fit; determination coefficient R 2 The closer it is to 1, the better the model fits, the smaller the mean square error (MSE), and the higher the model prediction accuracy; The equations fitted by the above five evaluation indicators are multiplied by their respective weight values ​​and then added together to obtain the final evaluation model.

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