A method for evaluating stress release reworkability of a coal reservoir

Abstract

The application discloses a kind of tectonic coal reservoir stress release reconstructability evaluation method, belong to coalbed gas development technical field.Method includes four steps of sample test, selection evaluation index, calculate evaluation index weight, evaluate tectonic coal reservoir stress release reconstructability.Method is quantitatively evaluated reconstructability of tectonic coal reservoir by stress release, design coal formation structure reconstruction and similar material simulation device to obtain sample parameters by testing tectonic coal sample, according to the change of gas permeability and gas desorption amount of tectonic coal reservoir after stress release under different formation environment occurrence conditions, select appropriate evaluation index, and calculate the influence weight of each evaluation index on gas permeability and gas desorption amount of sample, to establish tectonic coal reservoir stress release reconstructability evaluation method, which is highly consistent with actual engineering situation, strong operability, quantifiable, can effectively evaluate the potential of tectonic coal reservoir development coalbed gas.

Description

TECHNICAL FIELD

[0001] The present application relates to a kind of construction coal reservoir stress release reformation evaluation method, especially a kind of coal measure reservoir evaluation suitable for construction coal reservoir in stress release process to reservoir mechanics characteristics, construction coal coalbed gas percolation, desorption research. BACKGROUND

[0002] The wide development of tectonic coal and the abundant tectonic coalbed gas resources are the significant features of China's coal and coalbed gas resources, and the proportion of tectonic coalbed gas resources in the total coalbed gas resources is relatively large. Tectonic coal has the outstanding characteristics of gas enrichment, low permeability and softness, and is mostly coal and gas outburst coal seam. Due to the great harm and the difficulty of extraction and utilization, the coal is often discharged into the atmosphere in coal mine production. Therefore, the energy, safety and ecological significance of efficient development of tectonic coalbed gas is very prominent.

[0003] The stress release research of tectonic coal reservoir is of great significance to the development of tectonic coalbed gas. Since the permeability of tectonic coal reservoir is extremely low and the effect of hydraulic fracturing and other reconstruction methods is poor, the theory of desorption and gas production by hydrophobic pressure reduction is obviously not suitable for tectonic coal reservoir. The exploration and development practice also shows that the coalbed gas exploration and development technology based on the theory of desorption and gas production by hydrophobic pressure reduction cannot realize the efficient development of tectonic coalbed gas. Therefore, the stress release and volume expansion of coal and rock by mining pressure relief and permeability improvement of tectonic coal reservoir can not only significantly reduce the fluid pressure in coal seam, but also greatly improve the permeability of coal seam, so that a large amount of coalbed gas is desorbed and seeps to the surface wellbore, thereby realizing the efficient development of pressure relief coalbed gas surface well in coal mine area.

[0004] At present, there are various types of coal reservoir reconstruction methods. The reconstruction evaluation method of hydraulic fracturing process for the hydraulic fracturing method of conventional coal reservoir takes the crack length and crack height expansion in the process of hydraulic fracturing as the evaluation index, but the research on stress release and permeability improvement of tectonic coal reservoir is relatively less, which leads to the shallow development of tectonic coalbed gas, and no quantifiable and operable stress release reconstruction evaluation method of tectonic coal reservoir has been proposed, which cannot effectively guide engineering practice. SUMMARY

[0005] Technical problem: The purpose of the present application is to overcome the shortcomings in the prior art and provide a kind of construction coal reservoir stress release reformation evaluation method. By analyzing the influence of evaluation index on the gas permeability and gas desorption amount of reservoir, the reformation of tectonic coal reservoir is quantitatively evaluated, so as to improve the development level of tectonic coalbed gas.

[0006] Technical scheme: In order to achieve the above purpose, a kind of construction coal reservoir stress release reformation evaluation method of the present application, characterized in that it comprises the following steps:

[0007] S1, screening samples: collecting samples of coal types, coal quality characteristics and their change rules and process performance of the coal seam, screening the collected samples, and screening tectonic coal samples of different coal and rock quality;

[0008] S2, gas adsorption-desorption test of tectonic coal: using a coal formation structure reconstruction and similar material simulation device, simulating tectonic coal samples under different reservoir environmental occurrence conditions, and performing gas adsorption-desorption test on tectonic coal samples; the formation test temperature is set to three temperature points of 30℃, 35℃ and 40℃, and the tectonic coal gas adsorption-desorption test under stress loading and unloading conditions needs to be performed three times at three different temperature points of 30℃, 35℃ and 40℃;

[0009] S3, gas permeability test of tectonic coal: using a coal formation structure reconstruction and similar material simulation device, simulating tectonic coal samples under different reservoir environmental occurrence conditions, and performing gas permeability test on tectonic coal samples; the different formation environmental occurrence 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℃, 35℃ and 40℃ respectively;

[0010] S4, selecting evaluation indexes: according to the test results of tectonic coal samples, selecting evaluation indexes which have greater influence on gas permeability and gas desorption amount;

[0011] S5, calculating the weight of evaluation indexes: calculating the weight of each evaluation index after step S4 screening;

[0012] S6, establishing evaluation method: using regression analysis method to establish the regression model between each evaluation index and gas permeability and gas desorption amount, and combining the weight of each evaluation index in step S5, finally establishing the tectonic coal reservoir stress release reconstructability evaluation model.

[0013] In step S1, the tectonic coal samples of different coal and rock quality are cataclastic coal, fragmented coal and mylonitic coal, and the tectonic coal samples of different coal and rock types are bright coal, semi-bright coal, semi-dark coal and dim coal.

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

[0015] In step S2, the structure reconstruction and similar material simulation device for coal measures includes, in sequence, a gas cylinder, a gas pressure reference cylinder, a stress loading and unloading device, a hot air blower, a vacuum pump, a gas flow meter and a computer; a pressure sensor is arranged on a pipeline connecting the gas cylinder and the gas pressure reference cylinder, a pressure reducing valve is arranged on a pipeline between the gas cylinder and the pressure sensor, a first and second gas valve are arranged on a pipeline connecting the pressure sensor and the gas pressure reference cylinder, a second pressure gauge and a third gas valve are arranged between the stress loading and unloading device and the first gas valve, a fourth gas valve is arranged between the stress loading and unloading device and the vacuum pump, a third pressure gauge and a fifth gas valve are arranged between the stress loading and unloading device and the gas flow meter, and a first pressure gauge and a thermometer are arranged on the gas pressure reference cylinder; all loading plates can be controlled to load pressure by the computer, so that the stress environment of the constructed coal sample in a natural environment can be simulated more truly.

[0016] In step S2, the stress loading and unloading device is a true triaxial loading and unloading device, which includes three hydraulic cylinders and six loading plates in the form of a cube, and the six loading plates are connected with the three hydraulic cylinders, wherein the first hydraulic cylinder is connected with the fifth loading plate and the sixth loading plate, the second hydraulic cylinder is connected with the first loading plate and the second loading plate, and the third hydraulic cylinder is connected with the third loading plate and the fourth loading plate.

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

[0018] Different types of constructed coal are placed in the stress loading and unloading device according to the required proportion, and the total length, width and height of the sample are 4:3:3; the vacuum pump is used to perform vacuumization to-0.1MPa and maintain for 30min, so as 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, the gas pipeline and the stress loading and unloading device, so as to calculate the void volume of the constructed coal sample and the free space of the pipeline.

[0019] After the gas pressure in the reference cylinder is stabilized, the third gas valve between the reference cylinder and the stress loading and unloading device is opened, the coal sample starts to adsorb, the adsorption time is set to 12 to 15 hours, and the gas pressure in the reference cylinder changes by less than 0.01MPa within 0 to 2 hours after the end of the adsorption process; then, the third gas valve is closed and the stress loading and unloading device is adjusted to simulate the decrease of effective stress, and the pore gas pressure and strain change values in the coal sample column are recorded.

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

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

[0022] In step S3, the tectonic coal permeability test under stress loading and unloading conditions is performed.

[0023] Based on the tectonic reconstruction and similar material simulation device of coal measure strata, the tectonic coal permeability test under stress change conditions is performed, and the influence of different coal body structures, coal rock types, gas content and temperature on the tectonic coal reservoir gas permeability change under stress release conditions is analyzed.

[0024] The first, third and fifth gas valves between the gas cylinder and the stress loading and unloading device are opened, the second and fourth gas valves are closed, the pressure reducing valve is opened and adjusted, and when the gas pressure displayed by the pressure reducing valve reaches 2-6 MPa, the gas flow meter reading is observed. When the reading jumping range of the gas flow meter is less than 0.1 cm 3 / min, it is considered that the gas flow reading is stable, and the computer is started to operate the stress loading and unloading device for pressure loading action;

[0025] The stress loading and unloading device pressure is loaded from 2 MPa to 17 MPa, and every 3 MPa is a pressure gradient. After each loading action is completed, the gas flow meter reading is stabilized, and the current gas flow meter data is recorded. Then the next pressure test is started.

[0026] When the stress loading and unloading device pressure is loaded to 17 MPa, the current data is recorded, and then the pressure is unloaded. Every 3 MPa is a pressure gradient, and the pressure is unloaded to 2 MPa. After each unloading operation is completed, the gas flow meter reading is stabilized, and the current gas flow meter data is recorded. Then the next unloading test is started.

[0027] The stress loading and unloading device is used to load pressure from 2 MPa to 17 MPa, and then unload from 17 MPa to 2 MPa, which is one cycle of loading and unloading; the permeability experiment of tectonic coal under stress loading and unloading conditions is set for 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 of coal structure, coal rock type, gas content, loading stress and temperature.

[0029] In step S5, the method for calculating the weight of the evaluation index is the analytic hierarchy process:

[0030] A comparison judgment matrix is established:

[0031]

[0032]

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

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

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

[0036]

[0037] The column vector M=(m1, m2, m3…, m n ) is normalized to obtain the characteristic vector W=(w1, w2, w3…, w n );

[0038]

[0039] The maximum eigenvalue λ max of the judgment matrix is calculated:

[0040]

[0041] The consistency index CI and the consistency ratio CR of the judgment matrix are calculated

[0042]

[0043] In the formula, A is the hierarchical judgment matrix, a ij is the element of the judgment matrix, m i is the component of the column vector, M is the column vector, W is the characteristic vector, w i is the weight value, λ max- maximum eigenvalue, B i To determine the consistency of the matrix, the i-th row vector, CI is a consistency index, RI is an average consistency index, CR is a consistency ratio, and n is the dimension;

[0044] Then determine the consistency of the matrix, when the consistency ratio CR is less than 0.1, the relative importance between the indicators can be accepted;

[0045] Gas-bearing - physical property system Coal petrographic type Coal body structure Reservoir gas content Geostress Geotemperature Weight value w1 w2 w3 w4 w5

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

[0047] First, collect the data points containing the evaluation index and the gas permeability and gas desorption amount, plot the data points in the plane rectangular coordinate system, and roughly determine the relationship type between them according to the shape of the scatter diagram; if the scatter diagram presents a linear shape, it is suitable for a linear model; if it is a curve shape, 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 equation coefficients, and ∈ is a constant;

[0055] Using the least square method to minimize the sum of squares, the equation coefficients are obtained; finally, the model is evaluated, and the determination coefficient R 2 , mean square error MSE is used to evaluate the model fitting degree; the determination coefficient R 2 closer to 1, the better the model fitting, the smaller the MSE, and the higher the model prediction accuracy;

[0056] The equations fitted by the above five evaluation indexes are multiplied by the weight values of each, and then added up to obtain the final evaluation model.

[0057] Beneficial Effects: By adopting the above-mentioned technical solution, this invention studies the influence of different factors on reservoir gas permeability and gas desorption in structural coal reservoirs through simulation experiments. Factors with significant impact on reservoir gas permeability and gas desorption are selected as evaluation indicators for modifiability. The weights of each evaluation indicator are determined using the analytic hierarchy process (AHP), and regression analysis is combined to establish a method for evaluating the modifiability of structural coal reservoirs based on stress release. This method comprehensively evaluates the modifiability of structural coal reservoirs. It fills the gap in existing coal reservoir modifiability evaluation methods that are not applicable to structural coal reservoirs. The method is flexible, highly operable, and allows for adjustments to evaluation indicators based on the reservoir characteristics of different study areas. The evaluation method highly matches actual engineering conditions, is highly operable, quantifiable, and can effectively assess the potential for coalbed methane development in structural coal reservoirs. By analyzing the influence of evaluation indicators on reservoir gas permeability and gas desorption, the modifiability of structural coal reservoirs is quantitatively evaluated, thereby improving the level of efficient coalbed methane development technology in structural coal and demonstrating good engineering application prospects for safe coal mining. This is of great significance for improving the level of efficient coalbed methane development technology and safe coal mining in my country. Attached Figure Description

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

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

[0060] In the diagram: 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 Implementation

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

[0062] like Figure 1 As shown, the present invention provides a method for evaluating the modifiability of structural coal reservoirs by stress release, and the specific steps are as follows:

[0063] S1, screening sample: collecting samples of coal type, coal quality characteristics and its change rule and coal process performance of mineable coal seam, screening the tectonic coal samples of different coal body structure and coal quality; the tectonic coal samples of different coal body structure are cataclastic coal, granular coal and mylonitic coal, and the tectonic coal samples of different coal rock types are bright coal, semi-bright coal, semi-dark coal and dim coal.

[0064] S2, gas adsorption-desorption test of tectonic coal: simulating sample under different stratum environment occurrence conditions, using coal measure stratum structure reconstruction and similar material simulation device to perform stress release test on tectonic coal sample; different types of tectonic coal are placed in the pressure chamber of the stress loading and unloading device 3 according to the required research proportion, and the total length, width and height of the sample should be 100mm*75mm*75mm; a vacuum pump 4 is used to vacuum the entire equipment to-0.1MPa and maintain for 30min to ensure the sealing property of the system and the gas discharge in the original coal sample; non-adsorbed helium is used 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; nitrogen is used to replace methane to perform adsorption and desorption test, because nitrogen has similar adsorption-desorption characteristics as methane in coal and is more stable in chemical property;

[0065] After the gas pressure in the reference cylinder 2 is stable, the third gas valve 7-3 between the reference cylinder 2 and the stress loading and unloading device 3 is opened, the coal sample starts to adsorb, and the adsorption time is set to more than 15 hours, and within 2 hours after the end of the adsorption process, the gas pressure change in the reference cylinder 2 should be less than 0.01MPa. Then, the third gas valve 7-3 of the stress loading and unloading device 3 is closed and the device pressure is adjusted to simulate the change of effective stress, and the pore gas pressure in the coal column and the strain change are recorded;

[0066] Once the pressure is stable, the fifth gas valve 7-5 is opened, the coal column starts to desorb, and the real-time flow rate and cumulative flow of the tail gas flow meter 10 are recorded. The desorption time is set to 7 hours, and when the temperature and atmospheric pressure change are less than 0.1℃ and 0.01kPa respectively, the test data is considered reliable. According to the in-situ stratum conditions, the test temperature is set to 30℃, 35℃ and 40℃.

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

[0068] The coal measure stratum structure reconstruction and similar material simulation device is used to simulate the occurrence of tectonic coal samples in different reservoir environment conditions, and gas adsorption-desorption tests are conducted on the tectonic coal samples; the stratum test temperature is set to three temperature points, i.e., 30℃, 35℃ and 40℃, and the tectonic coal gas adsorption-desorption test under stress loading and unloading conditions needs to be conducted three times at three different temperature points, i.e., 30℃, 35℃ and 40℃; the different reservoir environment conditions include reservoir gas content, ground stress and ground temperature.

[0069] The coal measure stratum structure reconstruction and similar material simulation device comprises a gas cylinder 1, a gas 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 sequentially and sealingly connected; a pressure sensor 6 is arranged on a pipeline connecting the gas cylinder 1 and the gas pressure reference cylinder 2, a pressure reducing valve 5 is arranged on a pipeline between the gas cylinder 1 and the pressure sensor 6, a first gas valve 7-1 and a second gas valve 7-2 are arranged on a pipeline connecting the pressure sensor 6 and the gas pressure reference cylinder 2, a second pressure gauge 8-2 and a third gas valve 7-3 are arranged between the stress loading and unloading device 3 and the first gas valve 7-1, a fourth gas valve 7-4 is arranged 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 arranged 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 arranged on the gas pressure reference cylinder 2; all loading plates can be controlled to load pressure by the computer 12, so that the stress environment of the tectonic coal sample in the natural environment can be more truly simulated.

[0070] The stress loading and unloading device 3 is a true triaxial loading and unloading device, which comprises three hydraulic cylinders and six loading plates in the shape of a cube, and the six loading plates are connected with the three hydraulic cylinders, wherein the first hydraulic cylinder 3-1 is connected with the fifth loading plate 3-8 and the sixth loading plate 3-9, the second hydraulic cylinder 3-2 is connected with the first loading plate 3-4 and the second loading plate 3-5, and the third hydraulic cylinder 3-3 is connected with the third loading plate 3-6 and the fourth loading plate 3-7.

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

[0072] Different types of tectonic coal are placed in the stress loading and unloading device 3 according to the required proportion, and the total length, width and height of the sample are 4:3:3; for example, the total length, width and height of the sample are 100mm*75mm*75mm; a vacuum pump 4 is used to perform vacuum pumping to-0.1MPa, and maintained for 30min, 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 2, the gas pipeline and the pressure chamber of the stress loading and unloading device 3 respectively, and the void volume of the tectonic coal sample and the pipeline free space are calculated; nitrogen is used instead of methane for adsorption and desorption test, because nitrogen has similar adsorption-desorption characteristics as methane in coal, and is more stable in chemical properties;

[0073] After the gas pressure in the reference cylinder 2 stabilizes, 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 starts to adsorb, and the adsorption time is set to 12-15 hours; within 0-2 hours after the end of the adsorption process, the gas 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 simulate the decrease of effective stress, and the pore gas pressure and strain change values in the coal sample column are recorded;

[0074] When the pressure reading change range of the pressure gauge 8-3 is less than 0.01MPa, it is considered that the pressure is stable, the fifth gas valve 7-5 is opened, and the coal sample column starts to desorb; at this time, the real-time flow rate and cumulative flow rate of the gas flow meter 10 at the tail end are recorded; the desorption time is set to 6-8 hours; when the temperature and atmospheric pressure change are less than 0.1℃ and 0.01kPa respectively, the test data is reliable, and the data is recorded; according to the in-situ formation conditions, the tectonic coal gas adsorption-desorption test under stress loading and unloading conditions needs to be carried out three times at three different temperature points of 30℃, 35℃ and 40℃ respectively;

[0075] The adsorption gas pressure of the reference cylinder 2 is set to 2-4MPa according to the experimental requirements, the effective stress of the tectonic coal sample is set to 2-3MPa, the pressure of the stress loading and unloading device 3 is loaded from 6MPa, the loading and 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] The first and second gas valves 7-1, 7-2 between the gas cylinder 1 and the reference cylinder 2 are opened, and after the gas pressure in the reference cylinder 2 reaches the required value, the first gas valve 7-1 between the reference cylinder 2 and the gas cylinder is closed, and the third gas valve 7-3 between the reference cylinder 2 and the stress loading and unloading device is opened;

[0078] The initial gas flow is dynamic, and then the gas flow is stable within 10 minutes, and the coal permeability is tested immediately after the flow changes are stable, and then the test conditions are adjusted to the next target value, and the permeability test is performed again.

[0079] The minimum confining pressure of the test is 2MPa, and the maximum is 17MPa, and every 3MPa is a pressure gradient, and the test temperature is set to 30℃, 35℃, and 40℃.

[0080] The coal bed structure reconstruction and similar material simulation device is used to simulate the occurrence of tectonic coal samples in different reservoir environments, and the gas permeability of the tectonic coal samples is tested; the different reservoir 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℃, 35℃ and 40℃ respectively; the tectonic coal permeability experiment under stress loading and unloading conditions:

[0081] Based on the coal bed structure reconstruction and similar material simulation device, the tectonic coal permeability experiment under stress change conditions is carried out, and the influence of different coal body structures, coal rock types, gas content and temperature on the change of tectonic coal reservoir gas permeability under stress release conditions is analyzed;

[0082] 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, and when the gas pressure displayed by the pressure reducing valve 5 reaches 2-6MPa, observe the reading of the gas flow meter 10, and wait for the reading of the gas flow meter 10 to jump within 0.1cm 3 / min, at this time, it is considered that the gas flow reading is stable, and the computer 12 is started to operate the stress loading and unloading device 3 to perform the pressure loading action;

[0083] The minimum pressure of the stress loading and unloading device 3 is from 2MPa to 17MPa, and every 3MPa is a pressure gradient, and after each loading action is completed, the gas flow meter 10 reading is stabilized, and the current gas flow meter 10 data is recorded, and then the next pressure test is started;

[0084] When the pressure of the stress loading and unloading device 3 is increased to 17MPa, the current data is recorded, and then the pressure relief operation is started, and every 3MPa is a pressure gradient, and the pressure is unloaded to 2MPa; after each pressure relief operation is completed, the gas flow meter 10 reading is stabilized, and the current gas flow meter 10 data is recorded, and then the next pressure test is started;

[0085] The stress loading and unloading device 3 is loaded from 2 MPa to 17 MPa, and then unloaded from 17 MPa to 2 MPa, which is one cycle of loading and unloading; the permeability experiment of tectonic coal under the stress loading and unloading condition is set for three cycles of loading and unloading, and the temperature of each cycle of loading and unloading is set to 30 DEG C, 35 DEG C and 40 DEG C respectively.

[0086] S4, selecting evaluation indexes: according to the test results of tectonic coal samples, selecting evaluation indexes which have greater influence on gas permeability and gas desorption amount; the evaluation indexes include five evaluation indexes of coal structure, coal type, gas content, loading stress and temperature.

[0087] S5, calculating the weight of the evaluation index: calculating the weight of each evaluation index after screening in step S4; the method 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 too much;

[0088] Establish a pairwise comparison judgment matrix:

[0089]

[0090]

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

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

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

[0094]

[0095] The column vector M=(m1, m2, m3…, m n ) is normalized to obtain the characteristic vector W=(w1, w2, w3…, w n );

[0096]

[0097] The maximum eigenvalue λ max of the judgment matrix is calculated:

[0098]

[0099] The consistency index CI and the consistency ratio CR of the judgment matrix are calculated

[0100]

[0101] In the formula: A - level judgment matrix, a ij - judgment matrix element, m i - component of column vector, M - column vector, W - eigenvector, w i - weight value, λ max - the largest eigenvalue, B i is the i-th row vector of the judgment matrix, CI is a consistency index, RI is the average consistency index, CR is the consistency ratio, and n is the dimension;

[0102] The consistency of the judgment matrix is checked again. When the consistency ratio CR is less than 0.1, the relative importance between the indexes can be accepted.

[0103] Gas-bearing - physical property system Coal petrographic type Coal body structure Reservoir gas content Geostress Geotemperature Weight value w1 w2 w3 w4 w5

[0104] S6, establishing an evaluation method: according to the weight calculation results of each evaluation index in step S5, a tectonic coal reservoir stress release reconstructability evaluation model is established. The tectonic coal reservoir stress release reconstructability evaluation model is established by 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 weight of each evaluation index in step S5 to finally establish the tectonic coal reservoir stress release reconstructability evaluation model:

[0105] First, collect data points containing evaluation indexes and 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 linear shape, a linear model is suitable; if it is a curve shape, such as a quadratic curve (parabola, hyperbola), 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 - fitting equation, β0, β1, β2 - equation coefficients, ∈ - constant;

[0113] Using the least squares method to minimize the sum of squares, the equation coefficients are obtained; finally, the model is evaluated, and the determination coefficient R 2 , mean square error MSE are used to evaluate the model fitting degree; the determination coefficient R2 The closer to 1, the better the model fitting, the smaller the MSE, the higher the model prediction accuracy;

[0114] The equations of the above five evaluation indexes are multiplied by their respective weight values and added up to obtain the final evaluation model.

[0115] The reconstructability of tectonic coal reservoirs is quantitatively evaluated by stress release, a coal formation structure reconstruction and similar material simulation device is designed to test tectonic coal samples and obtain sample parameters, according to the changes of gas permeability and gas desorption of tectonic coal reservoirs after stress release under different formation environment conditions, appropriate evaluation indexes are selected, and the influence weight of each evaluation index on the gas permeability and gas desorption of the sample is calculated, so as to establish a tectonic coal reservoir stress release reconstructability evaluation system.

Claims

1. A method of constructing a coal reservoir stress release reworkability evaluation, characterized by, It comprises the following steps: S1, screening samples: collecting samples of coal type, coal quality characteristics and its change rule and process performance of the coal seam, screening the collected samples, and screening the tectonic coal samples of different coal and rock quality; S2, gas adsorption-desorption test of tectonic coal: using the coal measure structural reconstruction and similar material simulation device, simulating the tectonic coal samples under different reservoir environmental occurrence conditions, and performing gas adsorption-desorption test on the tectonic coal samples; the test temperature of the coal measure stratum is set to three temperature points of 30℃, 35℃ and 40℃, and the tectonic coal gas adsorption-desorption test under stress loading and unloading conditions needs to be performed three times at three different temperature points of 30℃, 35℃ and 40℃; S3, tectonic coal gas permeability test: using the coal measure structural reconstruction and similar material simulation device, simulating the tectonic coal samples under different reservoir environmental occurrence conditions, and performing gas permeability test on the tectonic coal samples; The different reservoir environmental occurrence 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℃, 35℃ and 40℃ respectively; S4, selecting evaluation indexes: according to the test results of the tectonic coal samples, selecting evaluation indexes which have greater influence on gas permeability and gas desorption amount; S5, calculating the weight of the evaluation indexes: calculating the weight of each evaluation index after screening in step S4; S6, establishing evaluation method: using regression analysis method to establish the regression model between each evaluation index and gas permeability and gas desorption amount, and combining the weight of each evaluation index in step S5, finally establishing the tectonic coal reservoir stress release reconstructability evaluation model.

2. The method for evaluating the stress release reconstructability of a coal reservoir according to claim 1, characterized in that: In step S1, the tectonic coal samples of different coal and rock quality are cataclastic coal, fragmented coal and mylonitic coal, and the tectonic coal samples of different coal and rock types are bright coal, semi-bright coal, semi-dark coal and dim coal.

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

4. The method for evaluating the stress-release reconstructability of a coal reservoir according to claim 1, characterized in that: In step S2, the structure reconstruction and similar material simulation device of coal measures includes sequentially sealed and connected gas cylinder (1), gas pressure reference cylinder (2), stress loading and unloading device (3), hot air machine (11), vacuum pump (4), gas flow meter (10) and computer (12); the pipeline connected with the gas cylinder (1) and the gas pressure reference cylinder (2) is provided with a pressure sensor (6), the pipeline between the gas cylinder (1) and the pressure sensor (6) is provided with a pressure reducing valve (5), the pipeline connected with the pressure sensor (6) and the gas pressure reference cylinder (2) is provided with a first gas valve (7-1) and a second gas valve (7-2), the stress loading and unloading device (3) and the first gas valve (7-1) are also provided with a second pressure gauge (8-2) and a third gas valve (7-3), the stress loading and unloading device (3) and the vacuum pump (4) are provided with a fourth gas valve (7-4), the stress loading and unloading device (3) and the gas flow meter (10) are provided with a third pressure gauge (8-3) and a fifth gas valve (7-5), and the gas pressure reference cylinder (2) is respectively provided with a first pressure gauge (8-1) and a thermometer (9); all loading plates can be controlled to load pressure by the computer (12), so that the stress environment of the tectonic coal sample in the natural environment can be simulated more truly.

5. The method for evaluating the stress release reconstructability of a coal reservoir according to claim 4, characterized in that: The stress loading and unloading device (3) is a true triaxial loading and unloading device, and the true triaxial loading and unloading device includes three hydraulic cylinders and six loading plates in the form of a cube, and the six loading plates are connected with the three hydraulic cylinders, wherein the first hydraulic cylinder (3-1) is connected with the fifth loading plate (3-8) and the sixth loading plate (3-9), the second hydraulic cylinder (3-2) is connected with the first loading plate (3-4) and the second loading plate (3-5), and the third hydraulic cylinder (3-3) is connected with the third loading plate (3-6) and the fourth loading plate (3-7).

6. The method according to claim 1 or 4, wherein: the coal reservoir is constructed. In step S2, the gas adsorption-desorption test is performed on the tectonic coal sample: Different types of tectonic coal are placed in the stress loading and unloading device (3) according to the required proportion for research, and the total length, width and height of the sample are 4:3:3; the vacuum pump (4) is used to perform vacuumizing to-0.1MPa and maintain for 30min, so as 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 (2), the gas pipeline and the stress loading and unloading device (3) respectively, and the void volume of the tectonic coal sample and the free space of the pipeline are calculated; After the gas pressure in the reference cylinder (2) is stable, the third gas valve (7-3) between the reference cylinder (2) and the stress loading and unloading device (3) is opened, the coal sample starts to adsorb, the adsorption time is set to 12 to 15 hours, and the gas pressure change in the reference cylinder (2) should be less than 0.01MPa within 0 to 2 hours after the end of the adsorption process; then, the third gas valve (7-3) is closed and the stress loading and unloading device (3) is adjusted to simulate the decrease of effective stress, and the pore gas pressure and strain change values in the coal sample column are recorded; When the third pressure gauge (8-3) pressure reading change amplitude is less than 0.01 MPa, it is considered that the pressure is stable, the fifth gas valve (7-5) is opened, and the coal sample column starts to desorb, at this time, the real-time flow rate and cumulative flow of the gas in the tail end gas flow meter (10) are recorded; the desorption time is set to 6-8 hours, when the temperature and atmospheric pressure change are less than 0.1℃ and 0.01kPa respectively, the test data is reliable, the data is recorded, and according to the in-situ formation conditions, the stress loading and unloading conditions require three tests at three different temperature points of 30℃, 35℃ and 40℃ respectively; According to the experimental requirements, the reference cylinder (2) adsorbed gas pressure is set to 2-4 MPa, the effective stress of the tectonic coal sample is set to 2-3 MPa, and the stress loading and unloading device (3) pressure is loaded from 6 MPa, with a loading / unloading gradient of 3 MPa, and the maximum confining stress is 21 MPa.

7. The method according to claim 1 or 4, wherein: In step S3, the stress loading and unloading conditions of the tectonic coal permeability experiment: ​ Based on the tectonic reconstruction and similar material simulation device of coal measure strata structure, the tectonic coal permeability experiment under stress change conditions is carried out, and the influence of different coal body structure, coal type, gas content and temperature on the change of tectonic coal reservoir gas permeability under stress release conditions is analyzed; Open the first gas valve (7-1), the third gas valve (7-3) and the fifth gas valve (7-5) between the gas cylinder (1) and the stress loading and unloading device (3), close the second gas valve (7-2) and the fourth gas valve (7-4), open and adjust the pressure reducing valve (5), after the pressure reducing valve (5) shows that the gas pressure reaches 2-6 MPa, observe the reading of the gas flow meter (10), wait for the reading of the gas flow meter (10) to jump less than 0.1 cm 3 / min, at this time it is considered that the gas flow reading is stable, start operating the computer (12) to perform the pressurizing action of the stress loading and unloading device (3); The stress loading and unloading device (3) pressure is loaded from 2 MPa to 17 MPa, with a pressure gradient of 3 MPa each time, and after each loading action is completed, the gas flow meter (10) reading is stabilized, the current gas flow meter (10) data is recorded, and then the next pressure test is started; When the stress loading and unloading device (3) pressure is loaded to 17 MPa, the current data is recorded, and then the pressure is unloaded, with a pressure gradient of 3 MPa each time, until it is unloaded to 2 MPa; after each unloading operation is completed, the gas flow meter (10) reading is stabilized, the current gas flow meter (10) data is recorded, and then the next unloading test is started; The stress loading and unloading device (3) pressure is loaded from 2 MPa to 17 MPa, and then unloaded from 17 MPa to 2 MPa, which is one cycle of loading and unloading; 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℃, 35℃ and 40℃ respectively.

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

9. The method for evaluating the stress-release reconstructability of a coal reservoir according to claim 1, characterized in that: In step S5, the method for calculating the weight of the evaluation index is the analytic hierarchy process: A pairwise comparison judgment matrix is established: In the table, a, b, c, d, e, f, g, h, i and j are judgment matrix elements; The weight vector is calculated by square root method; Based on the hierarchical judgment matrix A n×n The column vector M = (m1, m2, m3…, m n ) is obtained, wherein: Normalization processing column vector M = (m1, m2, m3…, m n ), get the feature vector W = (w1, w2, w3…, w n ) calculating the largest eigenvalue λ of the judgment matrix max : The consistency index (CI) and consistency ratio (CR) of the judgment matrix are calculated wherein: A - hierarchy judgment matrix, a ij - judgment matrix element, 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. The consistency of the judgment matrix is verified, and when the consistency ratio CR is less than 0.1, the relative importance between the indexes can be accepted; 10. The method for evaluating the stress-release reconstructability of a 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 the gas desorption amount, and then combined with the weight of each evaluation index in step S5, a stress release reconstructability evaluation model of the coal reservoir is finally established: Firstly, the data points containing each evaluation index and the gas permeability and the gas desorption amount are collected, the data points are plotted in a plane rectangular coordinate system, and the relationship type between each evaluation index and the gas permeability and the gas desorption amount is roughly judged according to the shape of the scatter diagram; if the scatter diagram presents a linear shape, a linear model is suitable; if it is a curve shape, 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: y = a e βx + e In the formula: y is a fitting equation, β0, β1 and β2 are equation coefficients, and ∈ is a constant; Using the least square method to make the square difference and minimum, the equation coefficient of each term is obtained; finally, the model evaluation is carried out, and the determination coefficient R 2 , mean square error MSE is used to evaluate the model fitting degree; the determination coefficient R 2 The closer to 1, the better the model fitting is, the smaller the mean square error MSE is, and the higher the model prediction accuracy is; The fitting equation of each evaluation index is multiplied by the weight value of each evaluation index and then added up to obtain a final evaluation model.

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