Method for evaluating closure of cover layer of underground gas storage

Abstract

The invention discloses a method for evaluating the closure of a cover layer of an underground gas storage, and belongs to the field of oil-gas exploration. According to the technical scheme, the method comprises the following steps: evaluating the closure of the cover layer of the underground gas storage in three aspects of macroscopic, microscopic and mechanical aspects, and selecting seven key parameters closely related to the closure of the cover layer, wherein the seven key parameters comprise lithology, direct cover layer thickness, permeability, breakthrough pressure, dynamic breakthrough pressure, specific surface area and plasticity coefficient. The method is applied to the aspect of underground gas storage overburden closure evaluation, solves the problem that an existing underground gas storage overburden closure evaluation method cannot accurately evaluate the dynamic closure performance of the overburden in the operation process of the gas storage, and can quantitatively evaluate the static and dynamic closure performance of the overburden from the macroscopic, microscopic and mechanical dimensions; and a scientific basis is provided for site selection and operation of the gas storage.

Description

Technical Field

[0001] The present invention belongs to the field of oil and gas exploration, and particularly relates to a method for evaluating the sealing performance of a caprock in an underground gas storage. Background Art

[0002] Underground gas storage is an important means for natural gas storage, and its safety and operation efficiency depend on the sealing ability of the caprock of the gas storage. As a natural barrier for gas storage, the caprock needs to have good physical properties, mechanical properties and microscopic sealing performance. However, the periodic injection and production process of the gas storage will cause the formation pressure and stress field to change repeatedly, posing higher requirements for the long-term sealing performance of the caprock.

[0003] Existing studies usually start from a single parameter (such as caprock thickness, permeability, breakthrough pressure, etc.), or only consider the sealing performance of the caprock under static conditions, and it is difficult to accurately reflect the change of the dynamic sealing performance of the caprock during the operation of the gas storage. Chinese Patent CN110764159A discloses a method for evaluating the effectiveness of a caprock, which includes the following steps: dynamically evaluating the effectiveness of the caprock by using macroscopic indicators; evaluating the effectiveness of the caprock by using microscopic indicators; evaluating the effectiveness of the caprock by using formation water chemistry and organic geochemistry indicators in the formation water. The method of the present invention qualitatively studies the effectiveness of the caprock according to macroscopic elements such as lithology, burial depth, thickness and distribution of rock formations in different structural parts, combines microscopic research, quantitatively evaluates the sealing performance of the caprock with breakthrough pressure and the content of ultra-micropores less than 2.5 nm, analyzes the separation effect of the caprock on formation water through the chemical characteristics of formation water, and finally realizes the three-in-one comprehensive evaluation of the effectiveness of the caprock from macro-micro-hydrochemistry.

[0004] However, the evaluation method of the above patent cannot comprehensively evaluate the sealing performance of caprocks with different lithologies in the same study area, and does not consider the rock mechanical properties of the caprock and the change of rock mechanical properties during the multi-cycle injection and production process of the gas storage, so it cannot accurately evaluate the dynamic sealing performance of the caprock during the operation of the gas storage. Based on this, there is an urgent need to construct a systematic and scientific comprehensive evaluation method to quantitatively characterize the sealing ability of the caprock under complex working conditions. Summary of the Invention

[0005] Aiming at the deficiencies existing in the prior art, the technical problem to be solved by the present invention is to overcome the problem that the existing method for evaluating the sealing performance of the caprock of an underground gas storage cannot accurately evaluate the dynamic sealing performance of the caprock during the operation of the gas storage, and to propose an evaluation method for the sealing performance of the caprock of an underground gas storage that can quantitatively evaluate the static and dynamic sealing performance of the caprock from three dimensions of macro, micro and mechanics, and provide a scientific basis for the site selection and operation of the gas storage.

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

[0007] The present invention provides a method for evaluating the sealing performance of the caprock of an underground gas storage, which evaluates the sealing performance of the caprock of the underground gas storage from three aspects: macroscopic, microscopic, and mechanical; including:

[0008] Lithology evaluation step, including assigning values to the lithologies of gypsum rock, mudstone, argillaceous sandstone, and sandstone in descending order, denoted as L;

[0009] Direct caprock thickness evaluation step, including statistically analyzing the thickness of the direct caprock overlying the reservoir according to well logging data, assigning a value to H, and the higher the thickness of the direct caprock, the higher the value assigned to H;

[0010] Permeability evaluation step, including testing the rock permeability, assigning a value to K, and the higher the permeability, the lower the value assigned to K;

[0011] Breakthrough pressure evaluation step, including measuring the gas breakthrough pressure of the rock, assigning a value to P b and the higher the breakthrough pressure, the higher the value assigned to P b assigned;

[0012] Specific surface area evaluation step, including measuring the specific surface area of the rock, assigning a value to S, and the higher the specific surface area, the higher the value assigned to S;

[0013] Dynamic breakthrough pressure evaluation step, including measuring the dynamic breakthrough pressure, assigning a value to P bd and the higher the dynamic breakthrough pressure, the higher the value assigned to P bd assigned;

[0014] Plasticity coefficient evaluation step, including conducting a triaxial compression test, calculating the plasticity coefficient, assigning a value to T, and the higher the plasticity coefficient, the higher the value assigned to T;

[0015] Comprehensive weight distribution and sealing performance evaluation step, using the analytic hierarchy process to respectively determine the weight coefficients of the macroscopic sealing performance, microscopic sealing performance, and mechanical sealing performance of the caprock, pairwise comparing to determine the weight coefficients of lithology, direct caprock thickness, permeability, breakthrough pressure, specific surface area, dynamic breakthrough pressure, and plasticity coefficient, and using their assigned values L, H, K, P b 、S、P bd 、T to calculate the comprehensive evaluation factor Q of the caprock sealing performance and classify the caprock sealing performance into four grades.

[0016] Preferably, the values assigned to L for gypsum rock, mudstone, argillaceous sandstone, and sandstone are 4, 3, 2, and 1 in sequence;

[0017] When the direct caprock thickness is greater than 50 m, the value assigned to H is 4; when the direct caprock thickness is 20 - 50 m, the value assigned to H is 3; when the direct caprock thickness is 10 - 20 m, the value assigned to H is 2; when the direct caprock thickness is less than 10 m, the value assigned to H is 1;

[0018] When the permeability is less than 0.001 mD, assign K as 4; when the permeability is 0.001 - 0.01 mD, assign K as 3; when the permeability is 0.01 - 0.1 mD, assign K as 2; when the permeability is 0.1 - 1 mD, assign K as 1;

[0019] When the breakthrough pressure is greater than 15 MPa, assign P b as 4; when the breakthrough pressure is 12 - 15 MPa, assign P b as 3; when the breakthrough pressure is 10 - 12 MPa, assign P b as 2; when the breakthrough pressure is less than 10 MPa, assign P b as 1;

[0020] When the specific surface area is greater than 50 m 2 / g, assign S as 4; when the specific surface area is 30 - 50 m 2 / g, assign S as 3; when the specific surface area is 10 - 30 m 2 / g, assign S as 2; when the specific surface area is less than 10 m 2 / g, assign S as 1;

[0021] When the dynamic breakthrough pressure is greater than 20 MPa, assign P bd as 4; when the dynamic breakthrough pressure is 15 - 20 MPa, assign P bd as 3; when the dynamic breakthrough pressure is 10 - 15 MPa, assign P bd as 2; when the dynamic breakthrough pressure is less than 10 MPa, assign P bd as 1;

[0022] When the plasticity coefficient is greater than 6, assign T as 4; when the plasticity coefficient is 4 - 6, assign T as 3; when the plasticity coefficient is 3 - 4, assign T as 2; when the plasticity coefficient is less than 3, assign T as 1;

[0023] Using the analytic hierarchy process, first determine the weight coefficients of the macroscopic sealing property, microscopic sealing property, and mechanical sealing property of the caprock as 0.15, 0.75, and 0.10 respectively. After pairwise comparison, finally determine the weight coefficients of lithology, direct caprock thickness, permeability, breakthrough pressure, specific surface area, dynamic breakthrough pressure, and plasticity coefficient as 0.10, 0.05, 0.20, 0.40, 0.15, 0.06, and 0.04 in sequence. Calculate the comprehensive evaluation factor Q of the caprock sealing property according to the following formula:

[0024] Q = 0.1×L + 0.05×H + 0.2×K + 0.4×P b + 0.15×S + 0.06×P bd + 0.04×T

[0025] When Q > 2.5, the caprock sealing property is evaluated as Class I; when 1.5 ≤ Q ≤ 2.5, it is evaluated as Class II; when 1.0 ≤ Q < 1.5, it is evaluated as Class III; when Q < 1.0, it is evaluated as Class IV.

[0026] Preferably, collect the lithology data of the target caprock, combine the logging data and well logging data, and through thin section observation, geological description, and mineral composition analysis, confirm that the main lithology types of the caprock are gypsum rock, mudstone, argillaceous sandstone, or sandstone.

[0027] Preferably, conduct overburden pore permeability tests on caprock samples to obtain the permeability.

[0028] Preferably, conduct gas breakthrough pressure tests on caprock samples to obtain the breakthrough pressure of the caprock.

[0029] Preferably, according to the confining pressure and upper and lower limit pressures designed for the gas storage reservoir, conduct cyclic loading and unloading tests on caprock samples, and after completion, measure the breakthrough pressure of the caprock samples again to determine the dynamic breakthrough pressure.

[0030] Preferably, use the gas adsorption method to measure the specific surface area of caprock samples.

[0031] Preferably, conduct triaxial compression tests on caprock samples to obtain the stress-strain curve, and calculate the plastic coefficient with reference to the petroleum and natural gas industry standard SY / T 6942-2013.

[0032] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0033] The present invention provides a method for evaluating the sealing property of an underground gas storage reservoir caprock,

[0034] (1) Comprehensive parameters: Comprehensively consider the macroscopic distribution, microscopic pore throat structure, and mechanical properties of the caprock, and can comprehensively reflect the sealing ability of the caprock;

[0035] (2) Dynamic analysis: Introduce dynamic breakthrough pressure experiments to quantitatively evaluate the influence of multi-cycle alternating stress on the caprock sealing performance;

[0036] (3) Scientific weight assignment: Use the analytic hierarchy process to scientifically assign the weights of each parameter, and improve the accuracy and objectivity of the evaluation results;

[0037] (4) Strong applicability: The method is applicable to the sealing property evaluation of various types of caprocks (such as gypsum rock, mudstone, etc.), and has broad promotion value. Brief Description of the Drawings

[0038] Figure 1 It is a flowchart of the method for evaluating the sealing property of an underground gas storage reservoir caprock provided by an embodiment of the present invention. Detailed Embodiments

[0039] The technical solutions in the specific embodiments of the present invention will be described in detail and completely below in conjunction with the accompanying drawings. Obviously, the described embodiments are only partial specific implementations of the overall technical solution of the present invention, rather than all implementations. Based on the overall concept of the present invention, all other embodiments obtained by those of ordinary skill in the art fall within the protection scope of the present invention.

[0040] The present invention provides a method for evaluating the sealing performance of the caprock of an underground gas storage, as Figure 1 shown, evaluating the sealing performance of the caprock of the underground gas storage from three aspects: macroscopic, microscopic, and mechanical; including:

[0041] The lithology evaluation step, including assigning values to the lithologies of gypsum rock, mudstone, argillaceous sandstone, and sandstone in descending order, denoted as L;

[0042] The direct caprock thickness evaluation step, including statistically analyzing the thickness of the direct caprock overlying the reservoir according to well logging data, assigning a value to H, and the higher the direct caprock thickness, the higher the value assigned to H;

[0043] The permeability evaluation step, including measuring the rock permeability, assigning a value to K, and the higher the permeability, the lower the value assigned to K;

[0044] The breakthrough pressure evaluation step, including measuring the gas breakthrough pressure of the rock, assigning a value to P b , and the higher the breakthrough pressure, the higher the value assigned to P b ;

[0045] The specific surface area evaluation step, including measuring the specific surface area of the rock, assigning a value to S, and the higher the specific surface area, the higher the value assigned to S;

[0046] The dynamic breakthrough pressure evaluation step, including measuring the dynamic breakthrough pressure, assigning a value to P bd , and the higher the dynamic breakthrough pressure, the higher the value assigned to P bd ;

[0047] The plasticity coefficient evaluation step, including conducting a triaxial compression test, calculating the plasticity coefficient, assigning a value to T, and the higher the plasticity coefficient, the higher the value assigned to T;

[0048] The comprehensive weight distribution and sealing performance evaluation step, using the analytic hierarchy process, respectively determining the weight coefficients of the macroscopic sealing performance, microscopic sealing performance, mechanical sealing performance, etc. of the caprock, pairwise comparing to determine the weight coefficients of lithology, direct caprock thickness, permeability, breakthrough pressure, specific surface area, dynamic breakthrough pressure, and plasticity coefficient, and using their assigned values L, H, K, P b , S, P bd , T, calculating the comprehensive evaluation factor Q of the caprock sealing performance, and classifying the caprock sealing performance into four grades.

[0049] The following problems exist in the existing evaluation methods for caprock sealing performance: the evaluation parameters are single, and the lack of consideration of rock mechanical properties cannot comprehensively reflect the sealing performance of the caprock; there is a lack of research on the dynamic sealing performance of the caprock under multi-cycle alternating stress conditions; there is a lack of a scientific quantification method for the influence weights of multiple parameters, resulting in insufficient accuracy of the evaluation results. In view of the above problems, the comprehensive evaluation method for caprock sealing performance of underground gas storage proposed by the present invention quantitatively evaluates the static and dynamic sealing performance of the caprock from three dimensions: macroscopic, microscopic, and mechanical, providing a scientific basis for gas storage site selection and operation. Specifically, the above evaluation method systematically evaluates the sealing ability of the caprock from three dimensions: macroscopic distribution, microscopic pore throat structure, and rock mechanical properties by introducing multi-dimensional parameters, and selects seven key parameters closely related to the caprock sealing performance, including: lithology, direct caprock thickness, permeability, breakthrough pressure, dynamic breakthrough pressure, specific surface area, and plasticity coefficient, and constructs a comprehensive evaluation factor for quantifying the caprock sealing performance by using the analytic hierarchy process.

[0050] The above evaluation method for caprock sealing performance of underground gas storage has the following characteristics:

[0051] (1) Comprehensive parameters: Considering the macroscopic distribution, microscopic pore throat structure, and mechanical properties of the caprock comprehensively, it can comprehensively reflect the sealing ability of the caprock;

[0052] (2) Dynamic analysis: Introducing the dynamic breakthrough pressure experiment to quantitatively evaluate the influence of multi-cycle alternating stress on the caprock sealing performance;

[0053] (3) Scientific weight distribution: Using the analytic hierarchy process to scientifically distribute the weights of each parameter, improving the accuracy and objectivity of the evaluation results;

[0054] (4) Strong applicability: The method is applicable to the sealing performance evaluation of various types of caprocks (such as gypsum rock, mudstone, etc.), and has broad popularization value.

[0055] It should be noted that the above evaluation method of the present invention comprehensively evaluates the sealing performance of caprocks with different lithologies in the same study area, and can accurately simulate the dynamic sealing performance of the caprock during the operation of the gas storage. This method focuses on the influence of lithology differences on the caprock sealing performance. It adopts a more multi-dimensional evaluation method, focusing on the comprehensive influence of key parameters such as lithology differences, caprock thickness, permeability, and breakthrough pressure, and does not involve hydrochemical characteristics. By systematically evaluating the sealing ability of the caprock from multiple perspectives (including macroscopic distribution, microscopic pore throat structure, and rock mechanical properties), it provides a different technical path and can more accurately simulate the caprock sealing performance under different operating conditions of the gas storage.

[0056] In a preferred embodiment, the lithologies of anhydrite rock, mudstone, argillaceous sandstone, and sandstone are assigned values of 4, 3, 2, and 1 in sequence; specifically, the assignment of lithology parameters is carried out according to the standard in Table 1.

[0057] Table 1 Lithology Assignment Table

[0058] Level Excellent Good Medium Poor Lithology Gypsite Mudstone Argillaceous sandstone Sandstone L assignment 4 3 2 1

[0059] When the thickness of the direct caprock is greater than 50 m, H is assigned a value of 4; when the thickness of the direct caprock is 20 - 50 m, H is assigned a value of 3; when the thickness of the direct caprock is 10 - 20 m, H is assigned a value of 2; when the thickness of the direct caprock is less than 10 m, H is assigned a value of 1; specifically, according to the logging data, the thickness of the direct caprock overlying the reservoir is statistically analyzed and assigned according to Table 2.

[0060] Table 2 Direct Caprock Thickness Assignment Table

[0061] Level Excellent Good Medium Poor Direct caprock thickness m ＞50 20-50 10-20 ＜10 H assignment 4 3 2 1

[0062] When the permeability is less than 0.001 mD, K is assigned a value of 4; when the permeability is 0.001 - 0.01 mD, K is assigned a value of 3; when the permeability is 0.01 - 0.1 mD, K is assigned a value of 2; when the permeability is 0.1 - 1 mD, K is assigned a value of 1; specifically, the permeability is measured by the confining pressure porosity and permeability experiment and assigned according to Table 3.

[0063] Table 3 Permeability Assignment Table

[0064] Level Excellent Good Medium Poor Permeability / mD ＜0.001 0.001-0.01 0.01-0.1 0.1-1 K assignment 4 3 2 1

[0065] When the breakthrough pressure is greater than 15 MPa, P b is assigned a value of 4; when the breakthrough pressure is 12 - 15 MPa, P b is assigned a value of 3; when the breakthrough pressure is 10 - 12 MPa, P b is assigned a value of 2; when the breakthrough pressure is less than 10 MPa, P b is assigned a value of 1; specifically, the breakthrough pressure generally refers to the minimum pressure at which gas begins to break through solid particles and enter the pores, which is obtained by the gas breakthrough pressure test and assigned according to Table 4.

[0066] Table 4 Breakthrough Pressure Assignment Table

[0067] Level Excellent Good Medium Poor Breakthrough pressure / MPa ＞15 12-15 10-12 ＜10 <![CDATA[P b Assignment]]> 4 3 2 1

[0068] When the specific surface area is greater than 50 m 2 / g, S is assigned a value of 4; when the specific surface area is 30 - 50 m 2 / g, S is assigned a value of 3; when the specific surface area is 10 - 30 m 2 / g, S is assigned a value of 2; when the specific surface area is less than 10 m2 When it is / g, assign S as 1; specifically, use the gas adsorption method to measure the specific surface area of the rock and make the assignment according to Table 5.

[0069] Table 5 Specific Surface Area Assignment Table

[0070] Level Excellent Good Medium Poor <![CDATA[Specific surface area / m 2 / g]]> ＞50 30-50 10-30 ＜10 S assignment 4 3 2 1

[0071] When the dynamic breakthrough pressure is greater than 20 MPa, assign P bd as 4. When the dynamic breakthrough pressure is 15 - 20 MPa, assign P bd as 3. When the dynamic breakthrough pressure is 10 - 15 MPa, assign P bd as 2. When the dynamic breakthrough pressure is less than 10 MPa, assign P bd as 1; specifically, the dynamic breakthrough pressure refers to the gas breakthrough pressure measured after conducting 50 - cycle cyclic loading and unloading tests on the core, and make the assignment according to Table 6.

[0072] Table 6 Dynamic Breakthrough Pressure Assignment Table

[0073] Level Excellent Good Medium Poor Dynamic breakthrough pressure / MPa ＞20 15-20 10-15 ＜10 <![CDATA[P bd Assignment]]> 4 3 2 1

[0074] When the plasticity coefficient is greater than 6, assign T as 4. When the plasticity coefficient is 4 - 6, assign T as 3. When the plasticity coefficient is 3 - 4, assign T as 2. When the plasticity coefficient is less than 3, assign T as 1; specifically, the plasticity coefficient of the rock is a parameter that quantitatively characterizes the plasticity and brittleness of the rock. The calculation method is carried out in accordance with the petroleum and natural gas industry standard SY / T 6942 - 2013 and make the assignment according to Table 7.

[0075] Table 7 Plasticity Coefficient Assignment Table

[0076] Level Excellent Good Medium Poor Plasticity coefficient ＞6 4-6 3-4 ＜3 T assignment 4 3 2 1

[0077] Use the analytic hierarchy process to determine the weight coefficients of L, H, K, P b , S, P bd , T are 0.10, 0.05, 0.20, 0.40, 0.15, 0.06, 0.04 in sequence. Calculate the comprehensive evaluation factor Q of caprock sealing performance according to the following formula:

[0078] Q = 0.1×L + 0.05×H + 0.2×K + 0.4×P b + 0.15×S + 0.06×P bd + 0.04×T

[0079] When Q > 2.5, the caprock sealing property is evaluated as Class I; when 1.5 ≤ Q ≤ 2.5, it is evaluated as Class II; when 1.0 ≤ Q < 1.5, it is evaluated as Class III; when Q < 1.0, it is evaluated as Class IV. Specifically, combining the scores after normalization of each parameter, the comprehensive evaluation factor Q of the caprock sealing property is calculated using the above formula. According to the value of the comprehensive evaluation factor Q, the caprock sealing property is divided into four grades as shown in Table 8.

[0080] Table 8 Evaluation Grade Table of Caprock Sealing Property

[0081] Score Q＞2.5 1.5≤Q≤2.5 1.0≤Q＜1.5 Q＜1.0 Evaluation level Class I Class II Class III Class IV

[0082] In a preferred embodiment, the lithology data of the target caprock is collected. Combining the logging data and well logging data, through thin section observation, geological description, and mineral composition analysis, it is confirmed that the main lithology types of the caprock are gypsum rock, mudstone, argillaceous sandstone, or sandstone; the permeability is obtained by conducting overburden pore pressure and permeability tests on the caprock samples; the breakthrough pressure of the caprock is obtained by conducting gas breakthrough pressure tests on the caprock samples; according to the confining pressure and upper and lower limit pressures designed for the gas storage reservoir, a cyclic loading and unloading test is conducted on the caprock samples, and after completion, the breakthrough pressure of the caprock samples is measured again to determine the dynamic breakthrough pressure; the specific surface area of the caprock samples is measured by the gas adsorption method; a triaxial compression test is conducted on the caprock samples to obtain the stress-strain curve, and referring to the petroleum and natural gas industry standard SY / T 6942-2013, the plasticity coefficient is calculated.

[0083] In order to introduce the underground gas storage caprock sealing property evaluation method provided by the embodiments of the present invention more clearly and in detail, the following will be described in combination with specific embodiments.

[0084] Example 1

[0085] (1) Collect the lithology data of the target caprock. Combining the logging data and well logging data, through thin section observation, geological description, and mineral composition analysis, confirm the main lithology types of the caprock. There are 3 sets of caprocks developed in the target block, and the lithology assignment L is determined according to Table 1.

[0086] (2) Using the logging data, determine the thickness of the direct caprock of the 3 sets of caprocks, and determine the direct caprock thickness assignment H according to Table 2.

[0087] (3) Conduct overburden pore pressure and permeability tests on the 3 sets of caprock samples to obtain the permeability, and determine the permeability assignment K according to Table 3.

[0088] (4) Conduct gas breakthrough pressure tests on the 3 sets of caprock samples to obtain the breakthrough pressure values of the caprocks, and determine the breakthrough pressure assignment P according to Table 4 b .

[0089] (5) According to the confining pressure and upper and lower limit pressures designed for the gas storage reservoir, 50 cyclic loading and unloading tests were carried out on 3 sets of caprock samples. After completion, the breakthrough pressure of the caprock samples was measured again, and the dynamic breakthrough pressure assignment P was determined according to Table 5. bd 。

[0090] (6) The specific surface area of 3 sets of caprock samples was measured by the gas adsorption method, and the specific surface area assignment S was determined according to Table 6.

[0091] (7) Triaxial compression tests were carried out on 3 sets of caprock samples to obtain the stress-strain curves. Referring to the petroleum and natural gas industry standard SY / T 6942-2013, the plastic coefficient was calculated, and the plastic coefficient assignment T was determined according to Table 7.

[0092] (8) The comprehensive evaluation factor Q of the caprock was calculated using the formula, and the caprock sealing property grade was determined using Table 8. The calculation results are shown in Table 9, and it was found that the sealing property of caprock 2 is the best.

[0093] Table 9 Comprehensive evaluation factor table

[0094]

[0095] In the present invention, we provided an evaluation method more suitable for multi-lithology caprocks than the existing methods through the comprehensive evaluation factor method. To verify the effectiveness of this patent, the actual geological conditions in the study area were used for comparative verification. The results showed that there are 2 reservoir sections developed in the study area. The upper reservoir section is located below caprock 1 and caprock 2, and the lower reservoir section is located below caprock 3. Production data showed that the lower reservoir section is an atmospheric pressure gas reservoir and the upper reservoir section is a superpressure gas reservoir, which is consistent with the relatively poor sealing ability of caprock 3.

[0096] To further verify the feasibility of this method, the caprock of the Dong 3 Member in the Baogu 2 gas storage reservoir was selected for evaluation. The statistical results showed that the lithology of the overlying layer of the reconstructed layer section of this gas storage reservoir is mudstone, and the L assignment is 3; the average thickness of the direct caprock is 90 m, and the H assignment is 4; the average permeability is 0.0065 mD, and the K assignment is 3; the average breakthrough pressure is 8.4 MPa, and the P b assignment is 1, the specific surface area is 56.647 m 2 / g, the S assignment is 4; the dynamic breakthrough pressure is 8.2 MPa, and the P bd assignment is 1; the calculated result of the plastic coefficient is 2.1, and the T assignment is 1. Using the above evaluation formula, Q was calculated to be 2.2, and the caprock sealing type is type II caprock, which is a favorable caprock in the study area and is consistent with the on-site evaluation results. Therefore, this method is more accurate and intuitive in the evaluation of the sealing property of multi-lithology gas storage reservoir caprocks.

Claims

1. A method for evaluating the sealing performance of a cap rock of an underground gas storage reservoir, characterized in that: Comprehensively evaluate the sealing performance of underground gas storage caprock from three aspects: macroscopic, microscopic and mechanical; including: The lithology evaluation step includes assigning values ​​to the lithology of gypsum rock, mudstone, mudstone, and sandstone in order from high to low, which is recorded as L; The step of evaluating the thickness of the direct cap layer includes calculating the thickness of the direct cap layer overlying the reservoir according to the logging data, assigning a value to H, and the higher the thickness of the direct cap layer, the higher the value of H is assigned; The permeability evaluation step includes testing the rock permeability and assigning a value to K, wherein the higher the permeability, the lower the K value; The breakthrough pressure evaluation step includes testing the rock gas breakthrough pressure and b Assign a value, and the higher the breakthrough pressure P b The higher the value assigned; The specific surface area evaluation step includes measuring the specific surface area of ​​the rock and assigning a value to S, wherein the higher the specific surface area, the higher the value of S; The dynamic breakthrough pressure evaluation procedure includes measuring the dynamic breakthrough pressure and bd Assignment, and the higher the dynamic breakthrough pressure P bd The higher the value assigned; The plasticity coefficient evaluation steps include conducting triaxial compression tests, calculating the plasticity coefficient, and assigning a value to T, wherein the higher the plasticity coefficient, the higher the value of T. Comprehensive weight distribution and sealing evaluation steps, using the analytic hierarchy process, respectively determine the weight coefficients of macroscopic sealing, microscopic sealing, and mechanical sealing of the cap rock, and compare the weight coefficients of lithology, direct cap rock thickness, permeability, breakthrough pressure, specific surface area, dynamic breakthrough pressure, and plasticity coefficient. b , S, P bd , T, calculate the comprehensive evaluation factor Q of the caprock sealing performance, and divide the caprock sealing performance into four levels.

2. The method for evaluating the sealing performance of the cap rock of an underground gas storage reservoir according to claim 1, characterized in that: The L values ​​of gypsum rock, mudstone, argillaceous sandstone, and sandstone are assigned as 4, 3, 2, and 1, respectively; When the thickness of the direct cover layer is greater than 50m, H is assigned a value of 4; when the thickness of the direct cover layer is 20-50m, H is assigned a value of 3; when the thickness of the direct cover layer is 10-20m, H is assigned a value of 2; when the thickness of the direct cover layer is less than 10m, H is assigned a value of 1; When the permeability is less than 0.001mD, K is assigned a value of 4; when the permeability is 0.001-0.01mD, K is assigned a value of 3; when the permeability is 0.01-0.1mD, K is assigned a value of 2; when the permeability is 0.1-1mD, K is assigned a value of 1; When the breakthrough pressure is greater than 15 MPa, assign P b When the breakthrough pressure is 12-15 MPa, the value P b When the breakthrough pressure is 10-12 MPa, the value P b When the breakthrough pressure is less than 10 MPa, the value P b is 1; When the specific surface area is greater than 50m 2 / g, the value S is 4, when the specific surface area is 30-50m 2 / g, the value S is 3, when the specific surface area is 10-30m 2 / g, the value S is assigned to 2. When the specific surface area S is less than 10m 2 / g, assign S to 1; When the dynamic breakthrough pressure is greater than 20MPa, assign P bd When the dynamic breakthrough pressure is 15-20MPa, the value P bd When the dynamic breakthrough pressure is 10-15MPa, the value P bd When the dynamic breakthrough pressure is less than 10 MPa, the value P bd is 1; When the plasticity coefficient is greater than 6, the value T is assigned to 4; when the plasticity coefficient is 4-6, the value T is assigned to 3; when the plasticity coefficient is 3-4, the value T is assigned to 2; when the plasticity coefficient is less than 3, the value T is assigned to 1; The analytic hierarchy process was used to first determine the weight coefficients of the macroscopic sealing, microscopic sealing, and mechanical sealing of the cap rock, which were 0.15, 0.75, and 0.10, respectively. The weight coefficients of lithology, direct cap rock thickness, permeability, breakthrough pressure, specific surface area, dynamic breakthrough pressure, and plasticity coefficient were finally determined to be 0.10, 0.05, 0.20, 0.40, 0.15, 0.06, and 0.04, respectively. The comprehensive evaluation factor Q of the cap rock sealing was calculated according to the following formula: Q=0.1×L+0.05×H+0.2×K+0.4×P b +0.15×S+0.06×P bd +0.04×T When Q>2.5, the sealing property of the cap layer is evaluated as Class I; when 1.5≤Q≤2.5, the sealing property of the cap layer is evaluated as Class II; when 1.0≤Q<1.5, the sealing property of the cap layer is evaluated as Class III; when Q<1.0, the sealing property of the cap layer is evaluated as Class IV.

3. The method for evaluating the sealing performance of the cap rock of an underground gas storage reservoir according to claim 1, characterized in that: The lithology data of the target caprock are collected, combined with the logging data and well logging data, and through thin section observation, geological description and mineral composition analysis, it is confirmed that the main lithology types of the caprock are gypsum, mudstone, mudstone sandstone, or sandstone.

4. The method for evaluating the sealing performance of the cap rock of an underground gas storage reservoir according to claim 1, characterized in that: The permeability is obtained by carrying out a pressure-bearing porosity test on the caprock samples.

5. The method for evaluating the sealing performance of the cap rock of an underground gas storage reservoir according to claim 1, characterized in that: A gas breakthrough pressure test is carried out on the cap layer samples to obtain the breakthrough pressure of the cap layer.

6. The method for evaluating the sealing performance of the cap rock of an underground gas storage reservoir according to claim 1, characterized in that: According to the designed confining pressure and upper and lower limit pressures of the gas storage reservoir, a cyclic loading and unloading test is carried out on the cap layer samples. After completion, the breakthrough pressure of the cap layer samples is measured again to determine the dynamic breakthrough pressure.

7. The method for evaluating the sealing performance of the caprock of an underground gas storage reservoir according to claim 1, characterized in that: The specific surface area of ​​the cap layer samples was determined by gas adsorption method.

8. The method for evaluating the sealing performance of the cap rock of an underground gas storage reservoir according to claim 1, characterized in that: Triaxial compression tests were carried out on the caprock samples to obtain stress-strain curves, and the plasticity coefficient was calculated with reference to the petroleum and natural gas industry standard SY / T 6942-2013.

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