A numerical simulation analysis method for mechanical integrity of mid-deep gas storage reservoir caprock considering thermal effect

Abstract

The present application relates to a kind of middle deep gas storage cap layer mechanical integrity numerical simulation analysis method considering thermal effect.Through the three-dimensional reservoir numerical simulation modeling being carried out as a whole to storage, cap layer, using the method of setting seepage barrier, storage, cap layer in model is divided into two big seepage areas, so that storage, cap layer does not occur gas, oil, water seepage transport and formation pressure conduction, but by setting heat transfer parameter, can realize the temperature change of cap layer caused by "cold gas" injection in storage during the injection-production process of gas storage, accurately consider the influence of thermal effect on the earth stress and mechanical integrity of storage, cap layer.Proposed to use the new index of tension safety index with the characteristics of normalization, combined with the dynamic minimum horizontal principal stress of gas storage injection-production process at different time points obtained by numerical simulation calculation, accurately quantifies the evaluation of cap layer tension failure risk, so that the mechanical integrity risk of gas storage cap layer is more accurate and in line with actual formation conditions.

Description

Numerical Simulation Analysis Method for Mechanical Integrity of Cap Layer of Medium-Deep Gas Storage Considering Thermal Effects Technical Field

[0001] This invention relates to the field of underground natural gas storage technology, and more specifically, to a method for ensuring the mechanical integrity of the gas storage tank cap layer while taking into account thermal effects. Background Technology

[0002] The dynamic sealing performance of a gas storage caprock encompasses both microscopic capillary sealing and macroscopic mechanical integrity. Firstly, the cyclic injection and production processes of a gas storage facility disturb the geostress field, causing varying degrees of elastoplastic deformation in the caprock's microporous structure and altering its original capillary sealing performance. Secondly, these disturbances can even lead to localized stress concentrations in the caprock, resulting in macroscopic mechanical failure. Therefore, evaluating the macroscopic mechanical integrity of the caprock is crucial for the efficient and safe operation of a gas storage facility. Cyclic injection and production conditions cause periodic disturbances in the regional geostress field, altering the mechanical properties of the caprock and thus affecting its sealing performance. Therefore, research on the rock mechanical integrity of the caprock under alternating stress constitutes the core of evaluating the sealing capacity of a gas storage caprock and provides a method for assessing the mechanical integrity of the caprock under cyclic stress.

[0003] For the evaluation of the mechanical integrity of the caprock in deep gas storage facilities (under high temperature and high pressure conditions), current assessments mainly fall into two categories: analytical methods, which evaluate the mechanical characteristics of rocks through laboratory rock mechanics experiments, and conventional geomechanical simulations. However, conventional geomechanical simulations do not consider the influence of thermal effects and are generally only applicable to shallow to medium-depth gas storage facilities (where the formation temperature is relatively lower than that of medium to deep reservoirs). In medium to deep gas storage facilities, the near-wellbore temperature difference can reach up to 60°C during gas injection. Therefore, it is necessary to consider the dynamic changes in the geostress field of the gas storage geological body under the influence of temperature and pressure fields during gas injection. Thus, it is essential to use four-dimensional fluid-structure interaction numerical simulation (fluid numerical simulation and mechanical finite element simulation) technology to evaluate the mechanical integrity of the caprock in gas storage models considering thermal effects. Summary of the Invention

[0004] This invention provides a numerical simulation analysis method for the mechanical integrity of the caprock of a medium-deep gas storage reservoir, taking into account thermal effects. By treating the reservoir and caprock as a whole in a three-dimensional numerical simulation model, and using a seepage barrier to divide the reservoir and caprock into two major seepage regions, this method prevents gas, oil, and water seepage and transport, as well as formation pressure transmission, between the reservoir and caprock. However, by setting heat transfer parameters, it can realize the temperature change in the caprock caused by the injection of "cold gas" into the reservoir during the gas storage injection and production process. This accurately considers the impact of thermal effects on the geostress and mechanical integrity of the reservoir and caprock, overcoming the shortcomings of previous methods that could not realistically simulate underground conditions in gas storage reservoirs. This method differs from conventional methods in three ways: First, it establishes a seepage barrier based on a three-dimensional oil and gas reservoir seepage model encompassing the reservoir and caprock. By setting the vertical conductivity of the top grid of the reservoir and the bottom grid of the caprock to 0, it prevents the seepage and transport of gas, oil, and water between the reservoir and caprock, as well as the conduction of formation pressure. However, it simultaneously sets heat transfer parameters for the reservoir and caprock, achieving the goal of preventing seepage transfer but allowing temperature conduction between the reservoir and caprock. This accurately considers the impact of the thermal effect caused by the injection of "cold gas" from the gas storage facility on regional geostress and the mechanical integrity of the caprock. Second, it sets hydrodynamic zones for the reservoir and caprock in the three-dimensional oil and gas reservoir seepage numerical simulation model, making the reservoir an oil reservoir. The presence of gas, water, and three phases, along with the complete filling of formation water in the caprock, ensures that the established three-dimensional oil and gas reservoir seepage numerical simulation accurately reflects the true distribution of underground fluids. Thirdly, a dynamic minimum horizontal principal stress is introduced when evaluating the mechanical integrity of the caprock. This differs from previous studies that the maximum formation pressure during gas injection in gas storage facilities should be less than 80% of the minimum horizontal principal stress (a fixed value). Instead, a new index with normalization characteristics, the tension safety index, is proposed. Combined with the dynamic minimum horizontal principal stress at different time points during the gas storage injection and production process obtained from numerical simulation calculations, the risk of caprock tension failure is accurately quantified, making the mechanical integrity risk of the gas storage caprock more accurate and consistent with actual formation conditions.

[0005] The technical solution provided by this invention is: a numerical simulation analysis method for the mechanical integrity of the caprock of a deep gas storage facility considering thermal effects, comprising the following steps:

[0006] Step 1: Based on the geological characteristics of the target gas storage facility and the dynamics of multi-cycle injection and production operations, a three-dimensional numerical simulation model of oil and gas reservoir seepage covering objects such as reservoir, caprock, and fault is established using dynamic modeling methods.

[0007] Step 2: Using the hydrodynamic equilibrium zoning method, the three-dimensional oil and gas reservoir seepage numerical simulation model is divided into two major seepage regions: reservoir and caprock, so that the caprock is filled with formation water and the reservoir simultaneously contains oil and gas and formation water.

[0008] Step 3: Set up a seepage barrier between the reservoir and caprock to prevent gas, oil, and water seepage and transport, as well as formation pressure transmission, from occurring between the reservoir and caprock during gas storage injection and production and early oil and gas reservoir development.

[0009] Step 4: Utilize the dynamic data of previous oil and gas reservoir development to conduct historical fitting of three-dimensional oil and gas reservoir seepage numerical simulation, and adjust the model multiple times to accurately reflect the actual characteristics of the oil and gas reservoir.

[0010] Step 5: Based on the determined gas storage construction plan, use the wellbore temperature calculation equation to calculate the bottom hole temperature of a typical single well under the average daily gas injection rate.

[0011] Step 6: Based on data from indoor experiments or theoretical research and field tests, assign rock and fluid thermodynamic parameters, initial formation temperature and bottom hole temperature during single-well gas injection to the three-dimensional oil and gas reservoir seepage numerical simulation model, and set heat transfer parameters between the reservoir and caprock regions to calculate the reservoir formation pressure and reservoir and caprock temperature fields at different times of gas injection and production in the gas storage facility.

[0012] Step 7: Based on the three-dimensional oil and gas reservoir seepage numerical simulation model, set the rock mechanical parameters of the reservoir and caprock and the geostress boundary conditions, and establish a three-dimensional dynamic geomechanical model considering thermal effects;

[0013] Step 8: Using a three-dimensional dynamic geomechanical model, calculate the three-dimensional geostress field at different times during multiple injection and production cycles of the gas storage facility;

[0014] Step 9: Based on the tensile and shear failure criteria of the gas storage cap layer, comprehensively analyze the mechanical integrity of the cap layer considering thermal effects.

[0015] The three-dimensional oil and gas reservoir seepage numerical simulation model established in step 1 above based on the dynamic modeling method should have a planar range that is more than three times the size of the oil and gas-bearing area, and vertically it should include the reservoir, caprock, and overlying strata.

[0016] The hydrodynamic balance zoning method in step 2 above involves setting different depths of gas-water or gas-oil and oil-water interfaces in the reservoir and caprock, so that the depth of the gas-water interface in the caprock is lower than the deepest part of the caprock section, thereby ensuring that the caprock is filled with formation water.

[0017] The seepage barrier in step 3 above is set by setting the longitudinal conductivity of the grid at the top of the reservoir and the bottom of the caprock to 0, thereby preventing the seepage and transport of oil, gas, and water between the reservoir and the caprock, as well as the transmission of formation pressure.

[0018] The average daily gas injection rate of a typical single well in the gas storage facility in step 5 above is calculated according to the formula. Calculated;

[0019] in, The average daily gas injection rate per well, 10 4 m 3 / d; For the working gas volume of the gas storage facility, 10 4 m 3 ; The number of days for gas injection into the gas storage facility, expressed in days (d). The number of gas injection wells for the gas storage facility.

[0020] In step 5 above, the bottom hole temperature of a typical single well in the gas storage facility under the average daily gas injection rate is determined according to the formula. Calculated;

[0021] in, The bottom temperature is K; Here, K represents the wellhead temperature. The static temperature gradient of the wellbore is K / m; Let the depth be m; Let m be the relaxation distance, derived from the formula. Calculations show that The mass flow rate of the injected gas is expressed in kg / s. Let be the inner diameter of the tubing in the gas injection well, in meters (m). The relative density of the injected gas is dimensionless. is the wellhead pressure, MPa; c2, c3, c5, and c7 are given calculation parameters with values ​​of 0.4882, -0.3476, 4.724, and 0.2219, respectively; e is the natural constant with a value of 2.71828.

[0022] The mechanical parameters set in step 7 above are obtained through laboratory experiments, well logging, and geomechanical theory. Rock mechanical parameters include Young's elastic modulus, Poisson's ratio, compressive strength, and tensile strength.

[0023] The geostress boundary conditions set in step 7 above are obtained through on-site hydraulic fracturing tests or regional geostress surveys. The geostress boundary conditions include the maximum and minimum horizontal geostress gradients and the direction of the maximum horizontal geostress.

[0024] The tensile failure criterion in step 9 above analyzes the risk of integrity failure of the caprock due to tensile failure by comparing the tensile safety index with 0.8. The tensile safety index is calculated using the formula... Calculated;

[0025] in, To extend the safety index, a dimensionless value is used, with its range of variation between 0 and 1; The formation pressure of the caprock is MPa; The minimum horizontal principal stress of the caprock section is ____ MPa.

[0026] The evaluation of the tensile failure criterion in step 9 above is a dynamic analysis and research process, which involves the minimum horizontal principal stress during the gas storage injection and production operation. It is not a fixed value, but changes dynamically with the changes in formation pressure.

[0027] The shear failure criterion in step 9 above analyzes the risk of integrity failure of the caprock due to shear failure by comparing the magnitude of the shear safety index with 0. The shear safety index is calculated using the formula... Calculated;

[0028] in, The shear safety index is dimensionless and ranges between 0 and 1. Let be the shear stress borne by a certain region of the caprock under any formation pressure, in MPa; is the maximum shear stress that a certain region of the caprock can withstand under any formation pressure, in MPa; Cohesion, MPa; The internal friction angle is °; The maximum effective principal stress is expressed in MPa. The minimum effective principal stress is , MPa.

[0029] The comprehensive evaluation in step 9 above analyzes and evaluates the risk of failure of the mechanical integrity of the caprock by comparing the distribution characteristics of tensile and shear safety indices calculated based on numerical simulations using a three-dimensional geomechanical model. <0.8 and When >0, the cap layer did not experience mechanical integrity failure; when ≥0.8 and When >0, the caprock experiences mechanical integrity failure due to tensile failure; when <0.8 and When =0, the cap layer experiences mechanical integrity failure due to shear failure; when ≥0.8 and When the value is 0, the caprock experiences mechanical integrity failure caused by both tension and shear.

[0030] The beneficial effects of this invention are as follows:

[0031] ① By setting a seepage barrier based on the establishment of a three-dimensional oil and gas reservoir seepage model covering the reservoir and caprock, and by setting the longitudinal conductivity of the grid at the top of the reservoir and the bottom of the caprock to 0, the seepage and transport of gas, oil and water and the transmission of formation pressure between the reservoir and caprock are prevented. However, the heat transfer parameters of the reservoir and caprock are set at the same time, so that the seepage between the reservoir and caprock is not transmitted but the temperature can be conducted. The thermal effect caused by the injection of "cold gas" from the gas storage tank on the regional geostress and the mechanical integrity of the caprock is accurately considered.

[0032] ② By setting hydrodynamic zones for the reservoir and caprock in the three-dimensional oil and gas reservoir seepage numerical simulation model, the reservoir is filled with oil, gas and water phases, while the caprock is completely filled with formation water, so that the established three-dimensional oil and gas reservoir seepage numerical simulation accurately reflects the real state of underground fluid distribution.

[0033] ③ When evaluating the mechanical integrity of the caprock, a dynamic minimum horizontal principal stress is introduced. This differs from previous studies that the maximum formation pressure during gas injection in gas storage facilities should be less than 80% of the minimum horizontal principal stress (a fixed value). Instead, a new index with normalization characteristics, the tension safety index, is proposed. Combined with the dynamic minimum horizontal principal stress at different time points during the gas storage injection and production process obtained through numerical simulation, the risk of caprock tension failure is accurately quantified and evaluated, making the risk of caprock mechanical integrity more accurate and consistent with actual formation conditions. Attached Figure Description

[0034] Figure 1 is a schematic flowchart of the numerical simulation method for the mechanical integrity of the caprock of a deep oil and gas reservoir considering thermal effects, according to an embodiment of the present invention.

[0035] Figure 2 is a geological model of the Nanpu No. 2 gas storage facility according to an embodiment of the present invention;

[0036] Figure 3 is a distribution map of formation water in the Nanbao No. 2 gas storage model according to an embodiment of the present invention;

[0037] Figure 4 is a schematic diagram of the model after setting the seepage barrier according to an embodiment of the present invention;

[0038] Figure 5 shows the daily gas injection of 50×10 in an embodiment of the present invention. 4 m 3 Temperature change diagram at the end of gas injection;

[0039] Figure 6 shows the temperature change at the bottom of the well under different gas injection volumes according to an embodiment of the present invention.

[0040] Figure 7 shows the temperature difference between the well bottom and the formation at different gas injection volumes according to an embodiment of the present invention.

[0041] Figure 8 shows the formation pressure when the gas injection reaches the upper limit pressure of 35 MPa according to an embodiment of the present invention.

[0042] Figure 9 shows the formation pressure when the gas injection reaches the upper limit pressure of 40 MPa according to an embodiment of the present invention.

[0043] Figure 10 is a cross-sectional view of the safety index of the gas injection end tension considering the temperature in an embodiment of the present invention.

[0044] Figure 11 is a cross-sectional view of the safety index of the gas injection end tension without considering temperature in an embodiment of the present invention.

[0045] Figure 12 is a cross-sectional view of the end-of-gas shear safety index considering temperature injection according to an embodiment of the present invention;

[0046] Figure 13 is a cross-sectional view of the end-of-gas shear safety index without considering temperature in an embodiment of the present invention.

[0047] Figure 14 is a comparison chart of the tensile and shear safety index of the present invention with and without considering temperature. Detailed Implementation

[0048] Preferred embodiments of the invention will now be described in more detail. While preferred embodiments of the invention are described below, it should be understood that the invention can be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided so that the invention will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art.

[0049] The specific embodiments of the present invention will be further described in detail below with reference to the accompanying drawings.

[0050] Referring to Figure 1, an embodiment of the present invention provides a numerical simulation analysis method for the mechanical integrity of the caprock of a medium-deep gas storage facility, considering thermal effects, comprising the following steps:

[0051] Step S101: Based on the geological characteristics of the target gas storage facility and the dynamics of multi-cycle injection and production operations, establish a three-dimensional numerical simulation model of oil and gas reservoir seepage, encompassing reservoirs, caprocks, and faults. Specifically:

[0052] ① Compile a basic overview of the oil and gas reservoir, its development and operation dynamics, and obtain information on reservoir depth, reservoir and caprock porosity, permeability, original formation pressure, temperature, etc. Table 1 is a statistical table of basic parameters of the Nanbao No. 2 oil and gas reservoir in the Bohai Rim region used in this embodiment of the invention.

[0053] Table 1. Statistical Table of Basic Parameters of Nanbao No. 2 Oil and Gas Reservoir in the Bohai Rim Region

[0054] ② Based on the collected basic information, a three-dimensional oil and gas reservoir seepage numerical simulation model was established using a dynamic modeling method with numerical simulation software, which includes the reservoir, caprock, and surrounding faults.

[0055] In this embodiment of the invention, the Nanpu No. 2 gas storage facility was modeled according to the data in Table 1. The model has 852,876 grids, and the grid plane size is 40m×40m. Due to the uneven thickness of each sub-layer in the vertical direction, it is set according to different sub-layers. The average grid height of the caprock is 17m, and the average grid height of the reservoir is 10m. The number of grids in the IJK directions are 149, 106, and 54, respectively. The caprock is divided into one layer Ed3 in the K direction with 20 grids and a caprock thickness of 340m. The reservoir is divided into 17 sub-layers with 24 grids and a reservoir thickness of 240m. Figure 2 is a geological model of the Nanpu No. 2 gas storage facility.

[0056] Step S102: Using the hydrodynamic equilibrium zoning method, the three-dimensional oil and gas reservoir seepage numerical simulation model is divided into two major seepage regions: the reservoir and the caprock. This ensures that the caprock is filled with formation water, and the reservoir simultaneously contains both oil and gas and formation water. Specifically:

[0057] ① Hydrodynamic equilibrium partitioning refers to dividing the model into non-interfering regions using numerical simulation methods. The distribution of fluids and pressure in each partition follows dynamic laws. By dividing the model into hydrodynamic equilibrium partitions, the true distribution characteristics of fluids in the formation reservoir and caprock can be accurately simulated.

[0058] ② Divide the model into zones, setting the reservoir and caprock as separate regions.

[0059] In practical applications, the overall model of Nanpu No. 2 gas storage facility is divided into two regions according to the stratigraphic layers: the cap layer is Ed3, and the remaining layers (Es1I1-Es1III3) are the reservoir sections.

[0060] ③ Based on the data collected in step S101, set the reservoir gas-water interface so that the reservoir contains oil and gas and bound water, and the bound water and saturated oil in the reservoir are consistent with the real underground environment.

[0061] ④ Set a gas-water interface in the caprock and set the interface depth in the upper part of the caprock so that the caprock is filled with formation water, that is, the fluid in the caprock is 100% water.

[0062] In practical applications, the gas-water interface is set in the established geological model of Nanpu No. 2 gas storage (hereinafter referred to as Nanpu No. 2 gas storage model). The reservoir section (Es1I1-Es1III3) is input with -4130, and the caprock section (Ed3) is input with -3600. Figure 3 shows the distribution of formation water in the Nanpu No. 2 gas storage model.

[0063] Step S103: A seepage barrier is installed between the reservoir and caprock to prevent gas, oil, and water seepage and transport, as well as formation pressure transmission, between the reservoir and caprock during gas storage injection and production and early-stage oil and gas reservoir development. Specifically:

[0064] ① A seepage barrier is a mechanism in numerical simulation models where the longitudinal conductivity at the interface between the reservoir and caprock is set to 0 using software, thus hindering the movement of fluids between the two regions to achieve the effect of a seepage barrier.

[0065] ② Set the longitudinal conductivity for the reservoir and caprock regions. The longitudinal conductivity of the top layer of the reservoir region is 0, and that of the interior is 1, so that the fluid in the reservoir can move freely but not upward. The longitudinal conductivity of the bottom layer of the caprock region is 0, so that the fluid at the bottom of the caprock does not move downward.

[0066] In practical applications, for the Nanpu No. 2 gas storage model, the longitudinal conductivity multiplier of the overall model is first set to 1 to ensure that the longitudinal conductivity of the overall model is not changed when setting the seepage barrier later. Then, the reservoir area is selected separately, and the longitudinal conductivity multiplier of the topmost grid of the reservoir is set to 0. Similarly, the same operation is performed on the bottommost grid of the caprock, setting the longitudinal conductivity multiplier of the bottommost grid of the caprock to 0. Figure 4 is a schematic diagram of the model with the seepage barrier set.

[0067] Step S104: Using previous dynamic data on oil and gas reservoir development, conduct historical fitting of three-dimensional oil and gas reservoir seepage numerical simulation, and adjust the model multiple times to accurately reflect the actual characteristics of the oil and gas reservoir.

[0068] ①History fitting refers to adjusting uncertain parameters in the model (such as permeability, porosity distribution, fault conductivity, relative permeability curve, etc.) to make the simulation results consistent with actual production data (such as pressure, yield, moisture content, etc.), thereby verifying the reliability of the model.

[0069] In practical applications, the permeability of each grid in the Nanpu No. 2 gas storage facility is adjusted according to historical dynamic data. After multiple rounds of adjustments to the Nanpu No. 2 gas storage facility model, it accurately reflects the actual characteristics of oil and gas reservoir development.

[0070] Step S105: Based on the determined gas storage construction plan, calculate the bottom hole temperature of a typical single well at the average daily gas injection rate using the wellbore temperature calculation equation. Specifically:

[0071] ① Based on the determined injection and production plan, determine the total working gas volume, number of injection days, number of injection wells, and clarify the average daily gas injection volume of a single well.

[0072] In practical application, the Nanbao No. 2 gas storage facility is designed and deployed with 26 wells participating in injection and production operations, including 10 injection wells and 26 production wells. Based on the key parameters determined by the storage construction plan, the working gas volume of the Nanbao No. 2 gas storage facility is 10 × 10⁻⁶. 8 m 3 The gas storage facility has an injection period of approximately 200 days, and the calculated average daily injection volume per well is approximately 50 × 10⁻⁶. 4 m 3 Figure 5 shows the daily gas injection of 50 × 10⁻⁶. 4 m 3 Diagram showing the temperature change at the bottom of the well after gas injection.

[0073] ② The temperature of the gas injected to the bottom of the well at the daily gas injection rate of a single well is calculated using the pipe flow formula, and the temperature difference between the gas and the formation is obtained.

[0074] In practical applications, according to the formula The bottom temperature was calculated.

[0075] in, The bottom temperature is K; Here, K represents the wellhead temperature. The static temperature gradient of the wellbore is K / m; Let the depth be m; Let m be the relaxation distance, derived from the formula. Calculations show that The mass flow rate of the injected gas is expressed in kg / s. Let be the inner diameter of the tubing in the gas injection well, in meters (m). The relative density of the injected gas is dimensionless. is the wellhead pressure, MPa; c2, c3, c5, and c7 are given calculation parameters with values ​​of 0.4882, -0.3476, 4.724, and 0.2219, respectively; e is the natural constant with a value of 2.71828.

[0076] Figure 6 shows the temperature change at the bottom of the well with different gas injection volumes, and Figure 7 shows the temperature difference between the bottom of the well and the formation with different gas injection volumes.

[0077] Step S106: Based on data from indoor experiments, theoretical research, and field tests, the rock and fluid thermodynamic parameters, initial formation temperature, and bottom-hole temperature during single-well gas injection are assigned to the three-dimensional oil and gas reservoir seepage numerical simulation model. Heat transfer parameters between the reservoir and caprock regions are also set, and the reservoir formation pressure and reservoir / caprock temperature fields at different times during gas injection and production are calculated. Specifically:

[0078] ① Obtain the thermodynamic parameters of rocks and fluids through indoor experiments or based on relevant basic research. Table 2 shows the values ​​of rock thermodynamic parameters that need to be input into the Nanpu No. 2 gas storage model, and Table 3 shows the thermodynamic parameters of the fluids during model simulation.

[0079] Table 2. Rock thermodynamic parameters

[0080]

[0081] Table 3 Thermodynamic parameters of the model fluid

[0082] ② Add thermal effects to the pre-set model scheme and set the required keywords, including: rock thermal conductivity, rock specific heat capacity, rock thermal conductivity coefficient, rock thermal expansion coefficient, fluid specific heat capacity, formation temperature, and temperature of injected fluid to the bottom of the well. Then perform simulation in the software.

[0083] In practical applications, select the simulation scheme that requires temperature setting, and first set relevant keywords related to rock thermal effects. Keywords added for rock thermal effects include rock thermal conductivity of 160 kJ·day. -1 ·m -1 ·K-1 The rock's specific heat capacity is 0.84 kJ·kg⁻¹ -1 ·K -1 The thermal conductivity of the rock is 1.82 W·m. -1 ·K -1 The coefficient of thermal expansion of the rock is 7.5 × 10⁻⁶. -6 K -1 Then, set keywords related to fluid thermal effects, including the specific heat capacity of petroleum (2.1 kJ·kg⁻¹). -1 ·K -1 Natural gas specific heat capacity: 2.5 kJ·kg -1 ·K -1 Specific heat capacity of water: 4.2 kJ·kg -1 ·K -1 Then, the formation temperature was set to 157℃, and finally, the temperature of the injected fluid at the bottom of the well was set to 80℃.

[0084] Step S107: Based on the three-dimensional oil and gas reservoir seepage numerical simulation model, set the reservoir and caprock rock mechanical parameters and geostress boundary conditions, and establish a three-dimensional dynamic geomechanical model considering thermal effects. Specifically:

[0085] ① The Young's modulus, Poisson's ratio, compressive strength, and tensile strength of reservoir sandstone and caprock mudstone were obtained through indoor experiments, well logging, and geomechanical theory.

[0086] In specific applications, 18 typical core samples were obtained from the Nanpu No. 2 gas storage facility, including 10 mudstone samples and 8 sandstone samples. Uniaxial compression tests (8 mudstone samples and 6 sandstone samples) and Brazilian splitting tests (2 mudstone samples and 2 sandstone samples) were conducted. The experimental results showed that the strength of mudstone was between 47 MPa and 59 MPa, with an average uniaxial compressive strength of 59.46 MPa, an elastic modulus of 10.41 GPa, a Poisson's ratio of 0.09, and an average tensile strength of 2.54 MPa. The uniaxial compressive strength of sandstone was between 39 MPa and 48 MPa, with an average uniaxial compressive strength of 43.56 MPa, an elastic modulus of 11.25 GPa, a Poisson's ratio of 0.24, and an average tensile strength of 1.79 MPa.

[0087] ② Obtain geostress boundary conditions, including the maximum and minimum horizontal geostress gradients and the direction of the maximum horizontal geostress, through hydraulic fracturing or regional geostress surveys.

[0088] In practical applications, the average orientation of the minimum horizontal principal stress measured in the old well NP3659 of the Nanpu No. 2 oil and gas reservoir is north-southwest-east, while the orientation of the maximum horizontal principal stress is north-southeast-west. The stress gradients of the maximum and minimum horizontal principal stresses are 1.67 MPa / 100m and 1.50 MPa / 100m, respectively, and the stress gradient of the vertical stress is 2.21 MPa / 100m.

[0089] ③ Set the mechanical parameters of the reservoir and caprock, and set the horizontal principal stress gradient and direction in the stress boundary conditions.

[0090] In practical applications, the mechanical parameters of sandstone and mudstone obtained through experiments and surveys in the first two steps, along with the maximum and minimum horizontal principal stress gradients and the minimum horizontal principal stress angle, are input into the model.

[0091] Step S108: Using a three-dimensional dynamic geomechanical model, calculate the three-dimensional geostress field at different times during multiple injection and production cycles of the gas storage facility. Specifically:

[0092] Mechanical modeling is performed on the model, and the mechanical parameters obtained in step S107 are added to the reservoir and caprock respectively. The numerical simulation calculation time nodes are set, and the required stress fields at different times are output.

[0093] In practical applications, the mechanical parameters obtained in step S107 are added to the reservoir and caprock, and a numerical modeling strategy is added to output the required stress fields at different times. Figure 8 shows the formation pressure diagram when the gas injection reaches the upper limit pressure of 35 MPa, and Figure 9 shows the formation pressure diagram when the gas injection reaches the upper limit pressure of 40 MPa.

[0094] Step S109: Based on the tensile and shear failure criteria of the gas storage caprock, comprehensively analyze the mechanical integrity of the caprock considering thermal effects. Specifically:

[0095] ① Using the scheme calculated in step S108, calculate and output the formation pressure, vertical principal stress, and maximum and minimum horizontal principal stress.

[0096] ② Calculate the tensile and shear safety indices. The formula for calculating the tensile safety index is as follows: .

[0097] in, To extend the safety index, a dimensionless value is used, with its range of variation between 0 and 1; The formation pressure of the caprock is MPa; The minimum horizontal principal stress of the caprock is given in MPa.

[0098] Shear safety index is .

[0099] in, The shear safety index is dimensionless and ranges between 0 and 1. Let be the shear stress borne by a certain region of the caprock under any formation pressure, in MPa; is the maximum shear stress that a certain region of the caprock can withstand under any formation pressure, in MPa; Cohesion, MPa; The internal friction angle is °; The maximum effective principal stress is expressed in MPa. The minimum effective principal stress is , MPa.

[0100] In practical applications, the tensile safety index of the model at different times is calculated by inputting the pressure and minimum horizontal principal stress. The shear safety index at different times is calculated by inputting the cohesion, internal friction angle, maximum horizontal principal stress and minimum horizontal principal stress. Figure 10 is a cross-sectional view of the tensile safety index after gas injection considering temperature, Figure 11 is a cross-sectional view of the tensile safety index after gas injection without considering temperature, Figure 12 is a cross-sectional view of the shear safety index after gas injection considering temperature, and Figure 13 is a cross-sectional view of the shear safety index after gas injection without considering temperature.

[0101] ③Analyze the simulation results comprehensively.

[0102] By comparing the distribution characteristics of tensile and shear safety indices calculated based on numerical simulations using a three-dimensional geomechanical model, the risk of caprock mechanical integrity failure is analyzed and evaluated. <0.8 and When >0, the cap layer did not experience mechanical integrity failure; when ≥0.8 and When >0, the caprock experiences mechanical integrity failure due to tensile failure; when <0.8 and When =0, the cap layer experiences mechanical integrity failure due to shear failure; when ≥0.8 and When the value is 0, the caprock experiences mechanical integrity failure caused by both tension and shear.

[0103] In practical applications, a set of numerical simulation comparison schemes without considering temperature is first conducted. Regarding the caprock's tensile strength, the average tensile safety index of the original reservoir is 0.61, and that of the caprock is 0.71. Simulated injection and production (200 days of gas injection, daily gas injection rate of 50 × 10⁻⁶) is then performed. 4 m 3 After 20 years, the top of the reservoir will decrease to around 0.54. The final formation pressure at the gas storage injection point is approximately 40 MPa. The minimum horizontal stress distribution in the reservoir and caprock is between 60 and 70 MPa. The reservoir tensile safety index is between 0.63 and 0.67, and the average caprock tensile safety index is 0.72. Substituting these values ​​into the formula... The calculated ultimate bearing capacity of the caprock is approximately 45.08 MPa. Regarding the shear safety index, the average shear safety index of the original reservoir is 0.58, and that of the caprock is 0.41. At the end of gas injection at the Nanpu No. 2 gas storage facility, the reservoir shear index ranged from 0.35 to 0.52, while the minimum shear index of the caprock was approximately 0.37.

[0104] Based on the temperature considerations of this invention, the daily gas injection rate is 50 × 10 4 m 3Tensile safety index analysis was conducted on the gas storage reservoir and caprock. The caprock section showed a relatively high tensile safety index, with the highest reaching 50 × 10⁻⁶ per day injection rate. 4 m 3 The value reached 0.78. From the comparison scheme, it was found that, without considering temperature, the daily gas injection volume was 50 × 10⁻⁶. 4 m 3 The tensile safety index of the caprock is approximately 0.71. Considering temperature, the tensile safety index during the reservoir-caprock injection-production stage is higher than that without considering temperature. The calculated ultimate bearing capacity of the caprock's mechanical failure is approximately 41.03 MPa, which is about 4 MPa lower than the ultimate bearing capacity ignoring thermal effects. The decrease in the minimum horizontal principal stress reduces the rock's tensile failure resistance. Regarding shear safety, at a daily gas injection rate of 50 × 10⁻⁶... 4 m 3 The shear safety index of the cover layer is about 0.22. In the comparison scheme, the shear safety index of the cover layer without considering temperature is about 0.37 under the same daily gas injection volume. The shear safety index is significantly lower. Considering temperature, the shear safety is also greatly affected. Figure 14 is a comparison of the tensile and shear safety indices with and without considering temperature.

[0105] By comparing the tensile and shear safety indices with and without considering temperature, Figures 11, 13, and 14 show that the tensile and shear safety indices within the well-controlled area remain essentially unchanged. Without considering temperature, the tensile and shear safety indices are 0.71 and 0.37, respectively. Therefore, the conventional mechanical integrity method (without considering temperature) cannot reflect the impact of the interaction between the injection well bottom temperature and formation temperature within the well-controlled area of ​​a medium-deep gas storage facility on the in-situ stress, tensile, and shear safety indices. It only reflects the single factor of the gas storage facility's injection-production formation pressure disturbance on the regional in-situ stress and caprock mechanical integrity. However, during actual gas storage facility operation, the injection well bottom temperature causes a temperature drop in the formation temperature within the well-controlled area, and the in-situ stress, tensile, and shear safety indices are simultaneously affected by the injection-production pressure field and thermal stress. Figures 10, 12, and 14 show that the tensile and shear safety indices within the well control area are simultaneously affected by formation pressure disturbances and thermal stress. The tensile safety index within the well control area increases significantly, while the shear safety index decreases significantly. Considering temperature, the tensile and shear safety indices are 0.78 and 0.22, respectively, representing increases of 9.86% and decreases of 40.54%. Therefore, conventional numerical simulation analysis methods that do not consider thermal effects will significantly underestimate the risk of mechanical integrity failure of the gas storage caprock, leading to incorrect injection and production operation strategies, and even inducing mechanical integrity failure and gas leakage in the gas storage caprock. However, considering thermal effects, the evaluation results are more consistent with the characteristics of horizontal principal stress reduction caused by the cooling effect after "cold gas" is injected into the reservoir in medium-deep gas storage. This allows for accurate early warning and evaluation of the risk of mechanical integrity failure of the caprock under the dual effects of formation pressure disturbances and thermal stress, significantly reducing the risk of safe operation of the gas storage facility.

Claims

1. A numerical simulation analysis method for the mechanical integrity of the caprock of a medium-deep gas storage facility considering thermal effects, characterized in that, Includes the following steps; S1: Based on the geological characteristics of the target gas reservoir and the dynamics of multi-cycle injection and production operations, a three-dimensional oil and gas reservoir seepage numerical simulation model covering reservoir, caprock, and fault objects is established using dynamic modeling methods. S2: Using a hydrodynamic balance zoning method, the three-dimensional oil and gas reservoir seepage numerical simulation model is divided into two major seepage regions: reservoir and caprock, ensuring the caprock is filled with formation water and the reservoir simultaneously contains oil and gas as well as formation water. S3: Seepage barriers are set between the two major seepage regions of reservoir and caprock to prevent gas, oil, and water seepage and transport, as well as formation pressure transmission, between the reservoir and caprock during gas reservoir injection and production and early oil and gas reservoir development. S4: Using early oil and gas reservoir development dynamic data, historical fitting of the three-dimensional oil and gas reservoir seepage numerical simulation is performed, and the model is adjusted multiple times to accurately reflect the actual characteristics of the oil and gas reservoir. S5: Based on the determined gas reservoir construction plan, the wellbore temperature calculation equation is used to calculate the bottom hole temperature of a typical single well under the average daily gas injection rate. S6: Based on indoor experiments or theoretical research and field test data, rock and fluid thermodynamic parameters, initial formation temperature, and bottom-hole temperature during single-well gas injection are assigned to the three-dimensional oil and gas reservoir seepage numerical simulation model. Heat transfer parameters between the reservoir and caprock regions are also set, and the reservoir formation pressure and reservoir and caprock temperature fields at different injection and production times are calculated. S7: Based on the three-dimensional oil and gas reservoir seepage numerical simulation model, reservoir and caprock rock mechanical parameters and geostress boundary conditions are set to establish a three-dimensional dynamic geomechanical model considering thermal effects. S8: Using the three-dimensional dynamic geomechanical model, the three-dimensional geostress field at different injection and production times during multiple cycles of the gas reservoir is calculated. S9: Based on the tensile and shear failure criteria of the gas storage cap layer, comprehensively analyze the mechanical integrity of the cap layer when considering thermal effects.

2. The numerical simulation analysis method for the mechanical integrity of the caprock of a medium-deep gas storage facility considering thermal effects, as described in claim 1, is characterized in that... The three-dimensional oil and gas reservoir seepage numerical simulation model established in step 1 based on the dynamic modeling method should have a planar range that is more than three times the size of the oil and gas-bearing area, and vertically it should include the reservoir, caprock, and overlying strata.

3. The numerical simulation analysis method for the mechanical integrity of the caprock of a medium-deep gas storage facility considering thermal effects, as described in claim 1, is characterized in that... The hydrodynamic balance zoning method in step 2 involves setting different depths of gas-water or gas-oil and oil-water interfaces in the reservoir and caprock, respectively, so that the depth of the gas-water interface in the caprock is lower than the deepest part of the caprock section, thereby ensuring that the caprock is filled with formation water.

4. The numerical simulation analysis method for the mechanical integrity of the caprock of a medium-deep gas storage facility considering thermal effects, as described in claim 1, is characterized in that... The seepage barrier in step 3 is set by setting the longitudinal conductivity of the grid at the top of the reservoir and the bottom of the caprock to 0, thereby preventing the seepage and transport of oil, gas, and water between the reservoir and the caprock, as well as the transmission of formation pressure.

5. The numerical simulation analysis method for the mechanical integrity of the caprock of a medium-deep gas storage facility considering thermal effects, as described in claim 1, is characterized in that... Step 5: The average daily gas injection rate of a typical single well in the gas storage facility is calculated according to the formula. Calculated; where, This represents the average daily gas injection rate per well, in units of 10. 4 m 3 / d; The working gas volume of the gas storage facility is expressed in units of 10. 4 m 3 ; The number of days for gas injection into the gas storage facility, expressed in days (d). This refers to the number of gas injection wells in the gas storage facility, expressed in wells.

6. The numerical simulation analysis method for the mechanical integrity of the caprock of a medium-deep gas storage facility considering thermal effects, as described in claim 1, is characterized in that... Step 5: Bottom-hole temperature of a typical single well in the gas storage facility under the average daily gas injection rate, according to the formula... Calculated; where, This refers to the bottom-hole temperature, expressed in Kelvin (K). This refers to the wellhead temperature, expressed in Kelvin (K). The static temperature gradient in the wellbore is expressed in K / m. The depth is measured in meters (m). The relaxation distance, in meters, is given by the formula. Calculations show that The mass flow rate of the injected gas is expressed in kg / s. The inner diameter of the tubing in the gas injection well is in meters (m). The relative density of the injected gas is dimensionless. is the wellhead pressure in MPa; c2, c3, c5, and c7 are given calculation parameters with values ​​of 0.4882, -0.3476, 4.724, and 0.2219, respectively; e is the natural constant with a value of 2.71828.

7. The numerical simulation analysis method for the mechanical integrity of the caprock of a medium-deep gas storage facility considering thermal effects, as described in claim 1, is characterized in that... In step 7: the mechanical parameters set are obtained through indoor experiments, well logging, or geomechanical theory. The rock mechanics parameters include Young's elastic modulus, Poisson's ratio, compressive strength, and tensile strength. The geostress boundary conditions set are obtained through on-site hydraulic fracturing tests or regional geostress surveys. The geostress boundary conditions include the maximum and minimum horizontal geostress gradients and the direction of the maximum horizontal geostress.

8. The numerical simulation analysis method for the mechanical integrity of the caprock of a medium-deep gas storage facility considering thermal effects, as described in claim 1, is characterized in that... In step 9: the tensile failure criterion analyzes the risk of integrity failure of the caprock due to tensile failure by comparing the tensile safety index with 0.

8. The tensile safety index is calculated using the formula... Calculated; where, To extend the safety index, it is dimensionless, and its range is between 0 and 1; This refers to the formation pressure of the caprock, expressed in MPa. The minimum horizontal principal stress of the caprock is expressed in MPa. The evaluation of the tensile failure criterion is a dynamic analysis process, involving the minimum horizontal principal stress during the gas storage injection and production operation. It is not a fixed value, but changes dynamically with the changes in formation pressure.

9. The numerical simulation analysis method for the mechanical integrity of the caprock of a medium-deep gas storage facility considering thermal effects, as described in claim 1, is characterized in that... The shear failure criterion in step 9 analyzes the risk of integrity failure of the caprock due to shear failure by comparing the magnitude of the shear safety index with 0. The shear safety index is calculated using the formula... Calculated; where, The shear safety index is dimensionless and ranges between 0 and 1. The shear stress borne by a certain region of the caprock under any formation pressure is expressed in MPa. This represents the maximum shear stress that a certain region of the caprock can withstand under any formation pressure, expressed in MPa. Cohesion, measured in MPa; The internal friction angle is expressed in degrees (°). The maximum effective principal stress is expressed in MPa. The minimum effective principal stress is expressed in MPa.

10. The numerical simulation analysis method for the mechanical integrity of the caprock of a medium-deep gas storage facility considering thermal effects, as described in claim 1, is characterized in that... Step 9, the comprehensive evaluation, analyzes and evaluates the risk of failure of the caprock's mechanical integrity by comparing the distribution characteristics of tensile and shear safety indices calculated based on numerical simulations using a three-dimensional geomechanical model. <0.8 and When >0, the cap layer did not experience mechanical integrity failure; when ≥0.8 and When the value is greater than 0, the caprock fails due to tensile stress, resulting in mechanical integrity failure. when <0.8 and When the value is 0, the cap layer fails to maintain its mechanical integrity due to shear failure. when ≥0.8 and When the value is 0, the caprock experiences mechanical integrity failure caused by both tension and shear.

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