A stability evaluation method for converting an old salt mine cavity with inclined interlayers into a gas storage facility

Abstract

The invention discloses a stability evaluation method for converting an old cavity of a salt mine with inclined interlayer into a gas storage reservoir, which belongs to the field of resource utilization of abandoned salt mines. The method firstly uses a three-dimensional seismic exploration technology to determine the geological structure characteristics of the rock layer area where the old cavity of the salt mine is located. The three-dimensional morphology of the salt cavity of the vertical well and the inclined well of the old cavity of the salt mine is measured to obtain a three-dimensional image and volume of the old cavity of the salt mine. Then, a data well is drilled in the near field outside the old cavity of the salt mine, and the core of the data well is extracted for testing to obtain the physical and mechanical parameters of salt rock, mudstone and interlayer. A geomechanical numerical model of the old cavity of the salt mine is established. Finally, the dynamic stability of the gas storage reservoir under different working gas pressures and operating parameters is evaluated. The method realizes a quantitative evaluation of the stability of the gas storage reservoir converted from the old cavity of the docking well with inclined interlayer, and optimizes reasonable storage capacity parameters to improve the stability of the gas storage reservoir converted from the old cavity of the docking well.

Description

Technical Field

[0001] The invention belongs to the field of resource utilization of abandoned salt mines, and particularly relates to a stability evaluation method for converting an old salt mine cavity with inclined interlayers into a gas storage reservoir. Background Art

[0002] Abandoned old cavities in salt mines are generally docking wells, consisting of a vertical well and an inclined well. The wells are drilled into the target salt rock layer at a distance of 250m to 350m on the surface. The inclined wells are docked with the vertical wells through horizontal channels (the length of the channel is about 150m to 200m) to form a through U-shaped channel. However, a large number of disordered inclined interlayers make the salt cavities mostly deformed cavities with complex structures, and a large amount of residues are accumulated at the bottom of the cavities. When using such salt cavities to store oil and natural gas, it is crucial to ensure their safety and stability. At the same time, since abandoned salt cavities are generally located hundreds of meters or even thousands of meters underground, the situation is a hidden protection project and lacks effective and real-time monitoring methods. This also brings huge technical challenges to predicting the instability and destruction of salt cavities. At present, the safety and stability evaluation of old cavities in salt mines with inclined interlayers mainly has the following technical difficulties:

[0003] (1) Lack of stability evaluation of the structural morphology of the salt mine cavity, especially the accumulated sediment at the bottom of the cavity. Salt chemical enterprises generally use the method of horizontal connection of vertical wells and inclined wells to inject water and extract brine. The sediment formed collapses to the bottom of the cavity and cannot be detected by sonar technology, which brings a lot of uncertainty to the stability evaluation of the rebuilt gas storage reservoir.

[0004] (2) There is a lack of a list of mechanical parameters of salt mines and salt rocks required in the stability evaluation process. The mechanical parameters of salt rock, mudstone and interlayers are involved in the stability evaluation of salt cavern gas storage, and the mechanical properties of collapsed interlayers also change after long-term immersion in brine. The mechanical parameters of these rock properties need to be obtained through on-site drilling and coring for indoor experiments, and the required experimental time is relatively long. If a detailed and accurate list of parameters cannot be given before the stability evaluation of the gas storage, it will seriously affect and delay the development of the stability evaluation of the gas storage.

[0005] (3) There is a lack of stability evaluation index system applicable to old cavities in salt mines. There are many types of stability failures in gas storage salt cavities, including ductile failure, brittle failure, casing seal failure, and cavity airtightness failure (caused by cavity wall instability). This makes it impossible for conventional stability evaluation index systems to accurately predict the stability failure of abandoned old cavities for reconstruction of gas storage, nor to accurately optimize the storage capacity parameters of gas storage.

[0006] Based on the above reasons, it is urgent to invent a method for evaluating the stability of gas storage converted from old salt mine cavities with inclined interlayers, so as to solve the current difficulties in the stability evaluation process of salt cavern gas storages and provide technical guarantees for ensuring the safety of abandoned salt mine old cavity gas storages. Summary of the invention

[0007] In view of the deficiencies in the prior art, the present invention provides a stability evaluation method for converting an old salt mine cavity with an inclined interlayer into a gas storage reservoir.

[0008] A stability evaluation method for converting an old salt mine cavity with inclined interlayers into a gas storage reservoir specifically comprises the following steps:

[0009] Step 1: Use 3D seismic exploration technology to determine the geological structural characteristics of the rock layer where the old cavity of the salt mine is located;

[0010] The geological structural characteristics include: the sedimentary environment of the salt-bearing layer where the old cavity is located, the distribution of fault structures, the distribution characteristics of salt rocks, the characteristics of the top and bottom plates of the salt layer, and the distribution characteristics of interlayers;

[0011] Among them, the distribution characteristics of salt rocks include: thickness, grade and physical and chemical properties of salt layers;

[0012] Step 2: Perform three-dimensional morphological measurement on the salt cavities of the vertical well and the inclined well of the old cavity of the salt mine to obtain the three-dimensional image and volume of the old cavity of the salt mine;

[0013] The three-dimensional morphology measurement is detected by using a sonar instrument;

[0014] The specific steps of sonar detection are as follows:

[0015] S1: Place the sonar probe on a certain depth level in the salt cavity to emit sound pulses to the wall of the salt cavity, detect the echo signal, and transmit the signal back to the ground computer system to obtain the horizontal measurement distance of the cavity at a certain depth;

[0016] S2: Change the measurement depth to obtain the horizontal measurement distance of the salt mine cavity at different depths;

[0017] S3: Perform tilt measurements on the top, bottom and abnormal part of the salt mine cavity to obtain the measured distances at different tilt angles; the abnormal part is the irregular contour of the cavity wall with local cantilever and finger-like protrusion caused by the tilted interlayer;

[0018] S4: Using sonar ranging imaging analysis software, obtain the three-dimensional image and volume of the entire vertical well and inclined well cavity;

[0019] Step 3: Drill a data well near the old cavity of the salt mine, extract the core of the data well for testing, and obtain the physical and mechanical parameters of salt rock, mudstone and interlayer;

[0020] The test contents of the core of the data well include: density test, uniaxial compressive strength test, tensile strength test, internal friction angle test, cohesion test, steady-state creep rate test and breakthrough pressure test, permeability test and porosity test; among them, salt rock is subjected to all test contents; mudstone and interlayer are subjected to tests other than breakthrough pressure test;

[0021] Step 4: Establish a geomechanical numerical model of the old cavity of the salt mine;

[0022] Step 4.1: Based on step 2, the three-dimensional morphology of the salt cavity of the vertical well and the inclined well of the old cavity of the salt mine is measured, and the three-dimensional morphology of the sediment space of the docking well is constructed in combination with the characteristics of the salt-bearing strata, the drilling trajectory and the mining history; the characteristics of the salt-bearing strata, the drilling trajectory and the mining history are provided by the salt chemical enterprise;

[0023] Step 4.2: Check the rationality of the three-dimensional shape of the sediment space by the amount of salt mined and make corrections. If the difference is large, the volume occupied by the sediment in the cavity needs to be adjusted; the amount of salt mined is provided by the salt chemical enterprise; the rationality needs to be calculated, such as the volume formed by the cavity should be equivalent to the volume converted from the total amount of salt mined by the salt chemical enterprise over the years; the mass of pure salt rock is obtained by the concentration and displacement of brine, and then compared with the density of mineral salt to obtain the mined salt volume, and the rationality is determined based on the cavity volume being the same as the obtained salt mined volume;

[0024] Step 4.3: Establish a three-dimensional geomechanical numerical model: The model is a cuboid, the length of each side of the cuboid should be no less than 5 times the maximum diameter of the salt cavity in each side length direction, the inclined interlayer is located in the middle of the model, the docking well is approximately located in the center of the cuboid, and the cavity is watertight;

[0025] Step 4.4: The surface of the abandoned old cavity wall is subjected to uniform internal pressure; the 3D geomechanical model applies initial geostress, and the top surface of the model is loaded with the average gravity of the overlying rock layer; horizontal constraints are applied to the four vertical surfaces of the 3D geomechanical model to limit the horizontal deformation of the model; fixed constraints are applied to the bottom of the 3D geomechanical model to limit horizontal and vertical deformation;

[0026] Step 5: Evaluate the dynamic stability of the gas storage under different working pressures and operating parameters;

[0027] Step 5.1: Use the established three-dimensional geomechanical model to simulate and calculate the stress, deformation, damage zone, and seepage pressure of the gas storage reservoir surrounding rock;

[0028] Step 5.2: Determine the reservoir capacity parameters, including working pressure, cycle period and maximum gas production rate, and conduct numerical simulation analysis of the static state of the cavity, the internal pressure operation of the cycle injection and production, and the numerical simulation analysis of ground subsidence;

[0029] Step 5.3: According to the calculation formulas of dilatancy safety factor, equivalent strain, deformation, volume shrinkage, plastic zone, and natural gas seepage range, the calculated stress, deformation, and seepage pressure are plotted into a cloud map;

[0030] Step 5.4: Evaluate the stability of the gas storage reservoir based on the critical safety values ​​of the shear safety factor, equivalent strain, deformation, volume shrinkage, plastic zone, and natural gas seepage range, and adjust and optimize the gas storage reservoir capacity parameters based on the evaluation results.

[0031] Compared with the prior art, the present invention has the following beneficial effects: it is suitable for the stability evaluation of the gas storage rebuilt from the old cavity of abandoned salt mines; the evaluation steps are simple, the evaluation indicators are clear, and the operability is strong. It can realize the quantitative evaluation of the stability of the gas storage rebuilt from the old cavity of the docking well with inclined interlayers, optimize the reasonable storage capacity parameters to improve the stability of the gas storage rebuilt from the old cavity of the docking well, and at the same time, it can also give reasonable measures according to the stability evaluation results to prevent the possible operational instability and damage of the gas storage, which is more in line with the actual engineering geological conditions. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 The present invention is a schematic flow chart of a method for analyzing the stability of an abandoned salt mine old cavity with inclined interlayers converted into a gas storage reservoir.

[0033] Figure 2 This is a schematic diagram of a three-dimensional numerical model of a gas storage facility converted from an old salt mine cavity according to an embodiment of the present invention.

[0034] Figure 3 It is a simplified plan view of the mechanical boundary constraint model of the gas storage reservoir according to an embodiment of the present invention. DETAILED DESCRIPTION

[0035] The present invention will be further described below in conjunction with the accompanying drawings and embodiments;

[0036] A stability evaluation method for converting an old salt mine cavity with inclined interlayers into a gas storage reservoir is shown in the attached figure. Figure 1 As shown, the specific steps include:

[0037] Step 1: Use 3D seismic exploration technology to determine the geological structural characteristics of the rock layer where the old cavity of the salt mine is located;

[0038] The geological structural characteristics include: the sedimentary environment of the salt-bearing layer where the old cavity is located, the distribution of fault structures, the distribution characteristics of salt rocks, the characteristics of the top and bottom plates of the salt layer, and the distribution characteristics of interlayers;

[0039] Among them, the distribution characteristics of salt rocks include: thickness, grade and physical and chemical properties of salt layers;

[0040] This embodiment: collect historical data on salt mine mining and select docking wells for abandoned salt mines to be rebuilt;

[0041] Collect historical data on the mining of old cavities from salt chemical enterprises, including drilling and core analysis data of each well group, sonar cavity measurement data of salt cavities, production and operation parameters, etc. Based on the screening principles of geological conditions, salt cavity conditions and ground conditions, select and determine the abandoned salt mine docking wells to be converted into gas storage facilities.

[0042] Obtain 3D seismic data, geological characteristics of salt-bearing strata and other stratigraphic structural parameters;

[0043] The three-dimensional seismic exploration technology is used to determine the geological structural characteristics of the rock layer area where the old cavity is located, including: the sedimentary environment of the salt-bearing layer where the old cavity is located, the distribution of fault structures, the distribution characteristics of salt rocks (salt layer thickness, grade, physical and chemical properties), the top and bottom plate characteristics of the salt layer, the distribution characteristics of the interlayers and other geological characteristics.

[0044] Step 2: Perform three-dimensional morphological measurement on the salt cavities of the vertical well and the inclined well of the old cavity of the salt mine to obtain the three-dimensional image and volume of the old cavity of the salt mine;

[0045] The three-dimensional morphology measurement is detected by using a sonar instrument;

[0046] The specific steps of sonar detection are as follows:

[0047] S1: Place the sonar probe on a certain depth level in the salt cavity to emit sound pulses to the wall of the salt cavity, detect the echo signal, and transmit the signal back to the ground computer system to obtain the horizontal measurement distance of the cavity at a certain depth;

[0048] S2: Change the measurement depth to obtain the horizontal measurement distance of the salt mine cavity at different depths;

[0049] S3: Perform tilt measurements on the top, bottom and abnormal part of the salt mine cavity to obtain the measured distances at different tilt angles; the abnormal part is the irregular contour of the cavity wall with local cantilever and finger-like protrusion caused by the tilted interlayer;

[0050] S4: Using sonar ranging imaging analysis software, obtain the three-dimensional image and volume of the entire vertical well and inclined well cavity;

[0051] This example: Three-dimensional morphological measurement of salt caverns in vertical and inclined wells of abandoned salt mines

[0052] To carry out sonar cavity measurement of abandoned salt mine old vertical wells and inclined wells, the sonar probe can be placed on a horizontal plane at a certain depth in the salt cavity to emit sound pulses to the wall of the salt mine cavity, detect the echo signal, and transmit the signal back to the ground computer system to obtain the horizontal measurement distance of the cavity at a certain depth. By changing the measurement depth, the horizontal measurement distance of the salt mine cavity at different depths can be obtained. Then, the top, bottom and abnormal part of the salt mine cavity can be tilted and measured to obtain the measurement distance at different tilt angles, and finally the three-dimensional image and volume of the entire vertical well and inclined well cavity can be obtained.

[0053] Step 3: Drill a data well near the old cavity of the salt mine, extract the core of the data well for testing, and obtain the physical and mechanical parameters of salt rock, mudstone and interlayer;

[0054] The test contents of the core of the data well include: density test, uniaxial compressive strength test, tensile strength test, internal friction angle test, cohesion test, steady-state creep rate test and breakthrough pressure test, permeability test and porosity test; among them, salt rock is subjected to all test contents; mudstone and interlayer are subjected to tests other than breakthrough pressure test;

[0055] This embodiment: extract the core of the data well to conduct indoor tests to obtain the physical and mechanical parameters of salt rock, mudstone and interlayer;

[0056] Through the data well, coring operations were carried out to obtain the formation cores in the library construction area; the coring range was from the surface to 100m below the bottom of the salt rock layer, and the core samples of salt rock, mudstone and interlayer were obtained; according to the requirements of the "Engineering Rock Test Method Standard (GBT50266-2013)", the above cores were processed into standard samples required by different test types; these samples were tested for density, uniaxial compressive strength, tensile strength, internal friction angle, cohesion, steady-state creep rate, breakthrough pressure, permeability and porosity; the interlayer samples were soaked in saturated brine and then the above rock mechanics samples were carried out to obtain the basic physical and mechanical parameters of the sediment. In order to ensure the reliability of the experimental results, 3 to 5 samples were tested for rock mechanics under the same conditions.

[0057] Step 4: Establish a geomechanical numerical model of the old cavity of the salt mine;

[0058] Step 4.1: Based on step 2, the three-dimensional morphology of the salt cavity of the vertical well and the inclined well of the old cavity of the salt mine is measured, and the three-dimensional morphology of the sediment space of the docking well is constructed in combination with the characteristics of the salt-bearing strata, the drilling trajectory and the mining history; the characteristics of the salt-bearing strata, the drilling trajectory and the mining history are provided by the salt chemical enterprise;

[0059] Step 4.2: Check the rationality of the three-dimensional shape of the sediment space by the amount of salt mined and make corrections. If the difference is large, the volume occupied by the sediment in the cavity needs to be adjusted; the amount of salt mined is provided by the salt chemical enterprise; the rationality needs to be calculated, such as the volume formed by the cavity should be equivalent to the volume converted from the total amount of salt mined by the salt chemical enterprise over the years; the mass of pure salt rock is obtained by the concentration and displacement of brine, and then compared with the density of mineral salt to obtain the mined salt volume, and the rationality is determined based on the cavity volume being the same as the obtained salt mined volume;

[0060] Step 4.3: Establish a three-dimensional geomechanical numerical model: The model is a cuboid, the length of each side of the cuboid should be no less than 5 times the maximum diameter of the salt cavity in each side length direction, the inclined interlayer is located in the middle of the model, the docking well is approximately located in the center of the cuboid, and the cavity is watertight; the schematic diagram of the three-dimensional numerical model is as shown in the attached figure. Figure 2 As shown;

[0061] Step 4.4: The surface of the abandoned old cavity wall is subjected to uniform internal pressure; the 3D geomechanical model applies initial geostress, and the top surface of the model is loaded with the average gravity of the overlying rock layer; horizontal constraints are applied to the four vertical surfaces of the 3D geomechanical model to limit the horizontal deformation of the model; fixed constraints are applied to the bottom of the 3D geomechanical model to limit horizontal and vertical deformation; the plane diagram of the mechanical boundary constraint model of the gas storage reservoir is shown in the attached figure. Figure 3 As shown;

[0062] This embodiment: establishes a geomechanical numerical model of the old cavity of the salt mine with a butt-jointed well with inclined interlayers;

[0063] The sonar instrument is used to detect the salt cavity morphology of vertical wells and inclined wells respectively. Combined with the characteristics of salt-bearing strata, drilling trajectory and mining history, the three-dimensional morphology of the docking well sediment space is constructed. The rationality of the three-dimensional morphology of the mining space is verified by the amount of salt mined and corrected. Establish a three-dimensional geomechanical model; the model is a cuboid (the length of each side should not be less than 5 times the maximum diameter of the salt cavity in that direction), the inclined rock layer is located in the middle of the model, the vertical shaft and the docking well are approximately located in the center of the cuboid, and the cavity is watertight; the abandoned old cavity wall has a uniform internal pressure; the three-dimensional geomechanical model applies initial geostress, and the top surface of the model applies the average gravity of the overburden layer (the upper surface of the model extends to the ground during ground settlement analysis, which is the free surface); horizontal constraints are applied to the four vertical surfaces of the three-dimensional geomechanical model to limit the horizontal deformation of the model; fixed constraints are applied to the bottom of the three-dimensional geomechanical model to limit horizontal and vertical deformation; the three-dimensional geomechanical model needs to be meshed, and the mesh size independence, convergence of the calculation results and mesh quality of the three-dimensional geomechanical model need to be checked to ensure the reliability of the calculation results;

[0064] Step 5: Evaluate the dynamic stability of the gas storage under different working pressures and operating parameters;

[0065] Step 5.1: Determine the reservoir capacity parameters, including working pressure, cycle period and maximum gas production rate, and conduct numerical simulation analysis of the static state of the cavity, the internal pressure operation of the cycle injection and production, and the ground subsidence on the built three-dimensional geomechanical model;

[0066] Step 5.2: According to the calculation formulas of dilatancy safety factor, equivalent strain, deformation, volume shrinkage, plastic zone, and natural gas seepage range, the calculated stress, deformation, and seepage pressure are plotted into a cloud map;

[0067] Step 5.3: Evaluate the stability of the gas storage reservoir based on the critical safety values ​​of the shear safety factor, equivalent strain, deformation, volume shrinkage, plastic zone, and natural gas seepage range, and adjust and optimize the gas storage reservoir capacity parameters based on the evaluation results.

[0068] Assume that the vertical well and inclined well of the docking well have top burial depths of 1204m and 1178m respectively, bottom burial depths of 1284m and 1274m respectively, maximum diameter of 90m, well spacing of 300m, and completion depth of the docking well of 1335m. If the bottom of the vertical well is taken as the origin of the model in the vertical direction, the burial depth of 728m can be taken as the upper surface of the model, and the burial depth of 1785m can be taken as the lower surface of the model. The model takes 1200m along the direction of the line connecting the two wells and 900m in the other direction perpendicular to it, that is, the model is a cube of 1200m×900m×1057m. Assuming that the docking well is in the middle of the model, the model contains five interlayers with an inclination of 8° and a thickness of 2m. For example Figure 2 shown.

[0069] The upper surface (728 m) of the model is a stress boundary condition, and the average density of the overlying rock is 2.5×10 3 kg / m 3 Therefore, the vertical load component of the upper boundary of the model is 18.2MPa; the lower surface of the model (1785m) is constrained in the X, Y, and Z directions, and the longitudinal surfaces around the model are simply supported in the corresponding normal directions, such as Figure 3 The mechanical parameters of salt rock and interlayer are shown in Table 1.

[0070] Table 1 Mechanical parameters

[0071]

[0072] Assuming that the docking well is filled with saturated brine after the cavity is formed, the stability of the cavity is calculated by assuming that the brine in the cavity has a uniform internal pressure of 15MPa on the surface of the cavity wall. According to the buried depth of the docking well, the upper limit pressure of the operation should not exceed 80% of the overlying formation pressure and the fracture pressure of the gas storage reservoir or the pressure gradient of a certain gas storage reservoir is 1.7MPa / 100m, and the upper limit pressure can be 19MPa. The lower limit pressure gradient of a certain gas storage reservoir is 0.7MPa / 100m, that is, the lower limit pressure is 9MPa. According to the pressure limit, considering different working pressures, cycle periods, and gas production rate conditions, the shear safety factor, equivalent strain, deformation, volume shrinkage rate, and plastic zone volume of the surrounding rock are obtained to comprehensively evaluate the long-term operation stability of the gas storage reservoir.

[0073] The deformation, plastic zone volume, and natural gas seepage range can be obtained from numerical results, and the shear dilatancy safety factor is Where I1 and J2 are the first invariant of element stress and the second invariant of stress deviator respectively; the equivalent effect becomes Where ε' ij is the deviatoric strain; volume shrinkage Where V0 and V are the total volumes of the vertical well and inclined well cavity after cavity dissolution and in the current operating state, respectively.

[0074] This embodiment: evaluates the dynamic stability of the gas storage under different working gas pressures and operating parameters;

[0075] According to the above three-dimensional geomechanical model, boundary conditions, and load conditions, the storage capacity parameters (working pressure, cycle period, and maximum gas production rate) are determined, and static numerical simulation analysis of the dissolution cavity, cycle injection and production internal pressure operation, and ground subsidence numerical simulation analysis are respectively carried out to simulate and calculate the stress, deformation, damage zone, and seepage pressure of the surrounding rock of the gas storage reservoir; according to the calculation formulas of shear dilatancy safety factor, equivalent strain, deformation, volume shrinkage, plastic zone, and natural gas seepage range, the calculated stress, deformation, and seepage pressure are plotted into a cloud map; according to the critical safety values ​​of shear dilatancy safety factor, equivalent strain, deformation, volume shrinkage, plastic zone, and natural gas seepage range, the stability of the gas storage reservoir is evaluated, and the operating parameters of the gas storage reservoir are adjusted and optimized according to the evaluation results;

[0076] This embodiment: Determine the stability of the gas storage reservoir by combining on-site monitoring and stability evaluation indicators;

[0077] The high-pressure gas storage is measured once every five years using cavity measuring equipment to obtain the volume shrinkage rate and cavity wall deformation measurement results of the gas storage; the surface settlement of the gas storage is measured once a year using GPS and a level to obtain the surface settlement; the above-mentioned on-site monitoring data are used to make real-time adjustments to the critical safety values ​​of the shear safety factor, equivalent strain, deformation, volume shrinkage rate, plastic zone, and natural gas seepage range to ensure the scientificity and reliability of the determined gas storage stability evaluation indicators.

Claims

1. A stability evaluation method for converting an old salt mine cavity with inclined interlayers into a gas storage reservoir, characterized in that: The specific steps include: Step 1: Use 3D seismic exploration technology to determine the geological structural characteristics of the rock layer where the old cavity of the salt mine is located; Step 2: Perform three-dimensional morphological measurement on the salt cavities of the vertical well and the inclined well of the old cavity of the salt mine to obtain the three-dimensional image and volume of the old cavity of the salt mine; The three-dimensional morphology measurement is detected by using a sonar instrument; Step 3: Drill a data well near the old cavity of the salt mine, extract the core of the data well for testing, and obtain the physical and mechanical parameters of salt rock, mudstone and interlayer; Step 4: Establish a geomechanical numerical model of the old cavity of the salt mine; Step 4.1: Based on step 2, the three-dimensional morphology of the salt cavity of the vertical well and the inclined well of the old cavity of the salt mine is measured, and the three-dimensional morphology of the sediment space of the docking well is constructed in combination with the characteristics of the salt-bearing strata, the drilling trajectory and the mining history; the characteristics of the salt-bearing strata, the drilling trajectory and the mining history are provided by the salt chemical enterprise; Step 4.2: Check the rationality of the three-dimensional shape of the sediment space by the amount of salt mined and make corrections. If the difference is large, the volume occupied by the sediment in the cavity needs to be adjusted; the amount of salt mined is provided by the salt chemical enterprise; the rationality needs to be calculated, wherein the volume formed by the cavity should be equivalent to the volume converted from the total amount of salt mined by the salt chemical enterprise over the years; the mass of pure salt rock is obtained by the concentration and displacement of the mined brine, and then compared with the density of the mineral salt to obtain the mined salt volume, and the rationality is determined based on the cavity volume being the same as the obtained salt mined volume; Step 4.3: Establish a three-dimensional geomechanical numerical model: The model is a cuboid, the length of each side of the cuboid should be no less than 5 times the maximum diameter of the salt cavity in each side length direction, the inclined interlayer is located in the middle of the model, the docking well is approximately located in the center of the cuboid, and the cavity is watertight; Step 4.4: The surface of the abandoned old cavity wall is subjected to uniform internal pressure; the 3D geomechanical model applies initial geostress, and the top surface of the model is loaded with the average gravity of the overlying rock layer; horizontal constraints are applied to the four vertical surfaces of the 3D geomechanical model to limit the horizontal deformation of the model; fixed constraints are applied to the bottom of the 3D geomechanical model to limit horizontal and vertical deformation; Step 5: Evaluate the dynamic stability of the gas storage under different working pressures and operating parameters.

2. The stability evaluation method for converting an old salt mine cavity with inclined interlayers into a gas storage facility according to claim 1 is characterized in that: The geological structural characteristics in step 1 include: the sedimentary environment of the salt-bearing layer where the old cavity is located, the distribution of fault structures, the distribution characteristics of salt rocks, the top and bottom plate characteristics of the salt layer, and the distribution characteristics of interlayers; Among them, the distribution characteristics of salt rocks include: salt layer thickness, grade and physical and chemical properties.

3. The stability evaluation method for converting an old salt mine cavity with inclined interlayer into a gas storage according to claim 1 is characterized in that: The specific steps of sonar detection described in step 2 are as follows: S1: Place the sonar probe on a certain depth level in the salt cavity to emit sound pulses to the wall of the salt cavity, detect the echo signal, and transmit the signal back to the ground computer system to obtain the horizontal measurement distance of the cavity at a certain depth; S2: Change the measurement depth to obtain the horizontal measurement distance of the salt mine cavity at different depths; S3: Perform tilt measurements on the top, bottom and abnormal part of the salt mine cavity to obtain the measured distances at different tilt angles; the abnormal part is the irregular contour of the cavity wall with local cantilever and finger-like protrusion caused by the tilted interlayer; S4: Using sonar ranging imaging analysis software, the three-dimensional image and volume of the entire vertical well and inclined well cavity are obtained.

4. The stability evaluation method for converting an old salt mine cavity with inclined interlayers into a gas storage facility according to claim 1 is characterized in that: The test contents of the data well core in step 3 include: density test, uniaxial compressive strength test, tensile strength test, internal friction angle test, cohesion test, steady-state creep rate test and breakthrough pressure test, permeability test and porosity test; among them, salt rock is subjected to all test contents; mudstone and interlayer are subjected to tests except breakthrough pressure test.

5. The stability evaluation method for converting an old salt mine cavity with inclined interlayers into a gas storage facility according to claim 1 is characterized in that: Step 5 is as follows: Step 5.1: Use the established three-dimensional geomechanical model to simulate and calculate the stress, deformation, damage zone, and seepage pressure of the gas storage reservoir surrounding rock; Step 5.2: Determine the reservoir capacity parameters, including working pressure, cycle period and maximum gas production rate, and conduct numerical simulation analysis of the static state of the cavity, the internal pressure operation of the cycle injection and production, and the numerical simulation analysis of ground subsidence; Step 5.3: According to the calculation formulas of dilatancy safety factor, equivalent strain, deformation, volume shrinkage, plastic zone, and natural gas seepage range, the calculated stress, deformation, and seepage pressure are plotted into a cloud map; Step 5.4: Evaluate the stability of the gas storage reservoir based on the critical safety values ​​of the shear safety factor, equivalent strain, deformation, volume shrinkage, plastic zone, and natural gas seepage range, and adjust and optimize the gas storage reservoir capacity parameters based on the evaluation results.

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