Study on the influence of injection pressure and frequency on the deformation and damage law of the surrounding rock in the old cavity of salt mine

The long‐term cyclic gas injection and production mode of the gas storage will lead to accelerated deformation of the surrounding rock of the old cavity, and even the instability and failure of the cavity. Considering the geological conditions and cavity measurement data of the old cavity of the horizontal butted‐well in China, the deformation and failure characteristics of the long‐term cyclic injection‐production process of the surrounding rock of the old cavity under different internal pressure difference and injection‐production frequency are studied. The long‐term stability of the gas storage is predicted by six indexes: the deformation of surrounding rock, dilatancy safety factor, volume shrinkage rate, plastic zone volume, equivalent strain, and no tensile stress. The results show that with the increase of operation duration and cycle times, the deformation, volume shrinkage rate, and plastic zone volume of the surrounding rock gradually increase after the old cavity is reconstructed into the gas storage and the difference of results under different UGS and CAES gradually expands. The stability of the surrounding rock of the old cavity under the cycle action of CAES will be lower. The number of cycles has a limited impact on the stability of the surrounding rock of the old cavity, while the operation duration and injection‐production pressure are the main determinants of the stability of the surrounding rock of the old cavity. If the old cavity of the salt mine is reconstructed into CAES, the parameters such as the number of cycles, the operation duration, and the injection‐production pressure still need to be considered comprehensively. The research results can provide reference value for the reasonable design of the gas storage pressure range of the old cavity and the safety and stability evaluation of the surrounding rock.

China has abundant salt mine resources with a long history of salt rock mining.Numerous well-established salt mining enterprises are distributed throughout the country.Decades of mining have resulted in the existence of a substantial number of old cavities. 1After preliminary statistics, only Jiangsu Huai'an, Hubei Yunmeng, Pingdingshan Ye County, Yunnan Anning, Jiangxi Zhangshu, and other places there are more than 400 mouths of the old cavity existing.Reasonable development and utilization of these abandoned salt mine resources, using the underground space formed by brine mining of salinization enterprises to transform into oil and gas reservoirs can not only turn waste into treasure and improve the development and utilization efficiency of the abandoned salt mine resources, but also provide a strategic path for the abandoned salt mine enterprises to transform themselves out of difficulties and develop sustainably.At the same time, it can also accelerate the promotion of utilizing the old cavities of salt mines for the construction of carbon burial, hydrogen storage, helium storage, and pressurized gas storage, which has a broad application prospect.The abandoned old cavities of the Salt Chemical Company were first dominated by a single well with a single cavity, and gradually used convection wells for salt extraction since the 1970s, that is, one well injects fresh water and the other extracts brine and the direction of injection and extraction is regularly switched.Currently, the distance between two wells of convecting wells is about 200-400 m, of which the horizontal section is about 100-200 m.There are two main types of twin-well convective production modes, namely, two straight well frac connections and straight and inclined well horizontal butted-well connections(Figure 1). 2 Fractured convection wells utilize high-pressure water to form fractures along weak zones in the rock formation, and then expand, flush, and dissolve the fracture to achieve the purpose of connecting the two wells by expanding and extending the fracture.Horizontal butted-well are used to extract brine by drilling two wells, one straight and the other inclined, and the two wells are connected underground through a bare eye to circulate the brine.This type of production method is able to obtain a high concentration of brine compared to single well brine extraction and the salt layer is more fully utilized.
At present, only 10 single-well single-cavity old cavities in Jintan have been successfully transformed to utilize old cavities for natural gas storage, which is mainly due to the fact that on the one hand, according to the existing evaluation criteria for target site selection of salt cavern gas storages, it is difficult to find suitable geological resources of salt mines, which is because the salt mines are dominated by layered rock salt in China and there are a lot of unfavorable features such as many muddy interlayers and a thick single interlayer, especially for the old cavities formed by years of brine mining, it is more necessary to select the old cavities with good economy, high safety and prospect of renovation through sufficient investigation and evaluation before carrying out the renovation of the salt cavern reservoir.On the other hand, the main reason is that many old cavities in China have been connected to each other or even linked together due to long mining time and the pursuit of maximizing the use of resources by the salt mining enterprises.Some of the old cavities are connected to several single wells and single cavities, some are connected by fracturing and some are horizontally buttjointed and then connected to other wells during the mining process, so this kind of complicated and abandoned salt cavities can't be transformed and utilized in the same way as the single wells and single cavities.And while some of the old salt cavities are not suitable for storing natural gas for pipeline networks peaking due to their shallow burial depth, small working gas volume and high retrofitting cost, but they can be constructed into compressed air energy storage power stations for storing electric power resources in the valley electricity stage.Figure 2 is a schematic diagram of the production mode of the old cavity of the salt mine converted into UGS (Natural Gas Storage) and CASE (Compressed-Air Energy Storage), which can be roughly divided into four phases, namely, the dissolved cavity phase, gas injection to drive the brine, raising (lowering) the upper limit of the pressure and cyclic injection and extraction.The earliest underground salt layer is under the balanced original ground stress, well workers brine mining through drilling wells into the ground constantly injecting fresh water dissolving salt layer and salt rock fluidization, a part of the soluble salt rock to form brine and then return to the ground through the wellbore recycling, a part of the insoluble impurities are deposited in the bottom of the salt cavity, build reservoir formation pressure gradually decreased to the saturated brine pressure, forming a certain scale of abandoned old cavern filled with brine.To utilize these underground spaces for energy storage, it is necessary to carry out gas injection to drive the brine during the renovation, the pressure in the old cavity starts to rise gradually and continue to inject gas to reach the permissible pressure of UGS to start the cycle of gas injection and extraction, the conventional UGS operates with one injection and extraction cycle per year, whereas CASE operates with one injection and extraction cycle for 24 h per day and the difference in operating pressures is significantly smaller than that of UGS. Figure 3 is the schematic diagram of stratigraphic pressure relationships in salt cavern storages.
P max g and P min g are the upper and lower limits of the internal operation pressure of the old cavity storage natural gas storage, that is, the maximum and minimum internal gas operation pressure; P max a and P min a are the upper and lower limits of the pressure inside the cavity of the old cavity storage compressed air gas storage, that is, the maximum and minimum internal gas operating pressure.Debrining pressure is the internal pressure of brine in the old cavity, and Brine pressure is the internal pressure of brine in the old cavity.
For this reason, many scholars at home and abroad have researched and made much progress on the evaluation of the safety and stability of the surrounding rock for natural gas storage and compressed air energy storage in the old cavities of a complex salt mine.Wang et al. 3  years.Ying et al. 4 on the Jintan Salt Mine in district No. 1, No. 2 wells, the center of the cavity is at a depth of 1000 m below ground, to determine the Jintan Salt Mine No. 1, No. 2 wells of old cavity gas storage with 20 years of service operation, volume of cavity shrinkage rate of 22% or less, the operating pressure of gas storage in the range of 6-14.5 MPa.Tian et al. 5 proposed the geological conditions and cavity conditions for the conversion of an existing old brine extraction cavity into a gas storage on the basis of considering the requirements of the gas storage and the characteristics of the old cavity and also to evaluate the mechanical stability of the cavity.Zhang 6 proposed a screening procedure for the sonar measurement of brine mining cavities for the characteristics of old cavities in Jintan Salt Mine, taking into account several factors such as cavity volume, cavity shape, and so forth, and then analyzed the safety and stability of the cavities by using FLAC 3D (Fast Lagrangian Analysis of Continua) numerical calculation software.Yang and Yan 7 counted the distribution of old salinization cavities in China and analyzed the feasibility of converting complex old cavities into gas (oil) storage.Zhou et al. 8 analyzed the characteristics of the salt layer and the data of brine extraction wells in Yunying and evaluated the old cavities in the area in terms of sealing, stability, and economy of the salt cavities.Five horizontal buttress wells were identified as the preferred targets for old cavity reconstruction.Chen 9 took the X-Y horizontal convection well cavity building inspection and evaluation project in a mining area in Huai'an as the basis, established the 3D morphology construction method in different cases and comprehensively determined the minimum operating internal pressure of the cavity to be 8.0 MPa.Ba et al. 10 sorted out the current situation of the utilization of the old cavity of the domestic salt cavern storage, the existing problems, and the direction of the utilization of the old cavity, and concluded that the cavity group reconstruction and utilization is the next step in the development of the reconstruction and utilization of the old cavity.Zhang et al. 11 analyzed the volumetric shrinkage of the reconstructed horizontal chamber, the plastic zone, and displacement distribution of the surrounding rock and the salt layer roof plate through the numerical simulation results and obtained the influence of the drilling of new wells outside the cave on the stability of the surrounding rock and the salt layer roof plate of the horizontal old chamber.Sh et al. 12 for the use of convective wells to extract brine old cavity reconstruction of underground gas storage, analyze its applicability in convective and extract brine old chamber reconstruction of gas storage, provide improved testing methods for numerical simulation methods, new techniques for well casing modification and operational technical requirements in qualification evaluation conditions and stability analysis.
In the field of stability of compressed air salt cavern energy storage cavity.Yang 13 took the first underground salt rock gas storage in China, Jiangsu Jintan gas storage, as a demonstration project and carried out a study on the factors affecting the stability of underground salt rock gas storage in the process of compressed air energy storage.Xia et al. 14 used Abaqus to calculate the strain distribution and plastic deformation region around the perimeter of hard rock gas storage chambers with different burial depths and chamber shapes, and analyzed the stability of the chambers after excavation.Zhang et al. 15 conducted mechanical tests to determine the mechanical properties of layered salt layers using Huai'an salt mine as an example and then evaluated the stability and volumetric shrinkage of CAES under different conditions by numerical simulation.Perazzelli and Anagnostou 16 used a computational model similar to Park et al. to analyze the interaction between overlying rocks and bridge plugs of different geometric shapes in CAES caves, and studied the stability of rocks around concrete bridge plugs.Chen et al. 17 studied the parameters and delineation criteria for potential development sites of hybrid CAES systems, such as the use of abandoned mine cavities as compressed air container resources and the distribution of cross-transmission lines in China.Through research, it is found that more than 13 major regions in China have the ability to support hybrid systems.Zhang and Mei 18,19 analyzed the working principle and technical characteristics of traditional compressed air energy storage (CAES) technology and advanced adiabatic compressed air energy storage system based on salt cavern gas storage, and discussed the application prospect of compressed air energy storage technology.To demonstrate the feasibility of the construction of the compressed air energy storage power station in China, Fu et al. 20 took the deep salt layer of a salt mine in Jiangsu Province as the research object, and used FLAC 3D software to establish a compressed energy storage cavity model.The stability of the cavity under long-term gas injection and production conditions was numerically simulated, and the effects of volume shrinkage and gas storage pressure on the efficiency of the cavity were analyzed.Zhou et al. 21used the coupled a thermo-mechanical (TM) damage model to realize the numerical model in COMSOL, and studied the long-term stability of the underground compressed air energy storage lining rock cavern.To verify the feasibility of shallow underground gas storage, Jiang et al. 22 built an underground gas storage in Pingjiang Pumped Storage Power Station in Hunan Province, and carried out a compressed air charging and discharging cycle test.The test results show that the test reservoir has good sealing and surrounding rock deformation safety.Zhang et al. 23 studied the stability of the roof of the horizontal cavity pressurized gas storage reservoir and its influencing factors by numerical simulation.The results show that the upper limit of cyclic internal pressure is the main determinant of the stability of the key roof.Zhang et al. 24 calculated the specific parameters of the reconstructed energy storage power station based on the specific conditions of the underground salt cavern in a province of China, and analyzed the economy of the reconstructed energy storage power station.Chen 25 subdivided the heterogeneous initial damage zone of the surrounding rock according to the influence of coal mining, and established a thermal-fluidsolid coupling numerical model of compressed air energy storage chamber with sealed layer suitable for the heterogeneity of surrounding rock damage zone of abandoned coal mine roadway.The numerical simulation was carried out by COMSOL, and the influence of heterogeneity of excavation damage zone on the displacement and stability of chamber was analyzed.To improve the utilization rate of resources and accelerate the transformation and upgrading, Du et al. 26 proposed a hybrid compressed air energy storage (CAES) system combining wind energy and solar energy, and used the underground abandoned mine as the energy storage space of compressed air.
It can be seen that the research on the renovation and utilization of old cavities of brine extraction in China started relatively late and has less mature experience, especially for the reconstruction of underground gas storage for old cavities of horizontal buttresses, which is even more limited.Therefore, to ensure the safety and stability of the old cavity of the horizontal docking well during the reconstruction and operation when storing natural gas and compressed air, the thesis establishes a 3D geomechanical model of an old cavity to be reconstructed in China, and predicts the evolution of the deformation volume, the volume of the plastic zone, the effective strain (ES), the dilatancy safety factor (SF), the volumetric shrinkage rate and tensile stress of the surrounding rock of the old cavity under the action of high and low internal pressure cycles.The study also investigates the stability of the old cavity under different air pressures and different injection-production times.The research results can provide reference value for the safe and stable operation and optimization of the operation parameters of the old cavity reconstructed gas storage in the brine salt mine.

| STABILITY EVALUATION CRITERIA
To assess the stability of the surrounding rock for the reconstruction of a horizontal docking old cavity gas storage, a reference was made to the existing code for designing single-well salt cavities, "Salt Cavern Gas Storage Cavity Design Code," 27 and "Salt Cavern Underground Gas Storage Safety Technical Code." 28[31][32][33][34]

| Deformation of the surrounding rock
The settling of the top plate and surrounding rock deformation are crucial indicators of gas storage stability in salt rock.Such deformation characteristics effectively demonstrate every location within the salt cavity.Studies have shown that underground gas storage in salt rock must reach maximum creep deformation after 30 years of operation 29 : where D max is the maximum displacement, m and d max is the maximum diameter of the salt cavity, m.

| Expansion safety factor
The dilatancy safety factor (SF) for swelling is a crucial index that many practical engineering applications have verified.
Complex stress states cause rock expansion and salt rock failure, resulting in gas storage leakage and tightness loss.Yet, during gas storage's long-term operation, salt rock expansion damage is unacceptable.Thus, the approved expansion failure criterion for salt rock is 30 where SF is the safety factor; 0.27 is the expansion coefficient, which is obtained by experimental fitting; I 1 is the first invariant of stress tensor; J 2 is the second invariant of stress deviator.It can be calculated by the following equations: (3) where σ 1 , σ 2 , and σ 3 are the three principal stresses, MPa.
According to the published literature, 31 the shear dilatancy safety factor (SF) threshold and the corresponding rock state are shown in Table 1.

| Cavity volume shrinkage
The measurement of gas storage volume shrinkage is critical for determining availability and economic viability.This measurement represents the ratio of the reduction in gas storage volume to the original volume of the gas storage 32 : where V represents the initial volume of the salt chamber, m 3 and V t denotes the current volume of the salt chamber, m.For instance, it is commonly accepted that the shrinkage rate of cavity volume should not surpass 20% in 30 years of gas storage operation.Additionally, in the initial stage of gas storage operation, the theoretical resolution of the storage with a burial depth of 1000 m should not exceed −3 × 10 −4 /a −1 , and for a burial depth of 2000 m, the volume shrinkage rate should not exceed −3 × 10 −2 /a −1 .

| Plastic zone
Damaged modes of salt rock involve shear and tensile damage.The Mohr-Coulomb criterion better reflects the plastic damage to the rock body resulting in the plastic yield of salt rock.This, in turn, causes macroscopic damage phenomena, such as cracking and spalling in the surrounding rock of the gas storage.The generation of a plastic zone is primarily to indicate whether the surrounding rock has experienced plastic flow in the iterative calculation and the possibility of failure.The rock mass around the salt cavity is divided into the tensile failure plastic zone and the shear failure plastic zone.The rock mass around the salt cavity is divided into the tensile failure plastic zone and the shear failure plastic zone.The rock mass around the salt cavity is divided into the tensile failure plastic zone and the shear failure plastic zone.To determine plastic damage, Equations ( 6) and ( 7) are used for the Mohr-Coulomb criterion and the maximum tensile stress criterion. 33,34 where σ 1 is the minimum principal stress, MPa; σ 3 is the maximum principal stress, MPa; c represents the cohesive force, MPa; φ is the angle of internal friction, measured in degrees and σ t is the tensile strength of the rock mass, MPa.Shear damage occurs when the shear damage function f s > 0, while tensile damage occurs when f > 0 t .

| Effective strain
The permissible strain limit for the surrounding rock of gas storage during an operation cycle must not exceed 3% of the permissible value.The damage caused by creep in the gas storage can be determined by checking if the equivalent strain exceeds the permissible value.For instance, Germany mandates that the equivalent strain of the surrounding rock of the gas storage during an operation cycle should be no more than 3% of the permissible value. 31 where ε 2 , I′ 1 and I′ 2 are the first and second invariants of strain tensor, respectively.

| No tensile stress
The criterion of "no tensile failure occurs in the bare well and cementing section in the salt rock layer of the top of the cavity," that is, no tensile failure in the horizontal direction occurs in the roof of the salt rock during the operation of the gas storage.
where σ 1 is the maximum principal stress, MPa and σ t is the tensile strength of salt rock, MPa.The mining area of a salt mine is 4.18 km 2 and the depth of the main salt layer is in the range of 1300-2460 m, with an average thickness of 440.7 m.The production wells of the salt mine are in a good condition, with straight wells for water injection and directional wells for brine extraction, and the wells are switched for injection and extraction at irregular intervals.The old cavity to be remodeled has a complete production record of injection and extraction cavity building, and the volume of salt extracted and the volume of salt-bearing strata utilized can be estimated based on the water injection, brine extraction, brine concentration, and NaCl content, and the geometry of the horizontal section salt cavity can be further obtained by combining with the material balance method. 35The morphology of the straight well-inclined well connecting salt cavity of the gas storage is shown in Figure 4A, in which the blue part is the sonar detection morphology and the brown part is the insoluble residue, and the morphology consists of four parts, A, B, C and D.
A is a straight well salt cavity that approximates a cylinder, B is a horizontal connecting passage that approximates a cylinder, C is the portion of the inclined well from the point of inclination to the beginning of the horizontal section that approximates a cone, and D is the part of the inclined well salt cavity from the point of inclination above the point of inclination to the bottom of the top of the cavity that approximates a cylinder.
The buried depth of this horizontal buttress well from the top of the cavity to the bottom of the cavity is 1650-2044 m, in which the salt cavity is a residue layer from 1764m down.The mudstone interlayers of 2 m and above are considered in the modeling, and there are four groups of interlayers in total.The origin of the numerical model is located at the lowest point directly below the straight well, and the actual burial depth is 2044 m.The model is a cube with the range of −300 to 700 m in the X direction, −200 to 200 m in the Y direction, and −200 to 694 m in the Z direction.The upper surface of the model (1350 m) is the stress boundary condition, and the average density of the overlying rock layer is 2.165 × 10 3 kg/m 3 , that is, a uniform vertical load of 29.20 MPa is applied at the upper surface; and the lower surface of the model (2244 m) is constrained by a fixed constraint.The surrounding longitudinal surfaces are constrained by simple support constraints in the corresponding normal directions, that is, it is considered that the model has normal constraints at the front, back, left, and right surfaces, which are not allowed to produce normal movement, and the dissolution process in the cavity has a negligible effect on them.The 3D geomechanical numerical model of the gas storage is shown in Figure 4B.Among them, A, B, C, and D volumes are filled with insoluble slag, and the analysis of numerical results focuses on the safe and stable state of the surrounding rock in the pure brine-filled cavity, as shown in the blue part of Figure 4A.The simulation of the injection-production operation uses the Cpower creep model that comes with Flac 3D , that is, the stable creep strain rate is a power function of the stress bias and obeys the Norton energy law, the plastic strain, the damage criterion obeys the Mohr-Coulomb flow law and the yield criterion, where the standard form of the Norton power exponent model is as shown in the following equation: where ε t ( ) is the steady state creep rate; q J = 3 2 (J 2 is for the second invariant of the stress bias); A is the material property parameter, n is the stress exponential constant, and the specific values of A and n are shown in Table 2.
The basic mechanical parameters and power index coefficients of creep rate and bias stress in the steady state stage of salt rock and mudstone are shown in Table 3. Guo Yintong and Ma Linjian 36 concluded that the upper stress threshold for fatigue damage of salt rock is 75%-80% by uniaxial and triaxial cyclic loading, and the elastic modulus shows an exponential trend of decaying with the number of cycles or the loading time, and eventually decreases by 6%-10%, which can be close to 20% for uniaxial.According to the literature 37 to fit the nonlinear function of the elastic modulus and the number of cycles, the elastic modulus of each calculation unit was dynamically corrected during the calculation process using FISH language, and the different effects of the number of cycles on the injection and gas extraction were not considered for the time being.
where E 0 is the initial modulus of elasticity, MPa and n is the cycle index.

| Design of injection and mining operation program
The parameters of internal pressure in natural gas storage and compressed air storage operations are mainly for the internal pressure limit value and cycle period.The internal pressure limit is the pressure of the high pressure gas injected into the storage, generally to ensure the safety of the storage, the gas pressure will not be too high or too low, the pressure generally varies periodically in an interval, the internal pressure gradually reaches the upper limit of the internal pressure P max during gas injection and the internal pressure will gradually drop to the lower limit of the internal pressure P min during gas extraction.A cycle generally has four phases, the injection period, high pressure shutdown wells, the extraction period, and the low-pressure shutdown wells.Natural gas storage is mainly to cope with the seasonal peaking of natural gas, generally one cycle per year.The maximum internal gas pressure of UGS should not exceed 80% of the overlying formation pressure or rupture pressure, the minimum internal gas pressure can refer to the field experience of Jintan storage, it is appropriate to consider the pressure of 0.007 MPa/m, and the operating pressure can be varied in a range of up to 12-28 MPa.Compared with UGS, the cavern gas pressure changes frequently in CAES, generally 1 day for a cycle, mainly used for the role of daily grid peaking.The operating internal pressure tends to be lower, with a smaller range of variation, usually considered to be 30%-80% of the vertical stress of the original rock, and is also influenced by the operating limits of the surface turbine equipment.Therefore, two types of high internal pressure circulation injection (12-28 MPa) and low internal pressure circulation injection (10-14 MPa) were selected respectively, and the cycle frequency was 1, 5, 10, and 30 (i.e., run 1, 5, 10, and 30 cycles per year) four kinds of cycles, and the time consumed in one cycle was 300, 60, 30, and 10 days, respectively.The higher the cycle frequency, the shorter the operation time of each cycle.When the cycling frequency is 1 cycle/year, the high and low internal pressure cyclic injection and extraction schemes are recorded as U-1 and C-1, respectively, with a gas injection period of 120 days, a high-pressure shut-in period of 30 days, a gas extraction period of 120 days and a lowpressure shut-in period of 30 days and a recurring cycle of 300 days.Each parameter of the other cycle index is shown in Figure 5B-D in turn, the high and low internal pressure cyclic injection and extraction schemes are recorded as U-2 and C-2, U-3 and C-3, and U-4 and C-4, respectively with a gas injection period of 24, 12, and 4 days, a high-pressure shut-in period of 6, 3, and 1 days, a gas extraction period of 24, 12, and 4 days and a low-pressure shut-in period of 6, 3, and 1 days and a recurring cycle of 60, 30, and 10 days.10, and 30, respectively.As can be seen in Figure 6, the vertical displacements at the top of the old cavity and both sides of the cavity are negative in the 1st year of operation, and the maximum settlement at the top of the cavity is about 0.8 m.The vertical displacement at the bottom of the cavity decreases and then increases layer by layer as the distance from the bottom of the cavity increases, and the range of change is from 0.4 to 0 m and then to −0.3 m.As can be seen in Figure 7, the vertical displacements at the top of the old cavity in the 30th year of operation are all negative, and the maximum settlement at the top of the cavity is increased to 5.5 m, and the bottom of the cavity changes from −2 to −3 m.It can be seen that with the operation time from the 1st year to the 30th year, the settlement at the top of the cavity and the two sides of the cavity is getting bigger and bigger, and the vertical settlement deformation of the slag at the bottom of the cavity is gradually increasing from the surface of the slag downwards.It shows that the surrounding rock shrinks and sinks under the action of geostress, and there is a sudden change of displacement at the two old cavity walls, and the slag at the bottom of the cavity shows settlement and compaction due to air pressure and its own gravity.Under different injectionproduction frequencies, the distribution pattern of displacement contours of surrounding rock is basically similar, but with the increase of cycle index, the contours of displacement contour cloud map are adjusted, for

| Enclosed rock deformation analysis
example, the contours of CAES contour cloud map above 0.8 m at the top of the left cavity in the 1st year of operation and above 5.5 m at the top of the left cavity in the 30th year of UGS operation.Similarly, under UGS and CAES modes, CAES corresponds to a larger relative range of contour profiles at the top of the cavity, i.e., in the low internal pressure condition, even though the change in pressure gradient is small, it also leads to larger deformation of the perimeter rock in the near field of the cavity, and this difference is particularly significant at the top of the cavity.
To study the deformation law of the old cavity surrounding rock under different injection-production frequencies, the cavity roof of the inclined shaft was monitored as the key point, and the vertical displacements of the key point in the 30th year of operation under different working conditions were recorded, and the vertical displacements of the key point versus time were plotted, and the vertical displacements of the key point versus time under different injection-production frequencies and operating pressures are shown in Figures 8 and 9. From Figures 8 and 9, it can be seen that there is little difference between UGS and CAES results in the first year, and the deformation of the chamber roof is 0.785 m in the first year, and 0.868 and 0.865 m respectively, when the cyclic cycle index is 30 cycles per year, and the roof deformation of UGS is 0.003 m higher than that of CAES, and it can be seen that the difference between UGS and CAES gradually increases with the increase of the cycle index and the effect of the increase of the cycle index in the first year of operation on the deformation of UGS and CAES is limited.In the first year of operation, the increase of injection-production frequency has limited influence on the deformation of UGS and CAES.In the 30th year of operation, the deformation of UGS and CAES is 5.60 and 5.75 m respectively when one cycle injection-production is adopted.The deformation of UGS and CAES is 5.78 and 5.92 m respectively when the injection-production cycle is 30 cycles per year.At this time, the deformation of the cavity top of CAES is about 0.14 m higher than that of UGS, which is equivalent to an increase of 2.4%.It can be seen that the deformation of surrounding rock is positively correlated with the cycle frequency and the operation time, that is, the increase of the cycle frequency will accelerate the deformation of the surrounding rock.With the increase of the operation time, the deformation of the roof gradually increases, but the increase rate gradually decreases, that is, the deformation of the surrounding rock will gradually slow down with the operation time, which is conducive to the stability of the old cavity reconstruction gas storage and the deformation of the surrounding rock is more sensitive to the operation time.safety factor from the cavity wall outward is distributed with the layered distribution with the cavity contour, the farther away from the cavity, the larger the dilatancy safety factor is, and the value is greater than 1.5, that is, the cavity tends to be in a more secure state, and the CAES surroundings locally appear to be less than 1.There is a region where the dilatancy safety factor is 0.5-1, such as the left side of the left cavity in the 1st year and the two shoulders of the top of the cavity in the 30th year.With a lower value of SF, there is a higher possibility of dilative shear failure, which reduces the tightness of the surrounding rock due to the connectivity of local cracks.Compared with Figures 10 and 11, it can be seen that the influence of cycle index on the dilatancy safety factor of the surrounding rock of the gas storage is not significant, which is mainly due to the fact that the drum-expansion failure of the surrounding rock is basically completed after the first change of the internal pressure, and there will be no further intensification.running time for different operating pressures and injection-production frequencies were plotted according to Table 3 (shown in Figure 12).As can be seen from Figure 12, the volume shrinkage of the abandoned old cavities under different internal pressure intervals, operation time, and injection-production frequencies are different, and the volume shrinkage gradually increases with the increase of operation time.At the same time, the difference in volumetric shrinkage between UGS and CAES is becoming more pronounced.For example, the volume shrinkage values of UGS and CAES are less than 1% in the 1st year, and the volume shrinkage of UGS and CAES can reach more than 7.5% and 11% in the 30th year, respectively.Meanwhile, it can be seen that the cavity shrinkage rate of CAES is significantly larger than that of UGS, which indicates that although the pressure difference is small, the pressure difference is the upper limit minus the lower limit of the pressure value, that is, the pressure difference in CAES is 4 MPa, and the pressure difference in UGS is 16 MPa), due to the minimum pressure of CAES being 10MPa, which is 2MPa lower than UGS, CAES sill produces a large volume shrinkage rate, that is, the minimum internal pressure has a more significant effect on the stability of the cavity.And as the running time increases, the effect of being affected by the minimum internal pressure becomes more significant.When the cycle index is increased from 1 cycle per year to 30 cycles per year, it also leads to an increase in volume shrinkage rate (relative volume shrinkage= (volume shrinkage rate of 30 cycles per year-volume shrinkage rate of 1 cycle per year)/volume shrinkage rate of 1 cycle per year), which can exceed 1.8%, and the impact on CAES is more significant, that is, the increase in volume shrinkage is more pronounced after the combination of smaller low-pressure and larger cycle times.

| Plastic zone analysis
Figures 13 and 14 show the distribution of plastic zones of the old cavity in the cavity at 1st year and 30th year under different injection-production schemes, shear-n represents the current shear failure of this element at the end of the equilibrium convergence calculation; shear-p represents that this element has experienced shear failure in the equilibrium convergence calculation; tension-p represents that this element has been tensile failure in the equilibrium convergence calculation.As shown in Figures 13 and 14, the top of the cavity of UGS shows more tensile damage and a small amount of shear damage, while the top of the cavity of CAES shows relatively less damage, and the distribution of plastic zones of UGS and CAES is basically the same, with more concentrated shear damage at the junction of the interlayer in the top of the cavity, the body of the cavity and the bottom of the cavity on both sides, and the plastic zones of the interlayer at the top of the cavity appear to be contiguous.In other words, the two chambers may be strung along the macroscopic cracks in the top layer, but there are some differences in the extension range and the volume of the plastic zone damage.As the injection-production operation time continues, the total plastic zone range of the cavity surrounding rock gradually becomes larger, but the amplification is smaller.The generation of plastic zone will aggravate the damage and destruction of surrounding rock, cause the increase of permeability of surrounding rock, and the local gas leakage along the fissures of surrounding rock may occur, affecting the tightness of the storage, especially the accumulation of slag at the bottom of the cavity through the loose voids to cause the flow of gas in the two cavities.
Figure 15 is the plastic zone volume versus time for the old cavity in the 30th year of operation under different schemes.As can be seen in Figure 15, the volume of plastic zone increases with the increase of operation time, and the plastic damage has been dominated by shear damage, and the volume of the plastic zone of the surrounding rock fluctuates in the 30th year of operation, but the amplification is not significant, and the maximum amplification is less than 0.5% compared with 1st year.According to the calculation result in the 30th year, the volume of shear damage is 2.77 times of the cavity volume, the volume of plastic zone of UGS and CAES increases gradually with the increase of cycle index, the volume of plastic zone of UGS is significantly larger than the volume of plastic zone of CAES, for example, the volume of plastic zone in the 30th year under the cycle index of 1 cycle per year is 3.4% larger; and the incremental amplitude of the plastic zone of CAES is slightly larger than that of UGS, which indicates that the frequency of cycle has a certain effect on the volume of plastic zone.

| Equivalent strain
Figures 16 and 17 show the contour plots of the equivalent strain of the cavity in the 1st and 30th year under different injection-production schemes.In the 1st year, the equivalent strain at the bottom of the cavity decreases layer by layer along the direction away from the bottom plate of the cavity, while the equivalent strain at the top and sides of the cavity is extremely small; in the 30th year, the equivalent strain gradually surrounds the whole cavity from the bottom upwards along the wall, and the equivalent strain contour is basically the same as that of the cavity contour and are characterized by dense at the top of the cavity and loose at the bottom.Equivalent strain gradually decreases along the direction away from the cavity wall, and there is an area of more than 3% in the near field of the cavity surrounding rock, in which the bottom of the cavity is particularly serious, the red area in Figures 16 and 17 is the area where the equivalent strain is more than the critical value of 3%, which indicates that the creep deformation in this area is extremely significant.Also, the range of the red area in the case of UGS is smaller than that in the case of CAES.It is recommended to appropriately reduce the gas extraction rate or the minimum internal pressure residence time during the operation period, to minimize the unfavorable situation of excessive effective strain on the cave wall and cavity bottom.

| Tensile stress analysis
Figures 18 and 19 show the contour plots of the maximum principal stresses of the old cavity in the cavity at 1st year and 30th year under different injectionproduction schemes.As shown in Figures 18 and 19, the surrounding rock is mainly subjected to compressive stress, and no tensile stress has occurred.With the increase of operation time, the maximum principal stress at the top and bottom of the cavity surrounding rock gradually increases (in absolute value) along the direction close to the cavity wall, and the maximum principal stress contour gradually forms a closed curve around the cavity contour.This is due to the natural gas injected into the cavity at the beginning of the injection-production period, which led to sharp changes in the internal pressure, as well as changes in the maximum and minimum pressures, resulting in large fluctuations in the redistribution of stresses in the surrounding rock located in the near-field of the cavity, and then the adjustment of stresses in the surrounding rock tended to be gradually stabilized.Under different injection-production frequencies, the maximum principal stress distribution patterns of UGS and CAES are basically the same, and the difference is not significant.shrinkage, dilatancy safety factor and tensile stress, have been comprehensively considered.These evaluation indexes can better reflect the evolution law of creep, shear, tension, dilatation, and volume shrinkage of surrounding rock during the operation of the old cavity.2. With the increase of the cycle frequency, the deformation of the surrounding rock, volume shrinkage rate, and the volume of the plastic zone of the old cavity gradually increase, while the influence on the dilatancy safety factor of the surrounding rock and the equivalent strain of the near field of the tunnel wall is not significant.With the increase of operation holding time, the increase of the deformation of the cavity top and the shrinkage rate of the cavity in the two reconstruction modes of CAES and UGS and the difference between the two will be more significant.Therefore, it is recommended to focus on the three indicators of the deformation of the surrounding rock, volume shrinkage rate and the volume of the plastic zone when evaluating the old cavity reuse modification method.3. Compared with the cycle frequency, the minimum internal pressure during the injection-production process has a more significant impact on the stability of the surrounding rock in the old cavity, and the range of effect variations such as the deformation of the cavity roof, the shrinkage rate of the cavity and the equivalent strain range of more than 3% will be larger in the old cavity when the low-pressure differential gas storage (CAES) is compared with the highpressure differential gas storage (UGS), and the dilatancy safety factor of the cavity roof in the old cavity is less than 1 and the possibility of the occurrence of the dilative shear failure is larger, which indicates that the stability of the surrounding rock of the old cavity will be lower under the condition of low-pressure difference circulation gas storage.Therefore, if the old cavity of the salt mine is converted to UGS, the factors of injection-production pressure (minimum pressure) and operation time can be considered, and if it is used as CAES, the injectionproduction pressure (minimum pressure), the cycle frequency and the operation time should be considered comprehensively.

| CONCLUSIONS
based on the sonar measurement data of the old cavity, combined with the stratigraphic characteristics of the Jintan Salt Mine, set up a three-dimensional (3D) geomechanical model of the new cavity close to the old one, and investigated the influence of the distance between the new and old caverns and the operation time on the stability of the salt caverns.To break through this development bottleneck in the construction of salt rock gas storage in China, a series of scientific and technological research on the construction of old cavities in salt mines have been carried out in China in recent F I G U R E 2 Schematic diagram of the production model of UGS and CASE power stations.F I G U R E 3 Schematic diagram of stratigraphic pressure relationships in salt cavern storages.

T A B L E 1 1 |
The safety factor (SF) threshold and the corresponding rock state.Geological profile of the old cavity

F I G U R E 4
Numerical modeling of salt cavern gas storage.(A) Schematic diagram of the morphology of the salt cavity of the straight well-inclined well connection.(B) Three-dimensional geologic modeling and meshing of gas storage.(C) Interregional plot of the interlayer distribution.

Figures 6
Figures6 and 7show the contour maps of the vertical displacement of the cavity in the 1st and 30th year of the two modifications of the old cavity at cycle index of 1, 5,

F
I G U R E 5 Schematic diagram of the injection-production operation of the abandoned old cavity.(A) 1 cycle/year, (B) 5 cycles/year, (C) 10 cycles/year and (D) 30 cycles/year.

F
I G U R E 7 Cloud map of vertical displacement of cavity in the 30th year.F I G U R E 8 Vertical displacement versus time curves of the chamber roof of an inclined shaft.

F I G U R E 9
Vertical displacement versus time curves of the chamber roof of an inclined shaft.(A) Different cycle index and (B) under different operation pressures.

4. 2 |
Figures 10 and 11 show the contour maps of the dilatancy safety factor of the old cavity in the 1st and 30th year of the two modifications, respectively, and the dark blue area in the figure indicates that the rock body is in the state of collapsing and destroying.As shown in Figures 10 and 11, the dilatancy safety factor of the old cavity has a similar trend on the whole, and the dilatancy

F
I G U R E 11 Cloud map of the dilatancy safety factor of the old cavity in the 30th year.F I G U R E 12 Volume shrinkage versus operation time.(A) Under different operation pressures and (B) different injection-production frequencies.

F
I G U R E 13 Distribution map of the plastic zone of the cavity at 1st year.F I G U R E 14 Distribution map of the plastic zone of the cavity at 30th year.ZHANG ET AL. | 1607 F I G U R E 15 Diagram of the plastic collapse versus operation time.F I G U R E 16 Contour plots of the equivalent strain of the cavity in the 1st year.F I G U R E 17 Contour plots of the equivalent strain of the cavity in the 30th year.

F
I G U R E 18 Contour plot of the maximum principal stress of the cavity in the 1st year.

1 .
To evaluate the stability of the gas storage rebuilt by the old cavity of brine mining, six evaluation indexes, namely, deformation amount of surrounding rock, distribution of plastic zones, effective strain, volumetric F I G U R E 19 Contour plot of the maximum principal stress of the cavity in the 30th year.
T A B L E 2

Table 3
shows the volumetric shrinkage of the old abandoned cavities at 1, 5, 10, 20, and 30 years of operation for the two modes of alteration.Volume shrinkage versus