Salt cavern gas storage roof dolomite creep damage prediction method

Abstract

A salt-cavern gas storage roof dolomite creep damage prediction method relates to the technical field of rock mechanics and comprises the following steps: acquiring elastic mechanical parameters and geological parameters of salt-cavern gas storage roof dolomite, and establishing a rock creep damage model under the action of graded circulation gas injection load; obtaining a rock creep rate control model under the action of the staged cycle gas injection load, performing parameter identification on the rock creep rate control model, and predicting the creep damage state of the dolomite of the reservoir roof under the action of the staged cycle gas injection load by using the rock creep rate control model subjected to parameter identification in combination with the elastic mechanical parameters and the geological parameters; according to the method, the creep mechanical behavior of the dolomite of the top plate of the salt-cavern gas storage under the action of the high-frequency cyclic load can be accurately represented, the creep time threshold value of the top plate can be accurately predicted, and the influence rule of the graded load on damage accumulation is quantified; therefore, an accurate quantitative decision basis is provided for gas injection load regulation and control and safe operation cycle optimization of the salt-cavern gas storage.

Description

Technical Field

[0001] The present invention relates to the technical field of rock mechanics, and specifically to a method for predicting creep damage of dolomite in the roof of a salt cavern gas storage reservoir. Background Art

[0002] Compressed Air Energy Storage in Salt Caverns (CAES) is a technology that realizes electrical energy storage and flexible dispatching of the power system by transforming underground salt caverns and using compressed air. This technology is an energy storage technology under the action of high-frequency periodic air loads. By cutting peaks and filling valleys in the conventional power system, it significantly improves the flexibility, economy, and security of the power grid, and at the same time constitutes the core component of the underground energy storage system.

[0003] However, for the "broken roof" salt caverns formed by brine extraction, the core challenge faced by the dolomite in the roof during the compressed air energy storage operation stems from the structural damage under the action of high-frequency cyclic loads. During long-term cyclic gas injection operations, considering the creep characteristics and mechanical response characteristics of the roof dolomite, the high-frequency periodic air loads may cause fatigue effects and induce creep damage or local collapse. This damage evolution characteristic requires the construction of a technical system that integrates dynamic monitoring and numerical simulation and matches the cyclic gas injection operation to achieve an accurate quantitative assessment of the degree of dolomite damage accumulation. At present, the main research object of the conventional rock mechanics evaluation system is the creep process of rocks under natural conditions, and it is difficult to accurately describe the creep characteristics under the action of cyclic gas injection loads. Summary of the Invention

[0004] In view of this, the main purpose of the present invention is to provide a method for predicting creep damage of dolomite in the roof of a salt cavern gas storage reservoir, which can accurately evaluate the creep mechanical behavior of the dolomite roof and its long-term stability critical threshold under the action of cyclic gas injection loads based on finite element numerical simulation technology.

[0005] The technical solution of the present invention is a method for predicting creep damage of dolomite in the roof of a salt cavern gas storage reservoir, including the following steps:

[0006] Step S1: Obtain the elastic mechanical parameters and geological parameters of the dolomite in the roof of the salt cavern gas storage reservoir;

[0007] Step S2: Establish a rock creep damage model under the action of graded cyclic gas injection loads, obtain a rock creep rate control model under the action of graded cyclic gas injection loads, and identify the parameters of the rock creep rate control model;

[0008] Step S3: Combine the elastic mechanical parameters and geological parameters, and use the parameter-identified rock creep rate control model to predict the creep damage state of the dolomite in the roof of the reservoir under the action of graded cyclic gas injection loads.

[0009] The technical effects of the present invention are as follows:

[0010] The present invention can accurately characterize the creep mechanical behavior of dolomite in the roof of a salt cavern gas storage under high-frequency cyclic loading, and can focus on revealing the creep damage evolution mechanism and the critical threshold of long-term stability of the roof dolomite, so as to deeply analyze the key factors affecting creep damage characteristics, and accurately predict the creep time threshold of the roof accordingly. The influence law of stepped loading on damage accumulation is quantified, thus providing an accurate quantitative decision-making basis for the injection gas load regulation and the optimization of the safe operation period of the salt cavern gas storage. Brief Description of the Drawings

[0011] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required for use in the embodiments will be briefly introduced below.

[0012] Figure 1 It is the indoor experiment creep curve in the embodiment of the present invention;

[0013] Figure 2 It is the finite element model of the roof dolomite at the core scale in the embodiment of the present invention;

[0014] Figure 3 It is the comparison diagram of the numerical simulation results and the test results under each level of loading in the embodiment of the present invention;

[0015] Figure 4 It is the schematic diagram of the stepped loading setting in the embodiment of the present invention;

[0016] Figure 5 It is the diagram of the initial displacement magnitude under each level of loading of the stepped loading model in the embodiment of the present invention;

[0017] Figure 6 It is the displacement-time change diagram of the stepped loading model in the embodiment of the present invention;

[0018] Figure 7 It is the diagram of the creep strain of the stepped loading of the roof dolomite and the strain results of each component in the stepped loading model in the embodiment of the present invention. Detailed Embodiments

[0019] The present invention will be further described in detail below in conjunction with the embodiments and the drawings.

[0020] To make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. The described embodiments are part of the embodiments of the present invention, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0021] A method for predicting the creep damage of dolomite in the roof of a salt cavern gas storage, comprising the following steps:

[0022] Step S1: Obtain the elastic mechanical parameters and geological parameters of the dolomite in the roof of the salt cavern gas storage.

[0023] The elastic mechanical parameters and geological parameters of the dolomite in the roof of the salt cavern gas storage include, but are not limited to, physical parameters such as elastic modulus, Poisson's ratio, density, compressive strength, tensile strength, cohesion, and internal friction angle. Among them, the elastic mechanical parameters and geological parameters of dolomite can be used to calculate the in-situ stress through relevant experiments on the downhole core samples in accordance with the national standard "GB / T 29172-2012 Core Analysis Methods". The basic mechanical parameters such as elastic modulus, Poisson's ratio, density, compressive strength, tensile strength, cohesion, and internal friction angle and their deformation and failure characteristics can be obtained through tests such as Brazilian splitting, uniaxial compression, and triaxial compression.

[0024] Step S2: Establish a rock creep damage model under the action of graded cyclic gas injection loads, obtain a rock creep rate control model under the action of graded cyclic gas injection loads, and identify the parameters of the rock creep rate control model.

[0025] Under the action of cyclic loading and unloading, the internal damage of the rock shows dynamic evolution characteristics. Each stage of loading will promote the expansion of the original microcracks and the generation of new cracks. Although some cracks will close temporarily during unloading, the residual deformation accumulates continuously. As the number of cycles increases, the internal damage continues to accumulate, resulting in a stepwise increase in the creep rate. When the cyclic stress approaches the long-term strength threshold of the rock, the irreversibility of plastic deformation is significantly enhanced, and the residual strain increases significantly, accelerating creep failure. Although the viscous characteristics of the rock itself will allow some deformations to slowly recover over time, frequent load cycles will break the viscoelastic equilibrium, resulting in a continuous increase in the residual deformation base value after each unloading, leading to an enhanced deformation hysteresis effect.

[0026] When the load level is low, the rock only shows instantaneous elastic deformation. When a certain stage of stress exceeds the stress threshold of the switching element, creep of the rock shows viscous behavior. When a certain stage of stress exceeds the long-term strength threshold, the viscous coefficient decays non-linearly, resulting in accelerated damage accumulation and even directly triggering the accelerated creep stage, significantly shortening the failure time of the rock.

[0027] Therefore, the rock creep damage model under the action of graded cyclic gas injection loads can be constructed by connecting the following elements in series: using a generalized Kelvin element to characterize the instantaneous elastic deformation and the initial rheological relaxation effect, introducing an unsteady viscous element with a stress threshold triggering mechanism to describe the sudden change characteristics of viscous behavior, and combining a non-linear viscoplastic element to simulate the long-term plastic flow and creep failure process.

[0028] (1) Basic principles of series and parallel connection of components

[0029] Components in series follow the principles of stress equivalence and strain superposition: the total stress is equal to the stress of each component, and the total strain is the sum of the strains of each component. Components in parallel follow the principles of strain equivalence and stress superposition: the total strain is equal to the strain of each component, and the total stress is the sum of the stresses of each component.

[0030] (2) Generalized Kelvin element (H-(H|N))

[0031] The generalized Kelvin element is composed of an elastic Hooke element and a Kelvin element in series, and the Kelvin element is composed of an elastic Hooke element and a viscous Newton element in parallel. Its state equation is:

[0032]

[0033] The creep equations corresponding to each component are obtained by solving:

[0034]

[0035] In equations (3) and (4):

[0036] σ1—the stress corresponding to the elastic Hooke element part;

[0037] σ2—the stress corresponding to the Kelvin element part;

[0038] E1—the elastic coefficient of the elastic Hooke element part;

[0039] E2—the elastic coefficient of the elastic element in the Kelvin element;

[0040] η1—the viscosity coefficient of the viscous element in the Kelvin element;

[0041] ε1—the strain corresponding to the elastic Hooke element part;

[0042] ε2—the strain corresponding to the Kelvin element part;

[0043] —the first derivative of the strain corresponding to the Kelvin element part;

[0044] t—time.

[0045] (3) Unsteady viscous element with a switch (K-N)

[0046] The unsteady viscous element with a switch is composed of a Kelvin element and an unsteady Maxwell element in series. Its state equation is:

[0047]

[0048] The corresponding creep equation is obtained by solving:

[0049]

[0050] In Equations (5) and (6):

[0051] σ3—the stress corresponding to the part of the unsteady viscous element with a switch;

[0052] η2—the initial value of the viscosity coefficient of the unsteady viscous element with a switch;

[0053] λ—a constant, with a value range between 0 and 1;

[0054] ε3—the strain corresponding to the part of the unsteady viscous element with a switch;

[0055] —the first derivative of the strain corresponding to the part of the unsteady viscous element with a switch;

[0056] t—time.

[0057] (4) Nonlinear viscoplastic element (NDY)

[0058] The dashpot in the nonlinear viscoplastic element is essentially a viscous Newtonian element, and its mechanical behavior follows Newton's viscosity law. Its constitutive equation is:

[0059]

[0060] Taking into account the creep damage behavior of rocks and the influence of stress action time comprehensively, a viscosity coefficient is introduced, which can be characterized as a function of the damage variable and stress action time. Its expression is:

[0061] η3(D) = η3(1 - D)(8)

[0062] Based on the negative exponential relationship between long-term creep damage and stress action time, the damage factor D can be defined as:

[0063] D = 1 - e -αt (9)

[0064] Therefore, the viscosity coefficient of the dashpot in the nonlinear viscoplastic element is:

[0065] η3(D) = η3·e -αt (10)

[0066] When the applied stress value is greater than the yield stress threshold, i.e., σ ≥ σ s , the constitutive equation of the nonlinear viscoplastic element is:

[0067]

[0068] Its creep equation is:

[0069]

[0070] In Formulas (7) to (12):

[0071] σ4—the stress corresponding to the non-linear viscoplastic element part;

[0072] σ s —the stress threshold value of the non-linear viscoplastic element;

[0073] α—the exponential coefficient of the damage characteristics of the rock material, α is a constant and α > 0;

[0074] η2(D)—the viscosity coefficient considering the damage effect;

[0075] —the first derivative of the strain corresponding to the non-linear viscoplastic element part, that is, the creep rate during the accelerated creep process;

[0076] η3—the initial value of the viscosity coefficient of the non-linear viscoplastic element;

[0077] D—the damage factor variable, and its value range is 0 < D < 1;

[0078] ε4—the strain corresponding to the non-linear viscoplastic element part;

[0079] t—time.

[0080] (5) Base value of residual deformation under the action of staged cyclic gas injection load

[0081] The surrounding rock shows four-stage creep characteristics (instantaneous elastic deformation, decelerated creep, steady creep, accelerated creep) under the action of staged cyclic load. Although the viscous characteristics of the rock itself will cause part of the deformation to slowly recover with time, frequent load cycles will break the viscoelastic balance, resulting in a continuous increase in the base value of the residual deformation after each unloading, leading to an enhanced deformation lag effect.

[0082] Therefore, the base value of residual strain ε res (σ, N) is introduced, and this value is a function of the maximum load and the cumulative cycle period in the gas injection cycle load, and is defined as:

[0083]

[0084] Considering the condition of constant stress (σ < σ S ) and the time-related response, assuming that the number of cycles N is linearly related to time (N = kt), then the base value of residual strain can be rewritten as:

[0085]

[0086] (6) Rock creep damage equation under the action of staged cyclic gas injection load

[0087] ①σ < σ K < σ S

[0088] When a constant stress is applied that is less than the stress threshold of the switching element (σ < σ K < σ S ), the creep response only exhibits two stages: instantaneous elastic deformation and decelerating creep, and its strain rate decays with time and ultimately approaches zero. When the applied constant stress σ < σ K < σ S , the model only has a generalized Kelvin element, that is, at this time, it is a viscoelastic response dominated by the generalized Kelvin element.

[0089] The state equation at this time is:

[0090]

[0091] The total strain rate is the sum of the rates of each component:

[0092]

[0093] The one-dimensional constitutive equation at this time is:

[0094]

[0095] Therefore, under the action of a constant stress σ (σ < σ K < σ S ), the creep equation is as shown in Equation (20):

[0096]

[0097] ②σ K ≤ σ < σ S

[0098] When the applied constant stress exceeds the stress threshold of the switching element but is less than the stress threshold of the nonlinear viscoplastic element (σ K ≤ σ < σ S ), the creep response exhibits three-stage characteristics: instantaneous elastic deformation, decelerating creep deformation, and steady creep deformation. Its strain rate first decays and then converges to the steady-state creep rate, and finally approaches a certain constant. When the applied constant stress σ K ≤ σ < σ S , the unsteady viscous element with a stress trigger mechanism is activated and produces viscous flow. At this time, since the applied stress is still lower than the stress threshold of the nonlinear viscoplastic element, the constitutive relationship is an unsteady Burgers model with a time-varying viscosity coefficient (that is, composed of a generalized Kelvin element and an unsteady viscous element with a switch in series).

[0099] The state equation at this time is:

[0100]

[0101] The total strain rate is the sum of the rates of its components:

[0102]

[0103] The one-dimensional constitutive equation at this time is:

[0104]

[0105] The creep equation under the action of a constant stress σ (σ K ≤ σ < σ S ):

[0106]

[0107] ③ σ K < σ S ≤ σ

[0108] When the applied stress exceeds the stress threshold value of the non-linear viscoplastic element (σ K < σ S ≤ σ), the creep response exhibits four-stage characteristics: instantaneous elastic deformation, decelerated creep deformation, steady creep deformation, and accelerated creep deformation, ultimately leading to the failure of the rock sample. Given that neither the generalized Kelvin model nor the unsteady Burgers model can characterize the non-linear deformation characteristics in the acceleration stage, a non-linear viscoplastic element with a damage factor variable D is introduced to construct a creep constitutive relationship considering damage accumulation under high stress levels.

[0109] At this time, the creep constitutive model is composed of the series connection of the unsteady Burgers model and the non-linear viscoplastic element, and the corresponding state equation is:

[0110]

[0111] The total strain rate is the sum of the rates of its components:

[0112]

[0113] The one-dimensional constitutive equation at this time is:

[0114]

[0115] The creep equation under the action of a constant stress σ (σ K < σ S ≤ σ):

[0116]

[0117] In equations (13) to (26):

[0118] σ K — Stress threshold of the switching element, which has a stress threshold effect;

[0119] σ s — Stress threshold value of the nonlinear viscoplastic element, i.e., the long-term strength threshold;

[0120] σ — Applied constant stress;

[0121] σ1 — Stress corresponding to the elastic Hooke element part;

[0122] σ2 — Stress corresponding to the Kelvin element part;

[0123] σ3 — Stress corresponding to the unsteady viscous element part with a switch;

[0124] σ4 — Stress corresponding to the nonlinear viscoplastic element part;

[0125] E1 — Elastic coefficient of the elastic Hooke element part;

[0126] E2 — Elastic coefficient of the elastic element in the Kelvin element;

[0127] η1 — Viscous coefficient of the viscous element in the Kelvin element;

[0128] η2 — Initial value of the viscous coefficient of the unsteady viscous element with a switch;

[0129] η3 — Initial value of the viscous coefficient of the nonlinear viscoplastic element;

[0130] t — Time;

[0131] Φ — Rock damage coefficient, which is related to the properties of the rock itself;

[0132] δ — Cyclic accumulation coefficient;

[0133] N — Number of gas injection cycles, with each increase in one level of load being one cycle;

[0134] k — Coefficient;

[0135] n — Nonlinear exponent;

[0136] λ — Constant, with a value range between 0 and 1;

[0137] α — Exponential coefficient of the damage characteristics of the rock material, α is a constant and α > 0;

[0138] ε1 — Strain corresponding to the elastic Hooke element part;

[0139] ε2 — Strain corresponding to the Kelvin element part;

[0140] ε3 — Strain corresponding to the unsteady viscous element part with a switch;

[0141] ε4—the strain corresponding to the non-linear viscoplastic element part;

[0142] ε res —the base value of the residual strain;

[0143] ε—the total strain;

[0144] Integrating the equations in the above various states, the rock creep damage model under the action of the staged cyclic gas injection load is obtained as shown in Equation (1):

[0145]

[0146] In Equation (1), ε is the total strain; σ is the constantly applied stress; σ s is the stress threshold value of the non-linear viscoplastic element; σ K is the stress threshold value of the switching element; E1 is the elastic coefficient of the elastic Hooke element; E2 is the elastic coefficient of the elastic element in the Kelvin element; t is the time; η1 is the viscosity coefficient of the viscous element in the Kelvin element; η2 is the initial value of the viscosity coefficient of the unsteady viscous element with a switch; η3 is the initial value of the viscosity coefficient of the non-linear viscoplastic element; Φ is the rock damage coefficient; δ is the cyclic accumulation coefficient; k is the proportionality coefficient of the number of cycles and time; n is the non-linear index, n>0; λ is a constant between 0 and 1; α is the index coefficient of the rock material damage characteristics, α>0.

[0147] Taking the derivative of the rock creep damage model under the action of the staged cyclic gas injection load with respect to time, the rock creep rate control model under the action of the staged cyclic gas injection load can be obtained, specifically as shown in Equation (2):

[0148]

[0149] In Equation (2), is the first derivative of the creep strain with respect to time t; σ is the constantly applied stress; σ s is the stress threshold value of the non-linear viscoplastic element; σ K is the stress threshold value of the switching element; E2 is the elastic coefficient of the elastic element in the Kelvin element; t is the time; η1 is the viscosity coefficient of the viscous element in the Kelvin element; η2 is the initial value of the viscosity coefficient of the unsteady viscous element with a switch; η3 is the initial value of the viscosity coefficient of the non-linear viscoplastic element; Φ is the rock damage coefficient; δ is the cyclic accumulation coefficient; k is the proportionality coefficient of the number of cycles and time; n is the non-linear index, n>0; λ is a constant between 0 and 1; α is the index coefficient of the rock material damage characteristics, α>0.

[0150] Among them, the rock creep rate equation under the action of staged cyclic gas injection load involves 12 parameters, namely E1, E2, η1, η2, η3, λ, α, Φ, δ, N (N = kt), k, and n. The specific values of these parameters need to be determined through parameter identification based on Equation (1). The parameter identification process is as follows:

[0151] First, for the elastic coefficients E1 and E2, the instantaneous creep deformation ε1 of the rock is caused by the elastic Hooke element. The instantaneous elastic modulus E1 of the model can be determined according to the instantaneous strain, as shown in Equation (27) specifically:

[0152]

[0153] E2 can be obtained by extending the stable creep stage curve to intersect with the strain coordinate axis, and the intersection strain value ε 交 The relationship between E1 and E2 is shown in Equation (28) specifically:

[0154]

[0155] Secondly, for the remaining parameters, their value ranges need to be limited through numerical means in combination with their physical meanings: the value range of parameter λ is between 0 and 1; the value of η1 is in the normal order of magnitude range of the rock; η2 and η3 are related to the time when the rock fails and are also affected by the values of parameters λ and α.

[0156] Φ characterizes the anti-damage ability of the material itself. The larger its value, the greater the residual strain generated by the material under the same loading conditions; δ is the control factor for the contribution rate of a single cycle to damage. The larger its value, the faster the damage accumulation rate caused by each cycle of loading; the total number of cycles N reflects the cumulative effect of the staged load, k is the linear coefficient, and n is the nonlinear exponent, which is used to characterize the nonlinear amplification effect of the constant stress approaching the long-term strength threshold.

[0157] Based on the nonlinear model of Equation (1) relying on the indoor creep experiment results, nonlinear optimization can be carried out in Excel through piecewise modeling combined with Solver. Set the initial guess values of the parameters to be estimated that meet the above range requirements, select the corresponding piecewise equation according to the interval of the applied load, calculate the predicted values row by row, calculate the sum of squared residuals after the calculation is completed, and use the nonlinear GRG algorithm of Solver for parameter optimization, and run the solution iteration until convergence. If Solver does not converge, try to adjust the initial guess values or add parameter constraints.

[0158] Step S3: Combine the elastic mechanics parameters and geological parameters, and use the rock creep rate control model after parameter identification to predict the creep damage state of the dolomite in the reservoir roof under the action of staged cyclic gas injection load.

[0159] The prediction process mainly constructs a finite element model based on core experimental data and parameter identification results, and conducts numerical simulations of the spatio-temporal evolution of creep damage relying on a multi-physical field coupling platform. This model strictly adopts the standard rock sample size for indoor creep tests (Φ50mm×H100mm), configures boundary constraints according to in-situ stress conditions, constructs a creep rate control equation based on the non-linear visco-elastic-plastic constitutive equation of seven-element series, determines the element parameters by inverting indoor creep test data, and establishes a creep constitutive model for roof dolomite under staged loading conditions. This model can more accurately describe the creep behavior of roof dolomite.

[0160] Example:

[0161] This example takes the roof rock stratum of a typical salt cavern for brine extraction in Zigong area as the object, simulates the creep process using the creep rate control model example of rock obtained by the foregoing method, and analyzes the stability and creep characteristics of the roof surrounding rock under staged loads. Periodic air load (compressed air) energy storage is a technology that realizes energy storage and release by periodically compressing and releasing air. This technology relies on a gas turbine system to convert surplus electric energy into compressed air and store it in a geological gas storage during the low electricity load period, and release high-pressure gas to drive the turbine to generate electricity during the peak period. Among them, the test results of the elastic mechanics parameters and geological parameters of dolomite in the example are shown in Table 1:

[0162] Table 1 Rock mechanics and geological parameters of roof dolomite in the example

[0163] Parameter Value Unit Elastic modulus 22.93 GPa Poisson's ratio 0.25 - Density 2880 <![CDATA[kg / m 3 <!-- 9 -->]]> Compressive strength 123.00 MPa Tensile strength 2.19 MPa Cohesion 32.31 MPa Internal friction angle 42.55 ° Hydrostatic pressure in the original salt layer section 20 MPa

[0164] The results of parameter identification for the parameters in the rock creep rate equation in the model corresponding to the example are shown in Table 2:

[0165] Table 2 Parameter identification results of the rock creep rate equation example

[0166]

[0167]

[0168] The obtained indoor creep experimental curve is as Figure 1 shown, indicating that the load level that the specimen can withstand is 6 levels, corresponding to a limit stress of 120 MPa. Under the same stress level, the specimen mainly undergoes instantaneous deformation and stable creep. In the low stress stage (20 - 60 MPa), the instantaneous deformation accounts for the vast majority of the total deformation, while the creep amount is relatively small. With the increase of the load level, the instantaneous strain shows a trend of first decreasing and then increasing.

[0169] Next, based on the parameter identification results, relying on the seven-element nonlinear visco-elastic-plastic creep rate control equation considering damage, combined with the mechanical and geological parameters in Table 1, a numerical simulation of the spatio-temporal evolution of creep damage is carried out on a multi-physics coupling platform. The finite element model of the roof dolomite at the core scale in this embodiment is as Figure 2 shown. At the same time, as Figure 3 shown, the coincidence degree of the experimental data, the fitting data, and the simulation data is relatively good, and the model can accurately describe the creep behavior of the rock.

[0170] Since the salt cavern compressed air energy storage technology belongs to the high-frequency periodic air load energy storage technology, the air load borne by the salt cavern is periodically changing, and the deformation of the dolomite roof does not occur overnight, but shows four stages of instantaneous elastic deformation, decelerated creep deformation, steady creep deformation, and accelerated creep deformation over time. Therefore, a graded load is applied to the finite element model of the embodiment to simulate the periodic load during cyclic gas injection. As Figure 4 shown, the loads are 20MPa, 40MPa, 60MPa, 80MPa, 100MPa, 120MPa, and 140MPa respectively, and each load level is maintained for 48h.

[0171] As Figure 5 and Figure 6 shown, each increase in load will cause a new displacement increment in the rock. The growth is relatively fast in the early stage and then gradually flattens out. As time goes by, the displacement growth process slows down and the displacement growth rate decreases.

[0172] As Figure 7 shown, in the low stress stage (20 - 60MPa, 0 - 144h), after the load is applied and stabilized, the strain grows smoothly, and the strain growth rate accelerates between the previous load and the next load, which is approximately linear and belongs to the stage dominated by visco-elastic deformation; in the medium-high stress stage (80 - 120MPa, 144 - 288h), after the load is applied and stabilized, the strain grows smoothly, and the strain growth rate accelerates between the previous load and the next load, and the curve is slightly convex upward, reflecting that visco-plastic deformation gradually dominates; in the ultra-high stress stage (140MPa, 288 - 336h), the strain jumps significantly, showing an exponential accelerated creep characteristic, and finally the rock will be damaged.

[0173] The above is only a preferred specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed in the embodiments of the present invention should be covered by the protection scope of the present invention. Therefore, the protection scope of the present invention should be subject to the protection scope of the claims.

Claims

1. A method for predicting the creep damage of dolomite in the roof of a salt cavern gas storage, characterized in that It includes the following steps: Step S1: Obtain the elastic mechanical parameters and geological parameters of the dolomite in the salt cavern gas storage roof; Step S2: Establish a rock creep damage model under the action of a graded cyclic gas injection load, obtain a rock creep rate control model under the action of a graded cyclic gas injection load, and identify the parameters of the rock creep rate control model; Step S3: Combine the elastic mechanical parameters and geological parameters, and use the parameter-identified rock creep rate control model to predict the creep damage state of the dolomite in the storage roof under the action of a graded cyclic gas injection load.

2. A method for predicting the creep damage of dolomite in the roof of a salt cavern gas storage reservoir according to claim 1, characterized in that: The elastic mechanical parameters include elastic modulus, Poisson's ratio, compressive strength, and tensile strength; the geological parameters include density, cohesion, and internal friction angle.

3. A prediction method for the creep damage of dolomite in the roof of a salt cavern gas storage reservoir according to claim 1, characterized in that: The rock creep damage model under the action of the graded cyclic gas injection load is shown in Equation (1): In Equation (1), ε is the total strain; σ is the constant applied stress; σ s is the stress threshold of the non-linear viscoplastic element; σ K is the stress threshold of the switching element; E1 is the elastic coefficient of the elastic Hooke element; E2 is the elastic coefficient of the elastic element in the Kelvin element; t is the time; η1 is the viscosity coefficient of the viscous element in the Kelvin element; η2 is the initial value of the viscosity coefficient of the unsteady viscous element with a switch; η3 is the initial value of the viscosity coefficient of the nonlinear viscoplastic element; Φ is the rock damage coefficient; δ is the cyclic accumulation coefficient; k is the proportional coefficient of the number of cycles to time; n is the nonlinear index, n>0; λ is a constant between 0 and 1; α is the exponential coefficient of the rock material damage characteristic, α>0.

4. A method for predicting the creep damage of dolomite in the roof of a salt cavern gas storage reservoir according to claim 3, characterized in that: The rock creep rate control model under the action of the graded cyclic gas injection load is obtained by taking the derivative of the rock creep damage model under the action of the graded cyclic gas injection load with respect to time, and is specifically shown in Equation (2): In Equation (2), is the first derivative of the creep strain with respect to time t; σ is the constant applied stress; σ s is the stress threshold value of the non-linear viscoplastic element; σ K is the stress threshold value of the switching element; E2 is the elastic coefficient of the elastic element in the Kelvin element; t is the time; η1 is the viscosity coefficient of the viscous element in the Kelvin element; η2 is the initial value of the viscosity coefficient of the unsteady viscous element with a switch; η3 is the initial value of the viscosity coefficient of the nonlinear viscoplastic element; Φ is the rock damage coefficient; δ is the cyclic accumulation coefficient; k is the proportional coefficient of the number of cycles to time; n is the nonlinear index, n>0; λ is a constant between 0 and 1; α is the exponential coefficient of the rock material damage characteristic, α>0.

5. A method for predicting the creep damage of dolomite in the roof of a salt cavern gas storage reservoir according to claim 4, characterized in that: The parameter identification step is: for the object parameters, relying on the nonlinear model of Equation (1), take an initial value within the allowable range of the object parameters for parameter optimization.

6. A method for predicting the creep damage of dolomite in the roof of a salt cavern gas storage reservoir according to claim 5, characterized in that: The object parameters are E1, E2, η1, η2, η3, λ, α, Φ, δ, N, k, n.

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