Study On The Creep Mechanical Properties Of Salt Rock Under Different Loading Paths And Long-term Stability Of Salt Rock Gas Storage

It is the worldwide-recognized energy reserve method and the key deploymentdirection of our country to implement energy strategic reserve plan by storing oil andnatural gas in deep salt rock caverns. As one of the most important mechanicalproperties of salt rock, creep property is closely related to the long-term stability andsecurity of salt rock underground storage. So, it has very important significance inpractice to research the creep property of salt rock.In view of this, researches carried out in this paper would use a combinationmethod including testing research, theoretical analysis and numerical simulation. Firstly,triaxial compression creep tests for salt rock under different loading paths were carriedout, and the impact of loading path and stress level to creep property of salt rock wasanalyzed; Secondly, the nonlinear viscoelasto-plastic creep model and the creep modelunder low-frequency cyclic load for salt rock were present based on testing results; Atlast, the nonlinear viscoelasto-plastic creep model was programmed in computer andapplied to long-term stability analysis of salt rock underground gas storage.The main contents and research conclusions of this paper were as follows:①In order to know about the creep properties of salt rock, triaxial compressioncreep experiments with salt rock specimens under different stress loading paths werecarried out on RLW-2000rock servo controlling rheology testing machine. Based ontest results, deformation properties and variation law of creep rate with time underdifferent loading paths and stress levels in creep process were discussed in detail.②According to the basic nonlinear rheology theory of rock and thetime-dependence of viscosity coefficient in different creep stages, nonlinear Kelvinmodel, nonlinear Maxwell model and nonlinear Bingham model were put forward. Thenonlinear models had many special characteristics and better adaptability in describingthe rheological properties of rocks through comparing the characteristics of nonlinearmodels with common models.③A new nonlinear viscoelasto-plastic creep model for salt rock was established byconnecting nonlinear Kelvin model with nonlinear Maxwell model and nonlinearBingham model in the form of series, which could describe the nonlinear creep propertyof salt rock and the primary creep, steady creep and tertiary creep perfectly. Accordingto the least squares theory and triaxial compression creep test results of salt rock, the parameters of nonlinear creep model were obtained by inversion calculation. Thentheoretical results were calculated by substituting the parameters into the model, andthrough comparing, theoretical curves were in good agreement with test curves, whichindicated that the nonlinear creep model could well describe the creep property of saltrock.④The creep equation of salt rock under low-frequency cyclic load was deducedbased on Burgers model. According to the least squares theory and triaxial compressioncreep test results of salt rock under cyclic load, the parameters of the proposed modelwere obtained by inversion calculation. Through comparing theoretical curves with testcurves, it was found that the proposed creep model could well describe the creepproperty of salt rock under cyclic load, especially the fluctuation phenomenon of creepdeformation with load fluctuation.⑤According to the difference theory and good redevelopment platform of FLAC3Dsoftware, the finite difference form of the nonlinear viscoelasto-plastic creep model forsalt rock was deduced. Based on which, the redevelopment of nonlinear creep modelwas carried out in FLAC3Dwith taking the Mohr-Coulomb criterion as failure criterion,and then the correctness and applicability of the redevelopment computer program wasverified by experimental simulation and engineering example analysis.⑥The nonlinear viscoelasto-plastic creep model computer program was applied tolong-term stability analysis of salt rock underground gas storage. According to thesimulation results, the variation laws of plastic zones, surrounding rock massdeformation, volume closure ratio and dilation damage safety factor with time and gaspressure were analyzed, and the minimum gas pressure, maximum gas recovery velocityand reasonable pillar width were determined.