Stability analysis of complex behavior of salt cavern subjected to cyclic loading by laboratory measurement and numerical modeling using LOCAS (case study: Nasrabad gas storage salt cavern)

Stability analysis of salt caverns is a very complicated subject due to the coupled time-dependent thermo-mechanical behavior of salt during leaching and operational phase of the gas storage subjected to the cyclic loading. Because of purely plastic behavior of salt and the relevant convergence during injection and withdrawal, investigation of the salt cavern stability becomes more challenging. The objective of this study is stability analysis of Nasrabad salt cavern by numerical method using a comprehensive software entitled LOCAS having capability to model the complex time-dependent thermo-mechanical behavior of salt under cyclic loading of natural gas pressure. Measurement of geomechanical properties of salt is also the important requirement for modelling. First, geomechanical properties of Nasrabad salt including uniaxial and tri-axial compressive strength, tensile strength, uniaxial and tri-axial creep under different temperatures were measured. Thereafter, time-dependent behaviors and parameters of dilatancy criterion of the test results were analyzed by the advanced constitutive models for rock salt to obtain accurate parameters for modeling. Then, long-term stability was analyzed for Nasrabad salt cavern having different shapes, sizes, and depths under cyclic loading 3-8 MPa as minimum and maximum gas pressures. The results showed that an ellipsoidal cavern having initial volume of 100,000 m(3) at 450 m depth by 0.3% creep closure rate per year and volume loss of 0.8% of the initial volume per year as ideal conditions can store 8,000,000 m(3) natural gas with working capacity of about 6,000,000 m(3).