Failure analysis for gas storage salt cavern by thermo-mechanical modelling considering rock salt creep

Abstract

Salt cavern is ideal vessel for underground gas storage due to the high deliverability. During cyclic gas operations of injection-and-withdrawal, the temperature change in salt cavern imposes thermal stress on cavern wall. The temperature change, together with the pressure change in salt cavern, causes the effective stress on cavern wall to alter, leading to variation of creep rate. In this study, a coupled thermo-mechanical model is proposed for failure analysis of salt cavern in rock salt prone to creep deformation. Six cases that consider rock salt with different creep tendency have been conceived for the coupled thermo-mechanical model to investigate the stability of the salt cavern in terms of cavern convergence and failure indices. The results indicate that the affected region by cyclic pressure and temperature is up to 10 m inside the rock salt from the cavern wall. It is also revealed from the results that largest displacement occurs on the top of the cavern, indicating that the cavern top is most liable to deformation damage. Although convergence of cavern fluctuates over the injection and withdrawal sessions, long-term convergence of cavern depends on the creep tendency of rock salt rather than the cyclic loading; rock salt with stronger creep tendency leads to larger cavern convergence. The results also demonstrate that there is stress concentration on the top and bottom of the cavern; the cavern wall is likely subject to shear failure according to Mohr‒Coulomb failure criteria, however, the cavern is unlikely to fail due to expansion according to Drucker‒Prager failure criteria. Both Mohr‒Coulomb and Drucker‒Prager failure indices are smaller in the cases where rock salt has stronger creep tendency. The proposed thermo-mechanical model provides an approach for evaluation of long-term stability of underground salt caverns.