Failure mechanism of gas storage salt cavern cement sheath under the coupling effect of plasticity and fatigue damage

Abstract

Due to their good air tightness and stability, gas storage salt caverns are widely used in rapid peak shaving of natural gas. The wellbore is used to connect the cavern to the surface pipeline, and its integrity can usually be guaranteed after cementing and can be verified by logging. In gas injection and production, the tightness of the cement sheath will deteriorate, which may lead to gas leakage accidents. To explore the wellbore integrity during operations, a theoretical model considering fatigue damage and plasticity is established, and constitutive equations are developed for numerical simulation. The model in this paper has been verified by mechanical tests and has good stability and accuracy. Through experiments and simulations, the following conclusions are drawn: (1) The coupling of plasticity and damage can rapidly lead to failure of the wellbore cement sheath interface. Only calculating plasticity or damage may overestimate the integrity of the cement sheath. (2) Fatigue damage and plastic strain under cyclic loading will be concentrated locally, mainly at the interface between the cement sheath and the casing, that is, the inner cemented surface is more likely to fail. (3) The plasticity and damage evolution of the cement sheath have different sensitivities to different cementing defects. Low-quality cementing such as lack of cement sheath will lead to rapid accumulation of wellbore plastic strain and damage. However, casing deviation does not increase plasticity and damage evolution.