Failure mode of cement sheath in salt cavern gas storge wellbore based on coupling plasticity and damage evolution

Abstract

In this paper, the failure mechanism of the cement sheath in gas storage salt caverns under the cyclic internal pressure and cavity shrinkage was explored. First, an elastoplastic damage evolution model for the completion cement was established and further developed as a constitutive equation for numerical calculation. Second, the verification of the model is completed by mechanical tests and numerical examples, respectively. The uniaxial and triaxial compression tests, Brazilian splitting tests, and cyclic loading tests were performed to verify the theoretical stress-strain curves. Numerical examples were performed to verify the stability of the constitutive equation in numerical simulations. Finally, the constitutive equation is applied to the engineering, and the numerical model calculation of wellbore fatigue damage and salt cavity shrinkage was performed. The following conclusions are obtained: cyclic loading will increase the plastic strain of the cement sheath, and micro-annuluses will form on the inner surface after 25 years of operation. The shrinkage of the salt cavity will debond the outer side of the cement sheath near the open well. With the increase of cavity shrinkage, the range of wellbore failure increases upwards. When the cavity shrinkage rate exceeds 10%, partial debonding will occur on the cemented surface between the cement sheath and the rock salt formation, which may provide a channel for natural gas leakage.