Cement Sheath Integrity Evaluation Under Multiple Cyclic Loading Using Mechanical Equivalent Experiment for Gas Storage Wells in Eastern China

Abstract

Abstract Gas storage wells in eastern China have been in operation for nearly 30 years. Fatigue damage may have occurred in the cement sheath after years of injection-production cycles. If the risk of integrity failure of the cement sheath can be predicted accurately, optimization or remedial measures can be implemented over time to further prolong the life of gas storage wells. In this paper, we established a mechanical equivalent method to restore the wellbore load borne by the cement sheath to the self-developed wellbore simulation device, and carried out the cyclic load tests of 376, 141 and 54 rounds at the injection-production differential pressure of 5, 10 and 15 MPa, respectively. Based on this method and the twin-shear unified strength theory, the integrity failure mechanism of the cement sheath was analyzed. We found that after cyclic loading, the tensile strength of the cement sheath decreased. Fatigue-tensile failure was the main reason for the integrity failure of the cement sheath in gas storage wells. Residual tensile strength decreased with the increase in cycle time and injection-production differential pressure. Taking a gas storage well in eastern China as an example, it was found that if the well maintains the current injection-production differential pressure, there is a risk of cement sheath failure. In this regard, the optimization measures of reducing injection-production pressure difference were proposed to prolong wellbore life. At the same time, the suggestion of using a modified cement system in gas storage wells was also proposed. The research results can provide an important reference for predicting gas storage well life and designing mechanical properties of cement sheath.