Novel Prediction Method for the Service Life of the Cement Sheath in Gas Storage Wells

Summary Uncertainty about failure type and time of cement sheath integrity significantly threatens the long-term safe and stable operation of gas storage wells. In this study, a novel prediction method for the service life of a cement sheath that comprehensively considers structural failure, interface sealing failure, and gas seepage failure was established by combining theoretical models and experiments. The failure mechanism for the cement sheath integrity in a case gas storage well with sustained casing pressure was analyzed. The influencing factors and control measures for the service life of the cement sheath were discussed. The results indicated that, in the process of gas seepage along the intercommunication pores inside the cement sheath to the wellhead, pressure will gradually accumulate at the top of the cement sheath, resulting in sustained casing pressure. Maintaining the gas storage pressure below 22.5 MPa, using a cement sheath with a permeability below 1 × 10−3 md, and designing deeper well depth can effectively mitigate the aforementioned phenomenon. Based on the results, it is recommended to use a wellbore size of 215.9 mm (8-1/2 in.) with a 139.7 mm (5-1/2 in.) casing, along with a cement sheath with an elastic modulus close to 6 GPa, ensuring a service life of the cement sheath for at least 25 years and 45 injection-production cycles. The research results provide a novel method for predicting the service life of a cement sheath in gas storage wells, which helps to reveal the mechanism of sustained casing pressure further and put forward effective countermeasures.