Failure mechanism of cement plug-formation interface under the thermo- mechanical coupling effect in old wellbore for salt cavern gas storage

The effective sealing of old wells previously used for brine extraction is a key task for the transformation of salt caverns into gas storage tanks and their safe and long-term operation. The essence of the work is to explore the failure problem of the cement plug formation interface in the sealing section. In this paper, a three-dimensional finite element model of cement plug-formation was developed under the influence of mechanical and thermal coupling. This model was used to clarify the enhancement mechanism of thermal effect on the in-situ bonding of the interface. And, the effects of gas storage operation parameters, sealing section formation characteristics, and cement plug body parameters on the ultimate stripping length of the cement plug were studied. Moreover, the ultimate peel failure length of the interface under the coupling effect of multiple factors was determined. The research results indicate that under the influence of temperature and pressure conditions, the formation undergoes a phenomenon of radial shrinkage, increasing the contact stress between the cement plug and the formation, resulting in a denser cementation between the cement plug and the formation interface, thereby reducing the detachment limit.