Integrity analysis of wellbores in the bedded salt cavern for energy storage

Salt cavern used for energy (natural gas, hydrogen) storage has a significant advantage in peak shaving of gas supply due to their high injection-production efficiency and fast gas injection-delivery switching speed. As the only channel for gas injection and delivery, the wellbore is easily damaged by the alternating gas pressure and is a weak component. For salt cavern underground gas storages (UGSs) in bedded rock salt, the roof collapse can also cause tensile damage to the wellbore. Long-running UGSs may suffer leakage accidents due to wellbore failures, and wellbore safety issues need to be fully considered. This paper proposes a coupled analysis of cement sheath fatigue damage and rock salt creep, and its applicability is proved by mechanical tests. A three-dimensional numerical model was established that includes a wellbore, open well section (cavern neck), and bedded salt cavern. The creep and damage constitutive equations were used to calculate rock salt creep and cement sheath damage. The results show that the creep ability of bedded rock salt is lower than that of high-purity rock salt, confirmed by the creep test results and can be described by the creep constitutive equation in this paper. The cavity shrinkage of the bedded UGSs is generally lower than expected, resulting in better wellbore integrity. The low creep capacity of the impurity-containing rock salt will reduce the cavity shrinkage and protect the airtightness of the wellbore. Based on the integrity of the wellbore, the working gas volume of the UGSs in the bedded formation can be increased. Taking the Jintan gas storage as an example, under the operating pressure of 7–17 MPa, the damaged area of the cement sheath is limited to within 0.5 m of the lower end of the wellbore. When the lower limit pressure is reduced to 6 MPa, the damaged area is limited to within 2 m of the lower end of the wellbore, and the wellbore still maintains sufficient airtightness.