The permeability of the cement in the gas storage is very low (usually less than 1×10−15 m2), and a complete wellbore cement sheath can be considered to have sufficient gas tightness to ensure the safety of salt cavern storage. However, after a failure of the cement/casing or cement/rock interface, the wellbore will become the main path for leakage. High-frequency gas injection and extraction and cavern shrinkage are the main factors causing wellbore integrity failure, and the main forms of failure are radial cracks and micro-annulars. The cement sheath at the casing shoe location is contacted directly by the high-pressure gas in the cavern, which is a weak point in wellbore safety. Once debonding begins at the casing shoe of the wellbore, the stored medium in the cavern may seep into the wellbore defects, causing gas leakage. Therefore, revealing the failure mechanism of wellbore sealing in salt cavern UGS and being able to evaluate its sealing quantitatively is crucial to ensuring the safety of salt cavern UGS. Based on the above background and objectives, this chapter mainly reports on the following research: (1) the failure mechanism of wellbore sealing in salt cavern UGS; (2) wellbore seepage rules and leakage evaluation model; (3) permeability test of wellbore cement sheath; (4) wellbore leakage simulation and evaluation.