Leakage mechanism of cement sheath and plug under HPHT CO2 corrosion

The sealing integrity of cement sheaths and plugs is an essential barrier to maintaining long-term sealing in CO2 storage wells. Based on the typical working conditions in CO2 storage wells, a method for evaluating the integrity of the plug and sheath under high-temperature and high-pressure (HPHT) CO2 corrosion is proposed, by which the full-scale experimental device of the “cement plug-production casing-cement sheath-intermediate casing” system (CP-PC-CS-IC system) is developed. The interface performance (sealing, shearing bonding) and body performance (permeability and strength) of the sheath and plug are obtained under different temperatures and CO2 partial pressure, by which the failure mechanism of the sheath and plug is revealed under HPHT CO2 corrosion. The results show that the sealing performance of the plug is higher than the sheath after curing, by which the sealing performance decreases with the increasing curing temperature, corrosion temperature, and corrosion pressure. The HPHT corrosion damage sealing and bonding performance at the cement interface and UCS and permeability at the cement body. The cement interface is the main leakage channel for gas and liquid. The cement sheath is the weakest link in the CP-PC-CS-IC system under HPHT corrosion. The research results could provide a reference and basis for optimizing and controlling wellbore integrity in CCUS geological storage wells.