Leakage Mechanism of Cement Sheath and Plug Under HPHT CO2 Corrosion

Abstract

Abstract. The sealing integrity of cement sheaths and plugs is an essential barrier to maintaining long-term sealing in carbon dioxide (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 is developed. The interface performance (sealing and 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 HPHT corrosion not only damages the cement body, increases its permeability, and reduces its compressive strength, but also destroys the cement interface due to gas and liquid migration. However, the main leakage pathway is the cement interface rather than the permeation of its body, even though the permeability of cement increases after HPHT corrosion. The sealing performance of cement sheaths and plugs decreases with increasing curing temperature, corrosion temperature, and corrosion pressure, while the sealing performance of the plug is higher than the sheath under the same conditions because the two interfaces of the sheath provide more leakage channels. The research results could provide a reference and basis for optimizing and controlling wellbore integrity in CCUS geological storage wells.