Near Wellbore Cement Integrity of CO2 Geologic Storage in Mature Gas Field, Malaysia

Appraising CO₂ leakage risks and ensuring the long-term CO₂ containment integrity of existing and new wells are key factors in guaranteeing the sustainability of potential CO₂ geologic storage in a depleted mature gas field. Particularly, considering CO₂ leakage through cement itself (i.e., sheath and plug) and cement interfaces with casing is critical in assessing the CO₂ leakage risks and the long-term containment integrity sustainability.

To estimate CO₂ leakage through cement sheath/plug and cement-casing interfaces, two separate work scopes were identified and addressed. First, a corrosion-based cement damage model was developed to assess CO₂ leakage risks through cement sheath/plug based on the stress corrosion crack theory, which implies mechanical and chemical corrosion effects on mechanical materials (i.e., chemical-mechanical coupling). Thus, the stress corrosion-based cement damage model can account for corrosion effects on cement due to CO₂ exposure in a CO₂ geologic storage and assess CO₂ leakage through the damaged cement sheath/plug.

Later, a cement interface debonding model was developed to estimate CO₂ leakage risks through cement-casing interfaces. The interface debonding model is based on conventional static failure criteria, as well as analytical hydraulic fracturing methods simulating interface debonding as small-scale hydraulic fracturing caused by CO₂ injection. The cement interface debonding model can be applied for cement-casing bond integrity analysis to estimate CO₂ leakage risks and rates through the debonded interfaces.