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.

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