Cement Failure Caused by Thermal Stresses with Casing Eccentricity During CO_2 Injection

摘要

Carbon capture and sequestration (CCS) is one of the most promising technologies to mitigate greenhouse gas levels. To ensure an effective underground storage, well integrity is critical to isolating the injected fluid between different zones or back to the surface. Among the wellbore components, the cement sheath is the most important sealing element for zonal isolation. However, cement is vulnerable and prone to cracking that may provide leakage pathways for CO_2. Both laboratory study and field test show that thermal stresses caused by the temperature variation in the wellbore are a major factor for the mechanical integrity loss of cement. This work focuses on the mechanical response of the casing-cement-formation section above the injection zone. We firstly propose a wellbore flow model to predict the temperature distribution along the well depth. Then we calculate the induced stress in cement during injection by a finite element simulation. To identify the cement failure mode, we introduce failure factors by the Mogi-Coulomb criterion, tensile strength and interfacial strength corresponding to shear compressive failure, radial cracking and debonding at the casing/cement or cement/formation interfaces, respectively. A parametric study is conducted to investigate the influence of the injection temperature and rate as well as casing eccentricity on failure factors. The results show that radical cracking and debonding at the cement/formation interface are the main failure modes during CO_2 injection. Both the two failure factors would increase linearly as the injection temperature decreases while they grow non-linearly with the injection rate. In addition, the casing eccentricity exacerbates the risk of cement integrity loss by increasing failure factors. This study provides a failure assessment of CO_2 geological sequestration and guidelines for injection operations.