Simulation study on the evolution of cement plug sealing capability in CO2 geological sequestration wells

Abstract

CO2 geological sequestration technology injects CO2 into the target geological formation through a borehole. Usually, cement plugs are used to seal the well after finishing the injection. Ensuring the plugs' sealing capability is the focus after CO2 sequestration, which is also directly related to the final result of the sequestration. Since the cement's mechanical and poroperm behavior under corrosion has yet to be fully considered, this paper developed a 3D finite element model of the plug-formation, which considers the changes of the corrosion zone. This paper simulates the micro-annulus propagation on the bonding surface, quantifies the effects of corrosion time and corrosion environment on the propagation characterivdstics, and discusses the effects of potential changes in bonding strength and corrosion zone Poisson's ratio after corrosion on the plugs' sealing. The simulation results show that: with the increase of corrosion time, the plug is more prone to radial deformation, the micro-annulus propagation pressure decreases as a power function, and the micro-annulus axial propagation distance is longer. Compared with the liquid-phase corrosion environment, the micro-annulus is more easily propagated in the gas-phase corrosion environment, the micro-annulus aperture is larger, and the propagation distance is longer. When the bond strength and corrosion zone Poisson's ratio decrease after corrosion, the plug's sealing capability will weaken further. In CO2 sequestration wells, following the general abandonment requirements is not appropriate. Improving the cement formulation to reduce the corrosion rate, enhancing the cement's strength after corrosion, and extending the plug length will help maintain the plug's long-term sealing. This study explains the effect of CO2 chemical attack on the plug's sealing. The results have important implications for controlling the long-term sealing capability of CO2 geological sequestration wells.