ABSTRACT

Caprock integrity is an important subject, as it is closely related to the leakage of injected CO₂ into geological formations. Thus, it is necessary to evaluate how injected CO₂ affects caprocks during CO₂ storage projects. This study assessed CO₂ migration into a caprock from the injection reservoir and its interaction with the rock minerals of the caprock and brine. The caprock is the Sayindere Formation, which is a regionally extensive caprock in oil fields located in the southeastern part of Turkey including the Caylarbasi oil field, which has been previously identified and modeled as a potential CO₂ storage site. The upper part of the Sayindere Formation consists of clay, and the lower part consists of clayey limestone. In this work, we performed 2-D radial modeling of CO₂ injection at an annual rate of 1 million tons into the reservoir for 50 years and investigated its impact on the overlying caprock. The simulation was continued for further 1000 years without CO₂ injection to evaluate the caprock evolution over longer time periods. Calcite dissolution was observed in the reservoir as well as the caprock. However, no significant increase in the porosity and permeability associated with calcite dissolution was observed in either formation. Minor diffusion of the injected CO₂ into the lowermost layer of the caprock was observed over the simulation years. After 1000 years of post-injection, 70% of the injected CO₂ was trapped in the reservoir under the caprock as a plume in a free phase, and the rest was dissolved in the reservoir brine. Sensitivity analyses were performed to determine how permeability anisotropy and reactant surface area of minerals affect the numerical results. Minor variations in the spatial distributions of CO₂ saturation were observed when the permeability anisotropy was changed. No significant variation in the numerical results was observed with changes in the surface area of minerals. The simulation results suggest that the Sayindere formation is a very good caprock for potential CO₂ storage. This work provides information for future decision-making and for development of CO₂ storage demonstration projects in Turkey.