Impacts of hydrological heterogeneities on caprock mineral alteration and containment of CO2 in geological storage sites

For long-term geological CO2 storage, heterogeneity of hydrological parameters (i.e., porosity and permeability in this study) leads to the development of heterogeneities in physical parameters within the reservoir that are critical to the risk assessment and monitoring requirements of the project. Caprock heterogeneity is also a critical aspect of CO2 storage planning because of the implications for storage integrity and associated risk assessment. The objectives of this study are: (1) to assess the effects of heterogeneity on caprock mineral alteration and associated evolution of sealing quality; (2) to elucidate how hydrologic heterogeneities affect CO2 migration within caprock, and (3) to characterize the relative roles (importance) of porosity heterogeneity incorporated into permeability heterogeneity versus permeability heterogeneity only. A 2-dimensional (2D) model of the Xingouzui formation in the Jianghan Basin of China was developed for this analysis. Heterogeneity in porosity was parameterized employing a specific empirical relationship between permeability and porosity obtained by regression analysis of the field data in the Jianghan Basin. Homogeneous mineral compositions of reservoir and caprock from the Jianghan Basin were used in all simulations. Three model permutations were evaluated, including a homogeneous case, a case with heterogeneity in permeability only, and a case with heterogeneity in both porosity and permeability. Simulation results indicated significant differences among these cases with respect to forecasted mineral alterations, associated evolution of caprock sealing quality, and CO2 migration within the caprock. The impacts of hydrological heterogeneities depend not only on mineralogical composition but also on time scale. The relative roles of porosity heterogeneity and permeability heterogeneity were similar, though porosity variability led to slightly more retarded vertical CO2 migration and increased horizontal propagation, and enhanced mineral dissolution resulting in weakened caprock sealing quality.