Characterization of multiphase flow in shaly caprock for geologic CO2 storage

Abstract

Geologic carbon dioxide (CO2) storage is a promising strategy for reducing its atmospheric levels, but concerns regarding potential leakage through faults or high-permeability inclusions in caprock highlight the importance of understanding its multiphase flow mechanism. In this study, a comprehensive experimental approach is introduced to characterize the pore structure of heterogeneous and fractured caprock and its relationship to water/CO2 relative permeability. It is observed that intact caprock with higher clay content exhibits a higher decrease in water relative permeability in the drainage: the exponent values in the Brooks-Corey model are 3.89 for sandy Eau Claire Shale, 5.04 for the clayey shale, and 6.77 for intact Opalinus Clay - consistent with observations for porous and tight rock. CO2 relative permeability also exhibits an exponential increase with CO2 saturation, which is more pronounced for sandy and fractured caprock due to the higher endpoint CO2 relative permeability and CO2 saturation. Moreover, a method to estimate relative permeability based on the water-CO2 capillary pressure response derived from the pore structure analysis is adopted and its results are compared to the direct measurements. This approach provides a better fit for intact clay-rich caprock but tends to underestimate the relative permeability as a function of fluid saturation for heterogeneous and fractured caprock. The pore structure-based estimation allows for pre-assessing the multiphase flow through caprock, although the direct measurements are highly recommended, especially for fractured materials.