In geological sequestration, carbon dioxide (CO2) is injected and stored in aquifers confined with a thick low permeable layer known as a caprock. The formation of a normal fault in a sandstone- or shale-based caprock leads to the entrainment of clay smears within the fault zone which gets redistributed between footwalls and hanging walls. Few attempts were made to investigate the effect of clay smears on the cross-fault flow of groundwater or hydrocarbons. Also, the influence of interaction between clay minerals and CO2 on the integrity of the caprock was quantified only at a mineralogical scale. Given this, the present study developed a numerical model to account for the interaction of CO2 with clay minerals that occurs during the migration of CO2 through the caprock.
The model captures the volumetric strain of a clay-rich medium due to CO2 adsorption on the clay mineral’s surface, solid media compressibility, and change in the state of stress within the medium. The interaction between CO2 and clay minerals has affected the evolution of macro-permeability, gas storage capacity, gas pore pressure, and stress state of compacted clays under various confinement conditions. It is evident that under confined conditions, the permeability of the clay- rich media is reduced, and mean compressive stress increased due to gas adsorption-induced strain. On the contrary, in free and uniaxial conditions, the permeability of the clay-rich media increased. The magnitude of the swelling stress within the medium under uniaxial conditions depends on the degree of expansion caused by the adsorption-induced effect and applied overburden pressure. Overall, the study contributes to a comprehensive approach to evaluate the efficacy of geological formations having faults for long-term storage of CO2 in them.