Clay interaction with liquid and supercritical CO₂: The relevance of electrical and capillary forces

Caprocks with significant clay content are candidate seal layers for CO₂ geological storage. Changes in electrical and capillary forces are expected when CO₂ invades the water saturated pore space. Sedimentation experiments conducted to explore the response of kaolinite and montmorillonite to deionized water, brine, heptane, liquid CO₂ and supercritical CO₂ show that both montmorillonite and kaolinite aggregate when submerged in CO₂ and the final porosity in CO₂ is smaller than in brine. Differences in dielectric properties between CO₂ and water, and ensuing implications on van der Waals attraction and double layer repulsion explain the observed phenomena. On the other hand, capillary effects induced by the water-CO₂ interface are corroborated by clay-water paste desiccation experiments conducted using supercritical CO₂: water dissolution into the surrounding CO₂ causes suction and capillary contraction, the invasion of the CO₂-water interface into the sediment, and the formation of desiccation cracks. Volume contraction and crack initiation are consistent with the sediment response within an effective stress framework. Altered electrical forces and emergent capillary forces lead to coupled chemo-hydro-mechanical phenomena in seal layers that could facilitate CO₂ breakthrough and advection through high porosity caprocks; related phenomena are identified in the reservoir rock. Additional studies are needed to further assess coupled phenomena when the interparticle distance is a few monolayers of water.