Shale-caprock integrity is critical in ensuring that subsurface injection and storage of anthropogenic carbon dioxide (CO2) is permanent. The interaction of clay-rich rock with aqueous CO2 under dynamic conditions requires characterization at the nano-scale due to the low-reactivity of clay minerals. Geochemical mineral-fluid interaction can impact properties of shale rocks primarily through changes in pore geometry/connectivity. Several simulation results had predicted that influx-triggered mineral dissolution/precipitation reactions within clay-rich rock can continuously diminish micro-fracture networks, while pressure and effective-stress evolution initially increase then gradually constrict them. However, most of these studies have not investigated changes under experimental conditions and applying experimental techniques capable of detecting changes at nano-scale, which are ultimately representing pathways for molecular diffusion of fluids through porous media.
The experimental work reported in this paper applied specific analytical techniques in investigating changes in surface/near-surface properties of crushed shale rocks after exposure (by flooding) to CO2-brine for a time frame ranging between 30 days to 92 days at elevated pressure and fractional flow rate. Initial capillary entry parameters for the shale were estimated from digitally acquired pressure data evolution. Flooding of the shale samples with CO2-brine was followed by Nano-scale measurement of changes in internal specific surface area, pore volume and linear/cumulative pore size distribution. The BET Technique showed that changes in the shale caprock occurred due to geochemical interaction with aqueous CO2 will impact petrophysical properties of the rock. The intrinsically low permeability in shale may be altered by changes in surface properties as the effective permeability of any porous medium is largely a function of its global pore geometry. Diffusive transport of CO2 as well as carbon accounting could be significantly affected over the long term. The estimation of dimensionless quantities such as Peclet (Pe) and Peclet-Damkohler (PeDa) Numbers that are associated with geochemical reactivity of rocks and acidic fluid transport through porous media gave insight into the impact of diffusion and reaction rate on shale caprock.