Experimental investigations in CO2 sequestration and shale caprock integrity

More than sixty percent (60%) of conventional hydrocarbon reservoirs which are potential CO2 repositories are sealed by tight shale caprock. The geochemical reactivity of shale caprock during CO2 diffusive transport needs to be included in the reservoir characterization of potential CO2 sequestration sites as slow reactive transport processes can either strengthen or degrade seal integrity over the long term. Several simulation results had predicted that influx-induced mineral dissolution/precipitation reactions within shale caprocks can continuously reduce micro-fracture networks, while pressure and effective-stress transformation first rapidly increase then progressively constrict them. This experimental work 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 geochemical characterization of the effluent fluid and bulk shale rock through ICP-OES, XRD, EDS and pH measurements. Nano-scale measurement of changes in internal specific surface area, pore volume and linear/cumulative pore size distribution (using the BET Technique) showed that changes in the shale caprock due to geochemical interaction with aqueous CO2 can affect petrophysical properties. 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 in CO2 sequestration