Experimental Investigations of Caprock Integrity in CO2 Sequestration

AbstractThe geochemical reactivity of shale caprock during post-injection CO2 diffusive transfer should be included in the reservoir characterization of CO2 sequestration sites as slow reactive transport processes can either improve or degrade seal integrity over the long term. Simulation results reported by various researchers suggested that influx-induced mineral dissolution/precipitation reactions within shale caprocks can continuously close micro-fracture networks, while pressure and effective-stress transformation first rapidly expand then progressively constrict them. This experimental work applied specific analytical techniques in investigating changes in mineralogical and microstructural 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. The initial mineralogical composition in the samples showed bulk clay, quartz and feldspar as the major components of the formation. Initial capillary entry parameters for the shale were estimated from digitally acquired injection pressure profile. Flooding of the shale samples with CO2-brine was followed by geochemical characterization of the effluent fluid and bulk shale rock through pH and XRD measurements. Nano-scale measurement of changes in internal specific surface area and cumulative pore size distribution (using the BET Technique) revealed that changes in the shale caprock due to geochemical interaction with aqueous CO2 can affect petrophysical properties. The naturally low permeability in shale may be altered by changes in specific surface area as the effective permeability of any porous medium is largely a function of its total pore geometry/volume, and particularly the pore throat-diameter. The geochemical reactivity of shale caprocks through acidic fluid transport in interconnected pore networks gave insight into the impact of diffusion and reaction rate on shale caprock integrity in CO2 sequestration