Abstract

Clay-rich cap rocks such as shale should be investigated experimentally for their geomechanical and geochemical behavior when in contact with aqueous CO2 over a long period of time, as can be observed in geologic carbon sequestration. The reactivity of shale cap rock during diffusive transport of CO2-brine needs to be included in the reservoir characterization of potential CO2 sequestration sites because slow reactive transport processes can either strengthen or degrade seal integrity in the long term. This experimental work applied inductively coupled plasma–optical emission spectroscopy (ICP-OES) and the Brunauer-Emmett-Teller (BET) techniques in investigating changes in surface and near-surface properties of crushed shale rocks after exposure (by flooding) to CO2-brine for a time frame ranging from 30 days to 90 days at elevated pressure and fractional flow rate. Flooding of the shale samples with CO2-brine was followed by measurement of changes in internal specific surface area and linear pore-size distribution resulting from aqueous CO2 reactive transport. The intrinsically low hydraulic conductivity of shale may be altered by changes in surface properties because the effective permeability of any porous medium is largely a function of its global specific pore geometry and surface mineralogical properties. Capillary entry pressure for the shale, as well as the average pore throat size were estimated from digitally acquired pressure evolution data. This allowed for the estimation of dimensionless quantities such as Peclet (Pe) and Peclet-Damköhler (PeDa) numbers, which are associated with the geochemical reactivity of rocks and acidic fluid transport through porous media.

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