Impure CO2 reaction of feldspar, clay, and organic matter rich cap-rocks: Decreases in the fraction of accessible mesopores measured by SANS.

During CO 2 geological storage, low porosity and permeability cap-rock can act as a structural trap, preventing CO 2 vertical migration to overlying fresh water aquifers or the surface. Clay and organic matter rich shales, fine-grained sandstones and mudstones often act as cap-rocks and may contain substantial sub-micron porosity. CO 2 -brine-rock interactions can open or close pore throats through dissolution, precipitation or migration of clay fines or grains. This could affect CO 2 migration if the porosity is accessible, with unchanging or decreasing accessible porosity favourable for trapping and integrity. Two cap-rock core samples, a clay and organic-rich mudstone and a more organic-lean feldspar-rich fine grained sandstone, from a well drilled for a CO 2 storage feasibility study in Australia were experimentally reacted with impure CO 2 (+ SO 2 , O 2 ) and low salinity brine at reservoir conditions. Mercury injection capillary pressure indicated that the majority of pores in both cores had pore throat radii ~ 5–150 nm with porosities of 5.5–8.4%. After reaction with impure CO 2 -brine the measured pore throats decreased in the clay-rich mudstone core. Dissolution and precipitation of carbonate and silicate minerals were observed during impure CO 2 reaction of both cores via changes in water chemistry. Scanning electron microscopy identified macroporosity in clays, mica and amorphous silica cements. After impure CO 2 -brine reaction, precipitation of barite, Fe-oxides, clays and gypsum was observed. Ion leaching from Fe-rich chlorite was also apparent, with clay structural collapse, and fines migration. Small-angle neutron scattering measured the fraction of total and non-accessible pores (~ 10–150 nm radii pores) before and after reaction. The fraction of pores that was accessible in both virgin cap-rocks had a decreasing trend to smaller pore size. The clay-rich cap-rock had a higher fraction of accessible pores (~ 0.9) at the smallest SANS measured pore size, than the feldspar rich fine-grained sandstone (~ 0.75). Both core samples showed a decrease in SANS accessible pores after impure CO 2 -water reaction at CO 2 storage conditions. The clay-rich cap-rock showed a more pronounced decrease. After impure CO 2 -brine reaction the fraction of accessible pores at the smallest pore size was ~ 0.85 in the clay-rich cap-rock and ~ 0.75 in the feldspar-rich fine-grained sandstone. Reactions during impure CO 2 -brine-rock reaction have the potential to close cap-rock pores, which is favourable for CO 2 storage integrity. [ABSTRACT FROM AUTHOR]