Experimental Investigation of CO2–Brine–Rock Interactions and Permeability Impairment in Entrada Sandstone

This study investigates CO2–brine–rock interactions in Entrada Sandstone from the San Juan Basin under reservoir-relevant pressure and temperature conditions. A core plug from 8313 to 8315 ft was exposed to CO2-saturated synthetic formation brine with a salinity of 16,601 ppm, followed by static aging. Pre- and post-experiment analyses included porosity and permeability measurements, ICP-MS, SEM-EDS, XRD, CT imaging, and petrographic thin-section analysis. Permeability decreased from 4.22 to 1.60 mD, corresponding to a 62.08% reduction, whereas porosity decreased from 12.37% to 11.75%, a change interpreted as within experimental uncertainty. Effluent chemistry showed increases in Ca2+, Mg2+, K+, Si, Sr, and Mn, consistent with carbonate cement dissolution and feldspar/silicate alteration under CO2-acidified brine conditions. XRD analysis of suspended effluent solids identified quartz, montmorillonite, and illite–montmorillonite mixed-layer clays, indicating fines mobilization. These results suggest that permeability impairment was governed primarily by pore–throat blockage from mobilized clay particles, with possible contribution from secondary carbonate redistribution. The findings emphasize the importance of cement composition and clay mineralogy in evaluating injectivity risks for CO2 storage in clay-bearing sandstone reservoirs.