Evolution of Bentheimer Sandstone Wettability During Cyclic scCO₂-Brine Injections

Geologic sequestration in sedimentary formations has been identified as a potential technology to prevent climate-change inducing carbon dioxide (CO₂) from being emitted to the atmosphere. To achieve safe and effective storage underground, accurate understanding, and predictions of supercritical CO₂ (scCO₂) behavior in subsurface rock formations is required; including quantifying how much scCO₂ is trapped within pore spaces by capillarity (vs. how much remains mobile), and constraining the occurrence of physio-chemical reactions between scCO₂ and the mineral matrix. Experiments where multiple cycles of scCO₂ and brine are injected into rock samples have produced conflicting results regarding the consistency of trapping as cycles progress; likely due to differences in mineral content, pressure-temperature conditions, aqueous chemistry parameters, and experimental setups. We present a new set of experiments, replicating the conditions of a previous study, but with a new experimental design, apparatus, and timeline. We confirm previous results that demonstrated shifts in injection pressure and scCO₂ trapping behavior over multiple injection cycles, and we conduct additional analyses to discern the fluid-fluid macroscopic contact angle, interface mean and Gaussian curvatures, scCO₂ interfacial area, and topology of trapped scCO₂ ganglia. We also performed lattice-Boltzmann simulations approximating experimental conditions where solid wettability was systematically altered over multiple injections cycles; trends in scCO₂ ganglia characteristics compare well between experiment and simulation. The results indicate that this system undergoes a transition to a “patchy” mixed-wet state, and we observe that this wettability alteration renders scCO₂ more stable in the rock pore space, increasing capillary trapping over four injection cycles.