Evolution of Bentheimer Sandstone Wettability During Cyclic scCO₂-Brine Injections

Abstract

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.