Insights into wettability alteration during cyclic scCO₂-brine injections in a layered Bentheimer sandstone

Residual trapping of supercritical carbon dioxide (“scCO₂”) is a key mechanism contributing to the safety and security of geologic sequestration operations. Recent experimental studies have suggested that cycles of scCO₂ and brine injections cause surface chemistry reactions that increase residual trapping. We present results of a new experiment, analyzed with X-ray microcomputed tomography, aimed at pinning down specifics of the alteration mechanism. Four cycles of scCO₂ and brine injections were conducted in a Bentheimer sandstone core with a prominent low permeability region at the base (inlet side) of the core. Multiple successive scCO₂ injections were performed within cycles, and scCO₂ injection flow rate was varied. Saturation profiles near the low permeability layer remain approximately constant, while downstream scCO₂ saturation profiles change shape and generally increase for successive injections. Residual trapping increases over the four cycles, but only in the upper region of the core. Microstructural analysis of the scCO₂ phase indicates increasing presence of relatively high contact angle (i.e. less water-wetting) surfaces as the experiment progresses. These results suggest that surface chemistry alteration occurs during drainage injections, and may also occur during imbibition. However, the effect is only evident in the relatively high permeability region of the core; in the low permeability region, capillary heterogeneity dominates flow patterns and wettability alteration effects are not evident. These results support previous work demonstrating sandstone wettability alteration due to scCO₂/brine cycling, and provide new clarification as to the conditions under which this wettability alteration will cause shifts in scCO₂ flow and trapping.