Pore Scale Observations of Trapped CO₂ in Mixed-Wet Carbonate Rock: Applications to Storage in Oil Fields

Geologic CO₂ storage has been identified as a key to avoiding dangerous climate change. Storage in oil reservoirs dominates the portfolio of existing projects due to favorable economics. However, in an earlier related work (Al-Menhali and Krevor Environ. Sci. Technol. 2016, 50, 2727-2734), it was identified that an important trapping mechanism, residual trapping, is weakened in rocks with a mixed wetting state typical of oil reservoirs. We investigated the physical basis of this weakened trapping using pore scale observations of supercritical CO₂ in mixed-wet carbonates. The wetting alteration induced by oil provided CO₂-wet surfaces that served as conduits to flow. In situ measurements of contact angles showed that CO₂ varied from nonwetting to wetting throughout the pore space, with contact angles ranging 25° < θ < 127° in contrast, an inert gas, N₂, was nonwetting with a smaller range of contact angle 24° < θ < 68°. Observations of trapped ganglia morphology showed that this wettability allowed CO₂ to create large, connected, ganglia by inhabiting small pores in mixed-wet rocks. The connected ganglia persisted after three pore volumes of brine injection, facilitating the desaturation that leads to decreased trapping relative to water-wet systems.