Effect of shale surface wettability on the CO2 huff-n-puff behavior in oil-bearing nanopores: A molecular dynamics study

CO2 huff-n-puff is the most promising method for enhancing oil recovery from shale reservoirs and sequestering CO2. However, the fundamental understanding of the thermodynamic and transport phenomena related to the influence of wettability on the CO2 huff-n-puff process remains unclear. Here, we conduct the first investigation into the CO2 huff-n-puff process within distinct nanopore surfaces (i.e., water-wet calcite, oil-wet graphene, and neutral-wet hydroxyl quartz) to clarify the effect of wettability on CO2 and oil transport characteristics. First, the contact angles for water immersed in nC7 on mineral surfaces were measured to highlight the differences in surface wettability. Second, the occurrence characteristics of nC7 within three distinct types of nanopore were fully explored. Then, the dynamic transport phenomena of the CO2 huff-n-puff process were systematically compared to elucidate the influence of surface wettability. Notably, the results show that the enhanced recovery mechanisms of CO2 huff-n-puff in calcite and hydroxyl quartz nanopores primarily include extraction, competitive adsorption, miscible effect, and expansion displacement, while the competitive adsorption has no effect on CO2 huff-n-puff within graphene nanopores. Moreover, in calcite nanopores, CO2 can completely strip the nC7 adsorption layer, forming a concave meniscus advancing shape; in graphene nanopores, CO2 cannot strip the nC7 adsorption layer, forming a convex meniscus advancing shape; in hydroxyl quartz nanopores, CO2 can strip part of the nC7, forming a piston-like advancing shape. By combining the distribution of CO2 density and the advancing mode of CO2, we summarized the CO2 huff-n-puff process into three patterns in light of surface wettability. These findings can shed a new light on the fundamental understanding of enhanced oil recovery by CO2 huff-n-puff at nanoscale.