In-Situ Imaging of CO2 Trapping and Oil Recovery in Three-Phase Systems: Dependence on Pore Geometry and Wettability

Abstract

SummaryCarbon dioxide (CO2) injection into reservoirs can be combined with enhanced oil recovery. Pore geometry and wettability are critical factors in CO2 trapping and oil recovery. However, their relationship has not been tackled and the influence of wettability is always controversial. To elucidate this intrinsic link, we prepared simple models using glass beads and angular grains, changed wettability by aging, and imaged the three-phase displacement process at high resolution. The results showed that the oil flow occurred through the ways of direct drainage and double imbibition, which is caused by the trapped CO2 under immiscible conditions. More residual CO2 was trapped in the angular grain sample and the water-wet sample, and more residual oil was trapped in the angular grain sample and the oil-wet sample. Under water-wet conditions, CO2 was easier to be trapped as the state of gas-in-oil-in-water or gas-in-water. While under oil-wet conditions, more CO2 was directly surrounded by oil and more small-size residual oil dispersed near the throats. The statistics of the residual clusters showed that the capillary-trapping capacity Ctrap of the angular grain sample was high (12.97% > 8.69%), but the oil recovery R2 was low (89.89% < 93.85%). The altered wettability (water-wet to oil-wet) increased the number of isolated CO2 clusters and the proportion of oil and CO2 in the smaller pores while decreasing the Ctrap (8.93% < 10.34%) and R2 (84.48% < 91.04%). The dissolution trapping and capillary trapping were maximized in the angular grain sample due to the increased surface-to-volume ratio of CO2 clusters and higher capillary force. The effect of wettability on dissolution trapping was slight, but the residual CO2 saturation was higher under water-wet conditions, thus the water-wet media were more conducive to CO2 trapping. We suggested that reservoirs with strong heterogeneity should be selected as CO2 storage sites, and the wettability of oil-wet rocks should be changed by using tertiary oil recovery technology to enhance CO2 storage efficiency and oil recovery. Our study provided direct evidence of the influence of pore geometry and wettability on residual CO2 and oil distribution, and clarified that complex pore geometry and water-wet systems were more conducive to CO2 trapping under immiscible conditions.