Experimental investigation of residual saturation in mixed-wet porous media using a pore-scale approach

Wettability is an important factor in terms of flow distribution and the amount of oil left behind in a petroleum reservoir after primary, secondary, and tertiary recovery processes. In fact, most alkaline flooding operations are aimed at wettability reversal. Therefore, a good understanding of how wettability, and other relevant factors (e.g., fluid saturation and porosity), affects displacement efficiency and fluid distribution at the pore-scale can lead to successful predictions of flow properties at the macroscale. In this study, pore-scale two-phase fluid flow in a synthetic mixed-wet granular media was investigated using X-ray microCT. Analysis of residual fluid structures in granular media composed of a mixture of glass and plastic beads (ranging from 0.4 to 0.6 mm in diameter) showed that the number of trapped water blobs was 2.4 times greater than that of oil, whereas most of the blobs were between 0.0001 and 0.001 mm³ in size (i.e., 100 times smaller than in the case of uniform wettability), and were smaller than the mean pore volume (0.03 mm³). The ratio of surface area to blob volume showed a slight tendency of water blobs to wet a higher surface area of the grains than oil blobs, for blob volumes larger than the mean pore size; this phenomenon can be attributed to stronger wetting affinity of the glass grains to the water phase than that of the plastic grains to the oil phase. Furthermore, statistical analysis of the distance between residual oil and water blobs to each solid surface confirms preferential wetting affinity of oil and water to plastic and glass surfaces, respectively.