Laboratory investigation of capillary trapping under mixed-wet conditions

Remaining oil saturation established by waterflooding was measured in Indiana limestone in its original, water-wet state and under mixed-wet conditions established by adding organic acid to the oil phase. The porous plate technique was used to establish initial oil saturations ranging from S-nwi=0.23 to 0.93 under capillary-dominated conditions. For water-wet conditions, the residual oil saturation increased linearly with its initial saturation. In contrast, the remaining oil saturation under mixed-wet conditions, S-nw, displayed three distinct regimes. First, S-nw increased with its initial saturation up to S-nwi=0.58. Next, S-nw decreased from S-nwi=0.58 to 0.76. Finally, S-nw increased again as S-nwi approached one. The nonmonotonic dependence of S-nw on S-nwi at S-nwi>0.5 is well described by a concave-up quadratic function, and may be a salient feature of mixed-wet rocks.