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.