Laboratory investigation of capillary trapping under mixed-wet conditions

[1] 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 Snwi = 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, Snw, displayed three distinct regimes. First, Snw increased with its initial saturation up to Snwi = 0.58. Next, Snw decreased from Snwi = 0.58 to 0.76. Finally, Snw increased again as Snwi approached one. The nonmonotonic dependence of Snw on Snwi at Snwi > 0.5 is well described by a concave-up quadratic function, and may be a salient feature of mixed-wet rocks.