Impact of displacement rate on waterflood oil recovery under mixed-wet conditions

Under uniformly water-wetting conditions, it is well established that waterflood oil recovery remains independent of flood rate while the flow is capillary-dominated and furthermore, that the rate dependence emerges when the microscopic capillary number exceeds O(10^-5). In contrast, there is no equivalent framework for interpreting the flood rate-dependence of oil recovery under mixed-wet conditions representative of oil reservoirs. Indeed, not even the appropriate definition of capillary number under mixed-wet conditions is established. In this paper, we focus specifically on oil recovery from mixed-wet porous media at high initial oil saturation and its dependence on water injection rate. We present laboratory measurements of oil distribution and its evolution during secondary waterflood at 2.5, 5.0, and 500 ft day^-1. Oil distribution was directly imaged at the pore scale using a high-speed camera coupled to an optical microscope in a bed of calcite grains packed into a microfluidic chip. These measurements are benchmarked against bulk measurements of remaining oil saturation in limestone cores.
Data to date suggest that remaining oil saturation, S_o, after the same volume of water injection, displays a dependence on injection rate qualitatively similar to the classic capillary desaturation curve for uniformly water-wet media, with the rate dependence emerging at a higher injection rate under mixed-wet conditions than under water-wet conditions. However, unlike in uniformly water-wet media, the long-time (residual) S_o does not display a dependence on the rate. These findings may have important implications not only for oil recovery, but for a wide range of engineering applications involving mixed-wet porous media, e.g., geological CO₂ storage, remediation of nonaqueous phase liquid (NAPL)-contaminated soils and aquifers and irrigation.