Waterflood oil recovery in carbonate rock under mixed-wet conditions

Oil companies often inject water into oil reservoirs to ‘push’ the remaining oil in the reservoir towards production wells. This technique, known as waterflooding, can double the oil recovery from a reservoir. In this talk, we will consider the impact of reservoir wettability on waterflood oil recovery. Salient features of waterflood oil recovery where the oil contacted grain surfaces are strongly oil-wet are well established. However, there is no consensus on the optimal wettability for waterflood oil recovery: while some studies report the dependence of oil recovery for different contact angles, their findings are contradictory, with maximum recovery reported at neutral wettability in some studies and at strongly oil-wet conditions in others. I will present laboratory measurements which, combined with numerical simulation, explain the contradictory observations and show that residual saturation decreases monotonically with contact angle. I will also present evidence of enhanced permeability attributed to apparent oil/brine slippage, which is also a function of wettability. These findings may have important implications not only for EOR, but for other applications that involve mixed-wet porous media, such as irrigation, flood management, groundwater remediation, and geological CO2 storage.