Pore-scale modeling: Effects of wettability on waterflood oil recovery

We study the effects of wettability on waterflood oil recovery using a capillary-controlled pore-scale network model. We validate the model against experimental data in the literature on mixed-wet Berea sandstone and then apply it to study multiphase flow through four networks extracted from different types of rock: a sand pack, a poorly consolidated sandstone from the Middle East, a granular carbonate and Berea sandstone. We study the effects of initial water saturation, contact angle distribution and oil-wet fraction on recovery. For a uniformly-wet system, where the contact angle everywhere falls within a relatively narrow range, recovery increases as the system becomes less water-wet and reaches a maximum for oil-wet conditions where recovery is approximately constant for average intrinsic contact angles above 100°. As the initial water saturation increases, recovery decreases in water-wet systems whereas in oil-wet systems it initially increases and then decreases. For mixed-wet systems that contain water-wet and oil-wet regions of the pore space, the oil-wet fraction plays a more important role in determining recovery than the contact angle in the oil-wet regions. Optimal recovery occurs when a small fraction of the system is water-wet. Pore structure plays a relatively minor role in the generic behavior, although it does influence the initial saturation for maximum recovery and the magnitude of the recovery. These results are explained in terms of pore-scale displacement mechanisms and fluid configurations.