Physically-based network modeling of multiphase flow in intermediate-wet porous media

A capillary equilibrium-based network model is used to compute oil recovery and Amott wettability indices in intermediate-wet porous media. The pattern of wettability at the pore level is a generalization of the work of Kovscek et al. [Kovscek, A.R., Wong, H., Radke, C.J., 1993. A pore-level scenario for the development of mixed wettability in oil reservoirs, AICHE J., 39 (6) 1072–1085]. The model allows a single pore to contain both oil-wet and water-wet regions, with oil/water contact angles anywhere in the range 0–180°. We investigate models with varying contact angles, and where different fractions of the pore space are oil-wet. We show that oil recovery shows a complex, non-monotonic dependence with both contact angle and oil-wet fraction, and we explain the results in terms of competing displacement mechanisms at the pore scale. Strongly oil-wet media allow stable oil layers to form in the crevices of the pore space. These layers maintain the connectivity of the oil phase and allow low residual saturations to be achieved, but only by production well after breakthrough.