A pore‐level scenario for the development of mixed wettability in oil reservoirs

Abstract Understanding the role of thin films in porous media is vital to elucidate wettability at the pore level. The type and thickness of films coating pore walls determine reservoir wettability and whether or not reservoir rock can be altered from its initial state of wettability. Pore shape, especially pore wall curvature, is important in determining wetting‐film thicknesses. Yet, pore shape and physics of thin wetting films are generally neglected in flow models in porous rocks. Thin‐film forces incorporated into a collection of star‐shaped capillary tubes model describe the geological development of mixed wettability in reservoir rock. Here, mixed wettability refers to continuous and distinct oil and water‐wetting surfaces coexisting in the porous medium. This model emphasizes the remarkable role of thin films. New pore‐level fluid configurations arise that are quite unexpected. For example, efficient water displacement of oil (low residual oil saturation) characteristic of mixed‐wettability porous media is ascribed to interconnected oil lenses or rivulets which bridge the walls adjacent to pore corners. Predicted residual oil saturations are approximately 35% less in mixed‐wet rock than in completely water‐wet rock. Calculated capillary pressure curves mimic those of mixed‐wet porous media in the primary drainage of water, imbibition of water, and secondary drainage modes. Amott‐Harvey indices range from −0.18 to 0.36 also in good agreement with experimental values (Morrow et al., 1986; Jadhunandan and Morrow, 1991).