A PORE-LEVEL SCENARIO FOR THE DEVELOPMENT OF MIXED WETTABILITY IN OIL-RESERVOIRS

Understanding the role of thin films in porous media is vital to eluci date wettability at the pore level. The type and thickness of films co ating pore walls determine reservoir wettability and whether or not re servoir rock can be altered from its initial state of wettability. Por e shape, especially pore wall curvature, is important in determining w etting-film thicknesses. Yet, pore shape and physics of thin wetting f ilms 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 rese rvoir rock. Here, mixed wettability refers to continuous and distinct oil and water-wetting surfaces coexisting in the porous medium. This m odel emphasizes the remarkable role of thin films. New pore-level flui d configurations arise that are quite unexpected. For example, efficie nt water displacement of oil (low residual oil saturation) characteris tic of mixed-wettability porous media is ascribed to interconnected oi l 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 pres sure curves mimic those of mixed-wet porous media in the primary drain age of water, imbibition of water, and secondary drainage modes. A mot t-Harvey indices range from -0.18 to 0.36 also in good agreement with experimental values (Morrow et al., 1986; Jadhunandan and Morrow, 1991 ).