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 if wettability is to be elucidated at the pore level. The type and thickness of films coating pore walls determines reservoir wettability and whether or not reservoir rock can be altered from its initial state of wettability. Pore shape, especially pore wall curvature, is an important factor in determining wetting film thickness. Yet, pore shape and the physics of thin wetting films are generally neglected in models of flow in porous rocks. This paper incorporates thin wetting film forces into a collection of capillary tubes model to describe the geological development of so-called mixed-wettability in reservoir rock. Our 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 (i.e., low residual oil saturation) characteristic of mixed-wettability porous media comes about due to interconnected oil lenses or rivulets which bridge the walls adjacent to a pore corner. Predicted residual oil saturations are found to be approximately 35 percent less in mixed-wet rock when compared to completely water-wet rock. Calculated capillary pressure curves mimic those of mixed-wet porous media in the primary drainage, imbibition, and secondary drainage modes. Amott-Harvey indices range from −0.18 to 0.36 also in good agreement with experimental values. Introduction The wettability of reservoir rock is a critical factor in determining the displacement effectiveness and ultimate oil recovery by drive fluids, such as water. Since the most wetting fluid tends to occupy the smallest and, hence, most hydrodynamically resistive pore channels while the least wetting fluid distributes to the largest and least resistive pore channels, wettability is a prime factor controlling multiphase flow and phase trapping. Therefore, understanding how wettability is established at the pore level is crucial if predictive flow models are to be developed. Wettability in porous media is generally classified as either homogeneous or heterogeneous. For the homogeneous case the entire rock surface has a uniform molecular affinity for either water or oil. Conversely, heterogeneous wettability indicates distinct surface regions that exhibit different affinities for oil or water. Three broad classifications of homogeneous wetting exist: strongly water-wet, strongly oil-wet, and intermediate wet. If smooth representative rock surfaces can be prepared, then contact angles for water-wet surfaces, measured through the water phase, are near zero. Whereas, for oil-wet surfaces they are near 180 degrees. In the case of intermediate-wetting, the rock has neither a strong affinity for water nor oil, and contact angles range roughly from 45 degrees to 135 degrees. Two types of heterogeneous wettability are generally recognized. Mixed-wettability refers to distinct and separate water-wet and oil-wet surfaces which coexist and span a porous medium. P. 163^

In situ characterization of wettability alteration and displacement mechanisms governing recovery enhancement due to low‐salinity waterflooding

“…This might be attributed to the fact that the larger pores had more contact with the fresh oil and therefore their surfaces were more profoundly impacted. This observation is consistent with the trends proposed by Radke et al [1992]. The trend, however, had one exception and that was the wettability of the largest pore size group (i.e., group a), which had a lower degree of wettability alteration due to restoration compared to that in group b.…”

“…This observation is consistent with the trends proposed by Radke et al . []. The trend, however, had one exception and that was the wettability of the largest pore size group (i.e., group a ), which had a lower degree of wettability alteration due to restoration compared to that in group b .…”

“…Radke et al . [] suggested that this phenomenon might be associated with the fact that, during drainage, thick water films might remain in large pores coating pore surfaces. This coating protects the rock surface from reacting with the crude oil and might limit the wettability restoration.…”

Wetting Behavior of Tight Rocks: From Core Scale to Pore Scale

“…By exceeding critical capillary pressure, the water film collapses into molecularly adsorbed water monolayers. Then, the oil polar components such as asphaltenes contact directly with the pore walls, adsorb irreversibly onto the pore walls, and change the wettability to the oil‐wet state (Alshakhs & Kovscek, ; Buckley et al, ; Hirasaki, ; Morrow, ; Radke et al, ). Small pores and corners of the pore space remain water‐wet due to the high capillary pressure required for the water displacement, leading to mixed‐wet conditions.…”

Pore Network Modeling of Multiphase Flow in Fissured and Vuggy Carbonates

“…In order to understand effects of wettability, Blunt (1997) simulated displacement mechanisms, primary drainage, water injection and oil re-injection at pore scale. By using different contact angles and different fractions of oil wet, water wet and mixed wet pores based on Kovscek's study (Kovscek et al, 1992), importance of oil films and wettability alteration on recovery were investigated. Hui and Blunt (2000), investigated fluid configurations at pore scale for three phase flow in mixed-wet porous media.…”