Effect of Heterogeneous Wettability On Flow of Fluids Through Porous Media
Authors: Singhal A.K. , Mukherjee D.P. , Somerton W.H. 
Published: March 2010

Abstract
In order to study the effect of heterogenous wettability on the multiphase behavior, porous samples were prepared by mixing teflon powder and glass beads in different ratios. Flow and displacement tests were conducted using different fluid pairs, so that the effects of heterogeneous wettability could be tested over a large range of physical properties of the solid-fluid system.
It was found that as the fraction of surface wetting to the displaced phase increased, the ultimate recovery decreased linearly. There appeared to be a poor correlation between the ultimate recovery and Cassie's equivalent contact angle and the latter did not seem to be an adequate index of heterogeneous wettability. Also, as the wettability to the displaced phase increased the relative permeability ratio curve was found to shift, resulting in a higher residual saturation of the displaced phase.
Interfacial tension was shown to have the maximum influence on recovery, higher recovery being obtained at lower interfacial tension. There appeared to be an effect of viscosity ratio on the vertical position of relative permeability ratio curves, but not on ultimate recovery.

Introduction
The efficiency of a water flood, measured directly by the oil recovery it achieves, depends among other things, on the residual oil saturation and the relative permeability-saturation relationship of the reservoir rock. These properties, in turn, are influenced by wettability of the reservoir rock to the oil-water system.
Over the years, several suggestions (1, 2, 3, 4) have been made indicating that the actual reservoir rocks show different degrees of heterogeneous wettability to oil and water. This may be because the granular and cementing materials in a rock are usually of different chemical compositions. Also, in reservoir racks, which were initially water-wet, oil usually occurs in larger pores and water in smaller pores. Any polar components present in oil get deposited on rock surfaces and make the large pores oil-wet, whereas the small pores remain water-wet.(1) In recent years, significant progress has been made toward understanding the influence of wettability on the relative permeability behaviour of porous media having uniform surface properties(5, 6, 7, 8, 9). However, the role of heterogeneous wettability in multiphase flow through porous media remains little understood. Therefore, a need exists for its comprehensive study. In the present work, porous media made from mixtures of teflon and glass beads in different known ratios were used. Teflon and glass exhibit widely different wettability characteristics for certain liquids. Therefore, by controlling the ratio of these constituents in a mixture, it was possible to control the degree of heterogeneity in wettability.
Pairs of different immiscible pure liquids were used with these samples. Because each fluid pair showed a different contact angle on teflon and glass, it was possible to study the effect of heterogeneous wettability on flow behaviour for a wide range of properties in solid-liquid systems, the properties (viscosities, interfacial tensions, etc,) of the liquids used varied over a large range, making it possible to analyze their effects on relative permeability and recovery, and to compare the magnitudes of these effects with that of wettability.