Wettability: Fundamentals and Surface Forces

Abstract

Summary The wetting of mineral surfaces by water and oil is described by models of surface forces that become important when two surfaces approach each other. Force components are electrostatic, van der Waals. and structural. The electrostatic force depends on brine pH and salinity, crude oil composition, and the mineral. The surface forces are expressed as a disjoining pressure isotherm, and its integral is the specific interaction potential isotherm. The specific interaction potential isotherm can be used to determine the stable and metastable film-thickness profiles at the three-phase contact region for a given capillary pressure and/or curvature of the substrate. This profile gives the contact angle. Introduction Wettability has been recognized as an important factor in remaining oil saturation and in capillary pressure and relative permeability curves. This work describes some of the physics of the contact angle between mineral surfaces, water, and oil. Description of a particular system requires a chemical description of the mineral, brine, and oil. The wettability of a rock/brine/oil system cannot be described by a single contact angle because it is the multitude of contact angles at the various three-phase contact regions in the pore spaces that determines system wettability. A complete wettability description requires a morphological description of the pore space with the contact angles as a boundary condition for the fluid distribution. Regardless of the morphology, however, the wettability depends on the contact angles. This work focuses on the intermolecular surface forces that affect wettability. A tremendous amount of research on this subject exists outside the petroleum industry that can be applied to petroleum reservoirs. The state of the art is reviewed in Refs. 3 through 6. Mohanty applied these concepts to the fluid distribution in petroleum reservoirs, and Hirasaki elaborated on the development of Mohanty's applied theory. The intermolecular surface force approach originated with Derjaguin and Landau's and Verwey and Overbeek's (DLVO) theory of colloidal stability. This theory describes the stability and flocculation of lyophobic (solvent-fearing) colloids, considering electrostatic and van der Waals interactions. This approach has been used to describe the stability of the wetting-water film. The present work goes beyond the stability of the wetting-water film to examine the value of the contact angle when the wetting film collapses. To describe the contact angle, it is necessary to consider other surface forces, collectively called structural forces. Description of wettability in petroleum reservoirs requires the inclusion of capillary pressure and the curvature of the pore walls. This paper reviews the extensive literature on the physics and chemistry of wettability that resides outside the petroleum literature. The new information describes how fundamental surface forces affect wettability in petroleum reservoirs and the magnitude of advancing and receding contact angles for oil/water/mineral systems in terms of surface forces. This work is intended to provide a guide for researchers investigating wettability mechanisms. Thermodynamics of Wettability The thermodynamics of wettability requires a description of the region where three phases come together at the contact line, as shown in Fig. 1. When a pair of interfaces approach each other at a three-phase contact line, the interfaces interact with each other, and the distance separating the interface affects the system energy. Thus, this distance-called thickness-is a thermodynamic variable. The change in energy per unit area with change in distance as the pair of interfaces is brought from a large separation to a finite thickness is expressed as a force per unit area or disjoining pressure. The disjoining pressure is the force that tends to disjoin or separate the two interfaces. A negative disjoining pressure attracts the two interfaces.