The Wettability of Oil/Water/Silica Systems With Reference to Oil Recovery
Authors: Brown C.E. , Neustadter E.L. 
Published: March 2010

Abstract
The wetting behaviour of oil/water/silica systems is examined, principally by measurement of contact angles. Both pure and crude oils are used, with aqueous phases covering a range oj PH and salt content. A description of the wetting behaviour in terms of the system? physico-chemical properties is presented. In addition, dynamic wetting effects relating to immiscible displacement are described. The results obtained have been applied directly to laboratory testing of crude oil systems, and could be used more generally in considering the wettability of more complex systems, such as cores of reservoir material.

Introduction
It is widely appreciated that the recovery efficiency of crude oil from porous rock by displacement with water is dependent on the wettability of that rock (1,2). The term "wettability" refers to the relative tendency of the aqueous phase and of the oil phase to coat the solid, and hence to occupy the pore spaces of the rock under the action of capillary forces. Various methods of determining the wettability of porous media have been used, including imbibition tests(2) and measurement of capillary pressure(3). The geometric complexity of real rock samples results in complex wetting behaviour, which inhibits development of understanding of the chemical basis for the wettability. Therefore, systems of simplified geometry have Previously(4) been found useful in examining the fundamental basis of wettability.
An additional factor is that wetting in displacement or in imbibition is a dynamic process, intimately related to the microscopic displacement mechanism. In order to obtain in sight into these mechanisms, capillary systems of simplified geometry have been used(5, 6, 8). In model geometries, the wetting condition can frequently be conveniently characterized by a contact angle; it is the contact angle which is a boundary condition determining the interfacial shape and extent, and hence the capillary pressure. In fact, the notion of a contact angle is conceptually so attractive that it is often used as a parameter for wettability even when a uniquely defined contact angle no longer exists (for example, on a surface with asperities).
A knowledge of the wettability behaviour is therefore required in order to develop understanding of the displacement mechanisms, both in capillary systems of model geometry and in "real" systems such as cores of reservoir rock. The systems used in the present work are of model geometry, and are examined at laboratory rather than reservoir ambient condition. Such simplifications are essential to develop an understanding of the mechanisms controlling wettability.

B. Experimental
Methods Static contact angles were measured by two methods:The captive droplet technique(7) a small (approximately 3 mm3) drop of oil was formed at the end of a synnge held in an aqueous phase in a glass cell. The drop was pressed against an optically flat silica plate. The drop was photographed, and the contact angle measured from an enlarged projection of the photographic image.The capillary tube technique(8) a glass or silica capillary (bore less than 1 mm) tube was filled with oil and water.