Wettability Effects in Thermal Recovery Operations
Authors: Rao Dandina N. 
Published: December 2007

Do the three-phase oil-water-rock systems remain unaltered in wettability or do they become more strongly water- or oil-wet when the reservoir temperature is increased during thermal recovery operations? The published literature provides an affirmative support to each of the three possible answers to this question. Should we let this controversy continue or is it time to put together different pieces of the puzzle to reach at least a semblance of a solution? The latter is the purpose of this paper.

This paper aims to shed more light on the controversy surrounding the effect of temperature on wettability by bringing together the various schools of thought on this subject under a simple mechanistic explanation involving an inter- relationship between the spreading behaviour in solid- liquid- liquid (S-L-L) systems and the liquid-liquid interfacial tension, and thin wetting film stability considerations. This paper extends the Zisman-type correlation between contact angles and surface tension in solid-liquid-vapour (S-L-V) systems to the S-L-L systems. The success achieved in establishing such a correlation in this study is of practical significance in that it shows a way of identifying crucial wettability changes that may occur in secondary and tertiary recovery processes through simple laboratory experiments involving interfacial tension and contact angle measurements. This would help in recovery process selection and optimization.

The paper also describes a technique, which was discovered during the course of this experimental study, to prevent adverse wettability shifts in thermal operations. The technique involves in-situ deposition of small quantities of calcium carbonate (scale) particles near the wellbore region to maintain a strongly water-wet region of high relative permeability to crude oil. Encouraging results from an Alberta field test of this technology are also presented.

Introduction

Thermal recovery operations impose a major change in the important thermodynamic variable, namely temperature, of the reservoir rock-fluids system. Although reduction in oil viscosity may be its main purpose, the rise in temperature of the system affects almost all the variables (such as the density and viscosity of the individual phases, interfacial properties of oil/water/gas interfacial tensions and the interfacial energies involving the solid surface and its mineral transformations), that have either a direct or an indirect influence on the movement of oil-water-gas phases in the porous rock medium. In other words, the thermal energy imposed on the system introduces changes not only in fluid properties and fluid-fluid interactions but also in rock-fluids interactions. Characterizing wettability change with temperature would be one convenient and useful way to lump all of the above mentioned property variations into one overall effect of reaching a new thermodynamic state at an elevated temperature. However, a quick look at the literature on this subject of temperature dependence of wettability makes it obvious that the existing situation is not simple but quite complex. This complexity appears to be due not only to the three distinct schools of thought on the effect of temperature on oil-water relative permeabilities, but also due to the scatter of information across several scientific fields. It is the purpose of this paper to bring together some of these scattered information with the hope of gaining a better understanding of the probable mechanisms behind the widely differing experiences with temperature dependence of reservoir wettability.

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