Investigating the Role of Crude-Oil Components on Wettability Alteration Using Atomic Force Microscopy
Authors: Basu S. , Sharma M.M. 
Published: December 2007

The surface forces between a crude-oil or its components (asphaltenes, resins, oils) and mineral surfaces have been measured in brine. These force vs. distance curves have been analyzed to understand the role these components play in wettability alteration in oil reservoirs. The forces have been are compared with those obtained from the DLVO theory to explore the nature of interaction and surface characteristics of these components. The surface forces have been measured for different conditions of salt concentration and pH of the intervening brine film.

Introduction

The contribution of surface-active polar components in the crude-oil in determining the wetting preference of reservoir rock minerals for oil or water is well-documented. These compounds are mainly concentrated in the polar fractions of the crude-oil, i.e. the asphaltenes and resins. Many investigators have suggested that the wettability of reservoir rock is directly related to the adsorption and/or deposition of these petroleum heavy ends on mineral surfaces. Wettability alteration studies have mainly focused on understanding the adsorption of resins and asphaltenes on model mineral surfaces. It is conjectured that once adsorbed on the mineral surface, they permanently change the preference of the mineral surface for the oil phase. The variability in wetting properties of reservoirs has been attributed to factors such as the proportion of asphaltenes and resins in the crude-oil, the stability of the intervening water film, and the mineralogy of the rock surface.

The wettability reversal phenomenon has been studied in the past by investigating the conditions that cause the adsorption and desorption of crude-oil components on reservoir rock surfaces. The crude oil fractions that are most strongly adsorbed to mineral surfaces are the polar fractions. Many researchers have clearly demonstrated their effect in changing the wetting properties of mineral surfaces once they are adsorbed. However, it is unclear how these molecules gain access to the mineral surface. Their role in destabilizing the aqueous films, separating the crude-oil and the mineral surface, is also not known. There is a great deal of uncertainty about factors such as the size and complexity of molecular structural arrangements of these organic molecules and their physicochemical interactions with rock surfaces in a reservoir fluid medium. One measure of these interactions is the surface forces acting between two interacting phases across a fluid medium. It is well known that surface forces determine the contact angle at a three-phase contact line (Frumkin-Derjaguin theory) and that these forces primarily quantify the wetting properties of reservoir minerals. The force vs. distance curves provide information regarding the ability of these crude-oil fractions to break water films under different conditions, thereby, altering the wettability of the rock surface. Hirasaki presents a good review of the thermodynamic aspects of film stability.

In the past, studies on understanding the intermolecular and surface interactions causing the collapse of thin brine films confined between an oil and a mineral phase have met with limited success. This was primarily due to a lack of quantitative measurements of the surface force vs. distance curves. Recently, Basu and Sharma have demonstrated the feasibility of measuring these interaction forces between crude-oil and mineral substrates in an aqueous medium using an Atomic Force Microscope (AFM). The dewetting of preexisting brine films on glass and mica substrates for different conditions of oil composition, brine chemistry, and surface morphology were systematically studied. The results of their work clearly show the importance of surface forces on wettability reversal phenomenon in oil/brine/mineral systems in oil reservoirs. In this paper, the experimental technique developed in reference 14 has been used to measure the surface force versus distance curves for asphaltenes resins, and oils interacting with glass surfaces in brine. P. 203