Wettability controls the distribution and flow of immiscible fluids in an oil reservoir and thus plays a key role in any oil-recovery process. Once thought to be a fixed property of each individual reservoir, it is now recognized that wettability can vary on both microscopic and macroscopic scales. Polar oil components can adsorb onto the pore-bounding mineral surfaces by several different mechanisms. In this study, we explore a transition in the mechanism of wettability alteration that relates to the asphaltene fraction and its stability in the oil phase. Asphaltene stability was varied by adding n-heptane to samples of five crude oils. Conditions tested ranged from those in which asphaltenes were stable to mixtures in which aggregates formed and separated from the oil. The onset condition-in which the first asphaltene particles become visible-provides a reference point with respect to asphaltene stability in each crude-oil sample. Muscovite mica sheets, treated with brine and with the crude oil/heptane mixtures, were examined using contact angles between decane and water as a measure of altered wettability on the oil-treated surfaces. A significant increase in oil-wet conditions very near the onset of asphaltene precipitation was observed with four of the five oils, indicating the potential for wetting changes during the course of oil production if conditions of asphaltene instability are approached. Implications of changes in the mechanism of wetting alteration during the course of production from an oil reservoir are considered.
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