Mechanisms and consequences of wettability alteration by crude oils
Reservoir wettability is controlled by crude oil/brine/rock interactions that have not been well understood. Studies using either model compounds or crude oil fractions have had only limited success in reproducing the wetting alteration that can occur in nature. In this study, the approach is first to identify the key features of interfacial activity unique to crude oils, then to design and conduct wetting alteration experiments—using both flat solids and porous media—that demonstrate the mechanisms by which alteration can occur. Components of crude oils that are interfacially active are found in the highest molecular weight, most polar fractions of the oil, the resins and asphaltenes. How these components affect wetting depends on the compositions of not only the oil, but also the mineral surfaces and the aqueous phase that is always present in oil reservoirs. Wettability altering interactions can occur by several mechanisms. In the absence of water, adsorption of polar species can create intermediately-wet surfaces. If the oil is a poor solvent for its asphaltene fraction, adsorption of large asphaltene aggregates can make surfaces fairly oil-wet. Adsorption can also occur because of ionic interactions between oppositely charged acidic and basic sites at the oil/water and solid/water interfaces. There may also be interactions between similarly charged sites, if ion binding can occur. All of these mechanisms have been documented for a range of crude oils with varying asphaltene fraction, solvency, acid number, and base number.