Wettability changes induced by adsorption of asphaltenes

Abstract

Asphaltene fractions were precipitated from two crude oils (designated as Prudhoe Bay `95 and Wyoming `95) by addition of n-hexane. A solution of 250 mg/L of asphaltenes in toluene was used to induce wettability changes in Berea sandstone cores by flow of asphaltene solutions through the core. Concentration of asphaltenes in the effluent was measured by ultraviolet (UV) absorbance. Wettability changes were assessed from measurements of the rate of spontaneous imbibition for changes in initial water saturation and brine composition. Water wetness decreased markedly with increase in ionic strength, increase in cation valency, and decrease in initial water saturation. For comparable conditions, changing the brine anions (chloride, nitrate, carbonate, and sulfate) caused lesser but fairly consistent changes in wetting. Wetting changes induced by asphaltenes from the Wyoming `95 crude oil were always higher than for those prepared from the Prudhoe Bay `95 crude oil. Waterflood recoveries for NaCl, CaCl{sub 2}, and AlCl{sub 3} brines (0.09 M) increased with decrease in water wetness, with recovery reaching a maximum at an Amott wettability index of about 0.2. This result agrees with results reported previously for wettability changes induced by adsorption from crude oil.