Mechanisms for contact angle hysteresis and advancing contact angles

Mixed-wet crude oil/brine/mineral systems typically show a large contact angle hysteresis between the water-receding angle during primary drainage and the water-advancing angle during imbibition. Also, the water-advancing angle may have values that range from 50 degrees to 180 degrees. This investigation uses atomic force microscopy (AFM) to characterize mica surfaces that have first been equilibrated in 0.01 M NaCl, pH 6 brine and then aged in crude oil at elevated temperature. The wettability of the aged surfaces was measured with brine and crude oil. The mica surfaces that were to be examined by AFM were washed with cyclohexane to remove the bulk crude oil. The wettability of the surfaces washed with cyclohexane was measured with brine and decane. Two crude oils were used in this investigation. They were either used as the stock tank oil (STO) or diluted to 40% with n-heptane. This particular dilution was used because it is close to the asphaltene precipitation point of one of the crude oils. The AFM images show the mixed-wet surface to be patches of bare mica and patches of asphaltene with a characteristic areal dimension of about 200 nm. The elevations of the asphaltene patches are about 20 nm for the STOs but increase to above 200 nm when the crude oil is diluted to 40% with n-heptane. These mica surfaces equilibrated with crude oil diluted with heptane have larger advancing contact angles (e.g., 140 degrees) compared to the surfaces equilibrated with STO (e.g., 75 degrees). From this, we infer that the advancing contact angle becomes larger as the asphaltene solvency decreases because of increased coverage of the mica surface with larger asphaltene aggregates. (C) 1999 Elsevier Science B.V. All rights reserved.