Wettability Changes Induced by Adsorption of Asphaltenes
- J. Yan (U. of Wyoming) | H. Plancher (U. of Wyoming) | N.R. Morrow (U. of Wyoming)
- Document ID
- Society of Petroleum Engineers
- SPE Production & Facilities
- Publication Date
- November 1997
- Document Type
- Journal Paper
- 259 - 266
- 1997. Society of Petroleum Engineers
- 5.2 Reservoir Fluid Dynamics, 1.2.3 Rock properties, 4.3.3 Aspaltenes, 4.1.5 Processing Equipment, 4.1.2 Separation and Treating, 5.4.1 Waterflooding, 2.7.1 Completion Fluids, 1.6.9 Coring, Fishing
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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 UV absorbance. Wettability changes were assessed from measurements of 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 sulphate) 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, CaCl2, and AlCl3 brines (0.09 molar) 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.
Reservoir wettability characteristics are often attributed to adsorption of asphaltenes onto the mineral surfaces of reservoir rock. Asphaltenes are operationally defined as the precipitate resulting from addition of low molecular weight alkane to crude oil. Elemental analyses show asphaltenes to contain, in addition to carbon and hydrogen, atoms such as oxygen, sulfur, nitrogen and trace amounts of metals such as iron, vanadium and nickel.
Laboratory studies of wettability require some method of obtaining wettability control. At present, there is no widely adopted method of obtaining variation in wettability for oleic/aqueous liquid pairs. It is well known that a rich variety of wetting behaviors can be induced in rocks by treatment with crude oil. As with reservoir wettability, these changes are commonly ascribed to adsorption of asphaltenes. Previous studies of asphaltene adsorption have been mostly concerned with adsorption isotherms for dry and, in some cases slightly moist, mineral surfaces. In the present study, changes in wettability of Berea sandstone containing an initial water saturation (Swi) were induced by adsorption of asphaltenes from solution in toluene. Special attention was given to the effect of brine chemistry on wettability and oil recovery by imbibition and waterflooding.
Fluids. Brines were prepared from analytical grade NaCl, Na2CO3, Na2SO4, CaCl2, MgCl2, and AlCl3 salts and distilled water. Densities, viscosities, pH, and interfacial tensions are listed in Table 1. Soltrol 220, a 3.8 cP viscosity refined oil, was used as the non-wetting phase for the imbibition and water-flooding tests.
Sandstone Cores. Cylindrical 1.5 x 3 in. cores were cut from a single 6 x 12 x 12 in. block of Berea sandstone. After air-drying, the cores were oven dried at 110 C for two days and stored in a desiccator prior to use.
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