Investigating the Role of Crude-Oil Components on Wettability Alteration Using Atomic Force Microscopy
- Subhayu Basu (U. of Texas) | Mukul M. Sharma (U. of Texas)
- Document ID
- Society of Petroleum Engineers
- SPE Journal
- Publication Date
- September 1999
- Document Type
- Journal Paper
- 235 - 241
- 1999. Society of Petroleum Engineers
- 4.3.1 Hydrates, 4.3.3 Aspaltenes, 1.8 Formation Damage, 4.2.3 Materials and Corrosion, 5.8.5 Oil Sand, Oil Shale, Bitumen, 4.1.5 Processing Equipment, 4.1.2 Separation and Treating, 5.2 Reservoir Fluid Dynamics
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The surface forces between a crude-oil and its components (oil, resin, and asphaltenes) and a glass substrate have been measured in brine of different salinity and pH using an atomic force microscope. The force vs. distance curves are used to measure the critical disjoining pressure for wettability alteration, i.e., departure from water-wet conditions. The measured data are compared with the classical DLVO theory to explore the nature of interaction of these components with mineral substrates.
The results indicate that the resins and asphaltenes obey the DLVO theory, i.e., the stability of the brine film decreases with increasing brine salinity and increases with increasing solution pH. The opposite trend is observed with increasing salinity for the nonpolar (pentane soluble) fraction and for the crude oil itself. The deviation from DLVO predictions indicates that for nonpolar oils, hydrophobic interactions may play an important role in the interaction of oils with minerals and, therefore, in wettability reversal.
The contribution of surface-active polar components in crude oils in determining the wetting preference of reservoir rock minerals for oil or water is well documented. These compounds are mainly concentrated in the polar fractions of the crude oil, i.e., the asphaltenes and resins.1-5 Many investigators have suggested that the wettability of reservoir rock is directly related to the adsorption and/or deposition of these petroleum heavy ends on mineral surfaces.6-9 Wettability alteration studies have mainly focused on understanding the adsorption of resins and asphaltenes on model mineral surfaces. It is conjectured that, once adsorbed on the mineral surface, they permanently change the preference of the mineral surface for the oil phase. The variability in wetting properties of reservoirs has been attributed to factors such as the proportion of asphaltenes and resins in the crude oil, the stability of the intervening water film, and the mineralogy of the rock surface.
The wettability reversal phenomenon has been studied in the past by investigating the conditions that cause the adsorption and desorption of crude-oil components on reservoir rock surfaces. The crude-oil fractions that are most strongly adsorbed to mineral surfaces are the polar fractions. Many researchers have clearly demonstrated their effect in changing the wetting properties of mineral surfaces once they are adsorbed.9 However, it is unclear how these molecules gain access to the mineral surface. Their role in destabilizing aqueous films separating the crude oil and the mineral surface is also not known. There is a great deal of uncertainty about factors such as the size and complexity of molecular structural arrangements of these organic molecules and their physico-chemical interactions with rock surfaces in a reservoir fluid medium. One measure of these interactions is the surface forces acting between two interacting phases across a fluid medium. It is well known that surface forces determine the contact angle at a three-phase contact line (Frumkin-Derjaguin theory10,11) and that these forces primarily quantify the wetting properties of reservoir minerals. The force vs. distance curves provide information regarding the ability of these crude-oil fractions to break water films under different conditions, thereby, altering the wettability of the rock surface.12,13
In the past, studies on understanding the intermolecular and surface interactions causing the collapse of thin brine films confined between an oil and a mineral phase have met with limited success.14,15 This was primarily due to a lack of quantitative measurements of the surface force vs. distance curves. Recently, Basu and Sharma16 have demonstrated the feasibility of measuring these interaction forces between crude oil and mineral substrates in an aqueous medium using an atomic force microscope (AFM). The dewetting of pre-existing brine films on glass and mica substrates for different conditions of oil composition, brine chemistry, and surface morphology was systematically studied. The results of their work clearly show the importance of surface forces on wettability reversal phenomenon in oil/brine/mineral systems in oil reservoirs.17 In this article, the experimental technique developed in Ref. 17 has been used to measure the surface force vs. distance curves for asphaltenes, resins, and oils interacting with glass surfaces in brine. It is observed that the composition of the oil phase plays an important role in the stability of the brine films.
Asphaltenes and Resins
Crude oil can be fractionated into different components and in different ways based either on molecular weight or polarity. Fractionation in a refinery is typically based on molecular weight with the highest molecular weight components being in general less volatile than the lower molecular weight components. For the purposes of wettability alteration it is the polarity of the molecules that is of primary concern to us. For this reason we fractionate the crude oil on the basis of their solubility in various solvents, i.e., on the basis of molecular polarity. The fraction of crude oil, insoluble in heptane but soluble in benzene or toluene, is referred to as the asphaltene fraction; the heptane soluble fraction of the crude oil consists of oils and resins. The heptane soluble fraction is further fractionated by eluting a silica gel column with pentane to obtain the oil fraction, with benzene to obtain the aromatic fraction, and with methanol to obtain the resin fraction. This fractionation procedure with minor variations has been used extensively in the literature.18,19,22 The oils, therefore, consist of the nonpolar fraction of the crude oil. The resins constitute the low molecular weight polar fraction whereas the asphaltene fraction consists of the high molecular weight polar fraction of the crude oil.
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