Asphaltenes and Crude Oil Wetting - The Effect of Oil Composition
- J.S. Buckley (New Mexico Petroleum Recovery Research Center) | Y. Liu (New Mexico Petroleum Recovery Research Center) | X. Xie (University of Wyoming) | N.R. Morrow (University of Wyoming)
- Document ID
- Society of Petroleum Engineers
- SPE Journal
- Publication Date
- June 1997
- Document Type
- Journal Paper
- 107 - 119
- 1997. Society of Petroleum Engineers
- 1.2.3 Rock properties, 4.3.3 Aspaltenes, 4.1.2 Separation and Treating, 5.4.6 Thermal Methods, 1.8 Formation Damage, 4.1.5 Processing Equipment, 4.3.1 Hydrates
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It is widely believed that asphaltenes and other high molecular weight, polar components of crude oils are responsible for altering the wetting of reservoir rocks, but the mechanisms by which wetting is affected are still uncertain. The concentration of asphaltenes in an oil is not a good predictor of rock/oil interactions. The composition of the remainder of the oil phase is equally important, particularly with regard to its properties as a solvent for some of its largest constituents.
Solvent quality of the crude oil mixture is a function of composition, temperature, and pressure. Changing solvent quality by addition of an excess volume of low molecular weight paraffins is, of course, the basis for definition of the asphaltenes, the material that precipitates from the diluted mixture. The extent to which wetting is altered in oil/brine/rock systems by adsorption of asphaltic material from the crude oil is affected by changes in oil solvent properties, even in mixtures in which the asphaltenes are stable with respect to precipitation.
A simple approach, using measurements of mixture refractive index, is applied to quantify oil solvent quality for mixtures of crude oil or asphaltenes with hydrocarbon additives. Wettability alteration with systematic changes in oil composition are evaluated by contact angle measurements. Crude oils generally induce greater alteration as they become poorer asphaltene solvents.
Many factors can affect the interactions between components of crude oil, brine, and solid (COBS interactions). Prominent among these is the composition of the crude oil. The oil contains the interfacially active species that can alter wetting. Whether and when these species actually do affect wetting depends, in part, on the composition of the rest of the oil phase.
In order to induce wettability alteration in cores, Salathiel used stabilized East Texas crude oil diluted with n-heptane. The induced wetting condition as envisaged by Salathiel was that surfaces overlain by bulk water remained water-wet, whereas those contacted with oil became oil-wet. Salathiel referred to this condition as mixed wettability Because some oil continued to be produced even after injection of up to five thousand pore volumes of water, it was concluded that the water and oil phases were bicontinuous.
Fig. 1 presents a current view of the interactions between crude oil species and solid surfaces in the presence of brine. The pore space is represented as a brine-filled capillary tube of triangular cross section, into which the oil moves as the nonwetting phase. If the oil phase is a pure hydrocarbon, such as decane, interactions are restricted to dispersion forces; water is held as a film at the drained solid surface (Fig. 1a). If this film is in equilibrium with the bulk water in the corners, it has a disjoining pressure equal to the capillary pressure across the oil/bulk water interface. If the oil phase is crude oil, access to the solid surface may still be restricted by a thin film of water (Fig. 1b). Polar components in the crude oil will adsorb at the oil/water interface of both the bulk water and the thin film (Fig. 1c). The stability of the thin film depends on intermolecular forces arising from properties of the solid/brine and oil/brine interfaces. If the film is unstable, it will break, exposing solid surface to the components at the brine/oil interface (Fig. 1d). The influences of brine composition and other variables on stability of the intervening water layer have been reported previously.
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