Influence of Electrical Surface Charges on the Wetting Properties of Crude Oils

Abstract

Summary Reservoir wettability is important to oil recovery by waterflooding and many other oil recovery processes. The difficulties associated with determination of in-situ wettability, together with uncertainties about application of laboratory observations to field conditions, necessitate a more basic understanding of factors that control wettability. We previously reported that the adhesion of crude oil to a solid surface could be related to wettability alteration. In this work, conditions under which oil adheres to a particular solid surface are demonstrated for several crude oils. For a given oil, pH and ionic strength were varied to obtain a mapping of conditions under which adhesion occurs. Results were satisfactorily explained by double-layer calculations in combination with the ionizable surface group model. Lack of adhesion signifies the presence of a stable water film that results from double-layer repulsion between the crude oil and the solid surface. Introduction Results of laboratory waterflows show that wettability can have a profound effect on the efficiency of displacement of oil by water. Departure from strongly water-wet conditions, which are often taken as a convenient standard, can result in either a decrease or increase in oil recovery efficiency, reflecting a range of possible wettability changes. Laboratory waterfloods in which wettability change involved the use of or exposure to crude oil are compared in Fig. 1 with results for strongly water-wet conditions. Results are presented as percent of original oil recovered (or microscopic displacement efficiency), ED, vs. PV of water injected. There is growing opinion that mixed-wettability conditions pertain in many oil reservoirs. At areas of rock surface that are contacted by the crude oil, the potential exists for adsorption of water-insoluble polar components from the crude oil. Thus, in-situ wettability may depend directly on the initial distribution of oil and interstitial water with respect to the rock surface. The results shown in Fig. 1 illustrate the importance of performing laboratory waterfloods at properly representative reservoir conditions; however, maintaining or establishing the correct conditions is a major difficulty. A much improved understanding of the effects of crude oils on wettability is needed to give confidence in core recovery and handling procedures] aimed at preservation or restoration of reservoir wettability. Contact between crude oil and rock is dependent on the stability of water films between rock surface and the crude oil. The existence of stable water films in the range of 1- to 100-nm [10- to 1,000- A] thickness has been shown to depend on the presence of an electrical double-layer repulsion that results from surface charges at the solid/water and water/oil interfaces being of the same sign. In the regions of contact, thin films contour the solid surface. except as modified by surface roughness. Water held essentially as a skin at the rock surface by electrical double-layer or shorter-range forces will be referred to as pellicular water. Equilibrium with the bulk water, which will be at some capillary pressure, is satisfied by the disjoining pressure acting in the pellicular water. A schematic of the distribution of crude oil and bulk and pellicular water in pores of a triangular cross section is shown in Figs. 2a and 2b for smooth and rough pore walls, respectively. In this paper, discussion of film stability will refer to pellicular water unless otherwise stated. As long as water-soluble surfactants from the crude or in the formation water do not alter wettability, the stability of water films between crude oil and the rock surface and their ability to prevent adsorption of water-insoluble components over geologic time are key factors in maintaining reservoirs at strongly water-wet conditions. If the film is unstable, polar components from the oil will have the opportunity to adsorb directly onto the rock surface. If the adsorbed components cannot migrate from the region of contact, a mixed-wettability condition can be expected with the distribution of oil-wet surfaces complementing the regions overlain by bulk water. Whether migration occurs or not, instability of the wetting film followed by adsorption probably results in departure from a strongly water-wet condition in the region of contact. It has been shown that film stability is dominated by pH, brine concentration, and composition. In the present study, adhesion behavior observed for crude oils is related through electrical double-layer theory to the properties of the oil/water and water/solid interfaces. Theory and Background Electric Properties of the Oil/Water Interface. It has been previously demonstrated with bitumen that an oil/water interface has a negative electric charge that can be adequately explained by the ionizable surface group (ISG) model. In the application of this model, it is assumed that the negative charge of the interface is caused by the dissociation of carboxylic acids, which are naturally occurring surfactants. In contrast to observations reported previously for bitumen, electrophoresis measurements for three conventional crude oils studied in the present work showed a change from negative to positive interfacial charge at low pH, indicating the presence of basic and acidic surface-active groups at the oil/water interface. The theoretical model applied to bitumen was therefore extended to take into account the zwitterionic nature of the crude-oil/water interface by use of a method described by Harding and Healy: (1) (1b) where A- and BH+ represent acidic and basic groups at the interface. Their dissociation constants are defined by (2a) and (2B) where [Hs+] = hydrogen ion concentration in the vicinity of the surface, which can be related to the bulk concentration, [Hb+], by use of the Boltzmann relationship, (3a) where ro is a reduced potential given by (3b) where e=electron charge, 4) oro = surface potential, k=Boltzmann constant, and T=absolute temperature. SPERE P. 332^