Interpretation Of Mixed Wettability States In Reservoir Rocks

Abstract

Abstract It has recently been noted that the diffuse electrical double layers which exist at (1) the oil/ brine and (2) the mineral/brine interfaces in sand-stone reservoirs will in many cases be quite similar with respect to electric chare and potential. Extremely thin aqueous wetting films separating such interfaces are thus stabilized by the electrostatic repulsive force acting between the double layers. In the present paper the limits under which st able thin films can exist are examined in more detail. It is shown that there exists a lower limit to the pore size becomes a factor in determining the wettability of the rock/brine/oil system depends on the pore size distribution curve and on the initial brine saturation of the rock. Geological factors which may come into play in establishing the initial water saturation in play in establishing the initial water saturation in a given case are discussed. Introduction Recent studies dealing with the nature of aqueous wetting films in the Athabasca tar sand deposit have emphasized the role of several factor which contribute to the stability of these films. The work reported in the present paper extends the physico-chemical analysis of the wetting film stability physico-chemical analysis of the wetting film stability problem developed previously. This analysis is problem developed previously. This analysis is then applied to the interpretation of mixed wettability states in conventional petroleum reservoirs. In previous work evidence was presented which indicates that the diffuse parts of the electrical double layers at the mineral/brine and brine/ oil interfaces are similar with respect to the magnitude and sign of the electric potential and the electric charge density. As first pointed out by Langmuir, these electric double layers give rise to osmotic forces within thin saqueous wetting films. This causes the film boundaries to mutually repel each other, and such films are thereby stabilized. It as subsequently recognized that for these thin films the effect of the osmotic or electrostatic forces is modified by the existence of both dispersion (or van der Waals) forces and hydration (or adsorption) forces. Whereas the very short range hydration forces also stabilize aqueous wetting films, the dispersion forces tend to destabilize the films in question. In the study reported by Hall et al the effect of the hydration forces was accounted for in an approximate way by assuming that the so-called compact part of an electrical double layer was equivalent to two water molecules in thickness (about 0.55 nm). The dispersion force was neglected since the objective of the work was to obtain an estimate for the maximum thickness of films under conditions which are appropriate to Athabasca. Zeta potential data were used to calculate the magnitude potential data were used to calculate the magnitude of the electrostatic repulsive force for various aqueous phase salinities. For these calculations it was assumed that the diffuse double layer (zeta) potentials were the same for the two interface potentials were the same for the two interface forming the boundaries of a film. The wetting films were also assumed to be in hydrostatic and chemical equilibrium with the bulk aqueous phase, which in turn was taken to be in equilibrium with the bulk oil phase. Thus, the brine/oil interface formed at the boundary between a wetting film and the oil phase was characterized by the same value of the brine/oil interfacial tension as the interface between the bulk aqueous and oil phases. The difference between the pressures in the two bulk phases was taken as entirely due to the capillary pressure across highly curved brine/ oil interfaces. However, the difference in pressure modified by the repulsive force due to the electrical double layers. On the basis of the calculations corresponding to these assumptions, it was concluded that in Athabasca the films were in the range of 506 nm in thickness.