Modelling the Stability of Thin Water Films Using SEM Images

Abstract

Understanding reservoir wettability-including wettability alteration mechanisms-is important to model and to optimise oil recovery. When oil enters an originally water wet porous rock under primary drainage, the oil will gradually displace the water in the pores and a thin water film will be left between the pore walls and the oil. Under certain conditions, the water film may become unstable and collapse. Depending on the chemistry of the oil, water, and porous rock, this collapse may result in wettability alteration. Stability of the water film will control the wettability alteration of the porous rock. Equilibrium saturation configurations in the oil-water system at primary drainage are simulated in 2D for a realistic pore space geometry obtained directly from high resolution scanning electron microscope images and thus retaining geometric features of the porous rock under consideration. The eventual collapse of the wetting phase film is controlled by the curvature of the surface and the disjoining pressure isotherm. Given the critical disjoining pressure value depending on the temperature and the chemical properties of the system at hand, the model facilitates computation of the "wettability index", i.e., the fraction of the rock surface which will come in direct contact with oil and hence potentially become oil wet. Both synthetic and realistic images are analysed. The generic studies show the influence of the surface roughness of the pore walls on the fraction of the surface which can potentially be converted to oil wet for different drainage pressures. Introduction The reservoir rocks are filled with water and believed to be water-wet before oil enters these rocks during primary drainage (Tiab and Donaldson 2004, Dandekar 2006). This homogeneous wettability state may, however, change; either to a homogeneous oil-wet state, or to a heterogeneous state where some parts of the rock surface are water-wet while other parts are oil-wet (Donaldson and Alam 2008). A change in the wettability of a reservoir rock is caused by adsorption of components from the oil onto the grain surfaces of the rock. This adsorption is thus one necessary condition for wettability change. During primary drainage, oil has displaced most of the original water in place, leaving water only in crevices and corners and as a thin film along the pore surface of the reservoir rock. The stability of this water film depends on the disjoining pressure and the geometry of the pore surface. In order for the oil components to adsorb onto the rock surface, the water film must become unstable and collapse to a thin monolayer, providing direct access for the oil to the surface. The stability of the water film can be examined using the augmented Young-Laplace equation (Mohanty 1981, Hirasaki 1991a; Hirasaki 1991b; Hirasaki 1991c). This augmented version contains an extra term-the disjoining pressure-which incorporates the stabilizing forces in the film (Derjaguin and Churaev 1978; Derjaguin et al. 1987). When thin water films are present in a pore filled with oil and water, this equation is a requirement for equilibrium.

Pore-Scale Modeling of Waterflooding in Mixed-Wet-Rock Images: Effects of Initial Saturation and Wettability

“…3. Such behavior is in agreement with the work by Frette et al (2009), who calculated the pore-wall area with intact and collapsed water films during drainage on the basis of disjoining pressure and pore-wall curvature.…”

Dynamic Capillary Pressure Curves From Pore-Scale Modeling in Mixed-Wet-Rock Images

“…The stability of the water films depend on the disjoining pressure and the geometry of the solid surface, and the critical pressure when the thin films start to collapse can be examined by the augmented Young-Laplace equation (Hirasaki, 1991b,a;Kovscek et al, 1993;Frette et al, 2009). The local curvature of a pore wall has a significant effect on the critical pressure.…”

Estimation of the curvature of an interface from a digital 2D image

“…Some features, which can be naturally present in rock images, like grain-to-grain contacts, make the very applicability of the concept of curvature questionable. In a forthcoming paper [18] we will discuss the possible effect the segmentation process has on the computation of the curvatures.…”

“…Some examples are heart wall analysis (medical science) [15], diatom identification (biological science) [16], shape perception (robot engineering) [17] and thin film stability (petroleum engineering) [18].…”