Wetting Behavior of Tight Rocks: From Core Scale to Pore Scale

Abstract

AbstractPrevious measurements show mixed‐wet behavior of tight siltstone core plugs, initially saturated with air. However, the same plugs show water‐wet behavior when saturated with oil or water. This study aims at explaining these observations by investigating the effects of mineral heterogeneity and intermolecular forces acting on solid/liquid and liquid/liquid interfaces. First, we conduct pore‐scale visualizations to characterize the minerals surrounding the pores of the tight rock samples. Thin‐section and scanning electron microscopy/energy dispersive X‐ray spectroscopy analyses show the existence of multimineral pores, which can be responsible for the mixed‐wet behavior. Next, we apply the DLVO theory to investigate the intermolecular forces acting on solid/liquid and liquid/liquid interfaces. We use disjoining pressure‐distance profiles and contact angles calculated for different minerals to evaluate the stability of the thin film covering the rock grains and to explain the wettability of the core plugs during spontaneous imbibition tests. The disjoining pressure values calculated for the dry core plugs suggest similar wetting affinities toward oil and water. However, the contact angles calculated for the water‐saturated core plugs suggest the water‐wet behavior, which agrees with the countercurrent imbibition results. Finally, we measure the adhesion forces between the tip of a cantilever and (1) pure quartz slide, (2) pure calcite crystal, and (3) the rock thin section using an atomic force microscope. The values of adhesion forces measured for the thin section composed of multimineral pores are between those measured for pure quartz slide and pure calcite crystal.