Pore-scale distribution of crude oil wettability in carbonate rocks

Abstract

Carbonate reservoir rocks can exhibit highly variable and intricate pore systems at multiple scales, for which the distribution of wettability is largely unknown. To improve understanding of pore-scale wettability, a set of outcrop and reservoir carbonate plugs was treated by partial drainage of brine by crude oil and aging, for a variety of brine-oil combinations and conditions. Wettability alteration was imaged by high resolution scanning electron microscopy of the oil footprint remaining on internal rock surfaces after removal of oil and brine with mild solvents. The wettability distribution on the calcite microparticles, which comprised microporous regions and lined vugular macropores, showed a characteristic, but unconventional, mixed-wet pattern of distinct, coexisting oil-wet and water-wet subareas. Oil deposition was limited to the less crystalline (anhedral) faces of these particles, while neighboring crystalline (euhedral) facets remained water-wet. Supporting measurements of -potential, contact angle, and initial brine saturation demonstrated that this face-selective alteration was formed by spontaneous drainage during aging, which appeared to favor oil deposition on facet edges and surrounding anhedral faces, thus preventing brine drainage from euhedral facets. This unifying pattern may simplify the integration of realistic wettability distributions into pore models of carbonate cores to predict oil recovery. Spontaneous imbibition of brine was sometimes observed to cause retraction of oil deposits on anhedral faces. The visualization of such changes can aid in designing the ionic composition of the flood brine to induce a shift toward water-wetting and enhance recovery from carbonates. Show more