No 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 ζ-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.

No abstract

3D pore-scale imaging and analysis provides an understanding of microscopic displacement processes and potentially a new set of predictive modeling tools for estimating multiphase flow properties of core material. Reconciliation and integration of the data derived from these models requires accurate characterization of the pore-scale distribution of fluids and a more detailed understanding of the role of wettability in oil recovery.The current study reports experimental imaging progress in these endeavors for a preserved-state carbonate core from a Middle Eastern waterflooded reservoir. Micro-CT methods were used in combination with novel fluid X-ray contrasting techniques and image registration to visualize the 3D pore-scale distribution of residual oil in mini-plugs. Segmentation of the registered tomograms and their differences facilitated estimation of the residual oil saturation. These predictions from digital analysis agreed reasonably well with laboratory measurements of oil saturation from extraction of sister mini-plugs and spectrophotometry. The tomogram segmentations provide additional information beyond this average value, such as the fractions of oil associated with macroporosity and microporosity.After the tomogram acquisitions, one of the dried mini-plugs was cut and SEM imaged at this exposed face to provide 2D images of fine features below the micro-CT resolution limit, such as the characteristic dimpled texture of asphaltene films on calcite surfaces due to their local wettability alteration in the reservoir. A new registration procedure was developed to embed the SEM images from the cut plug into the tomogram of the original uncut plug at their correct locations, so that this highresolution wettability information could be integrated into the 3D pore network description and correlated to the local distribution of residual oil.