Nanoscale Morphology of Brine-Oil-Mineral Contacts in Connected Pores of Carbonate Reservoirs: Insights on Wettability from Cryo-BIB-SEM

Abstract We used Broad Ion Beam slope-cutting in combination with Scanning Electron Microscopy undertaken under cryogenic conditions (Cryo-BIB-SEM) to study mineral-oil-brine contacts in reservoir carbonates. This direct imaging method allows pore scale investigation of in-situ fluids and their distributions down to the nanometer scale. In this study we compare two types of carbonate reservoirs: a fine-grained Lixhe limestone (Belgium) and a coarse grained limestone from the Maastricht area (Netherlands). In both samples we first quantify the porosity using BIB-SEM and derive the spatially resolved pore connectivity of the rock from BIB-SEM on Wood's Metal (WM) injected sub-samples. In the second step sub-samples were saturated with the oil analogue n-hexadecane and NaCl-brine. Cryo-BIB-SEM in combination with high resolution EDS imaging and automated image analysis on the saturated samples allowed for a quantification of the oil droplet size, the lengths of oil-carbonate contacts and the 2D contact angle of carbonate with brine and oil. Our results show that these features (e.g., contact line length, contact angles, effect of surface roughness) are present on the scale of a few tens of nanometers to a few micrometers, which is in agreement with numerous theoretical and experimental studies. Hence, this technique permits the testing of predictions on the morphology and dynamics of contact lines in relation the mineral properties, which is not possible with other imaging methods, such as X-Ray Micro Computed Tomography (µ-CT), due to limits in resolution. Our results call for improvements in models of multiphase pore-scale flow in digital rocks.