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 (The Netherlands). In both rock types, we first quantify the porosity with BIB-SEM and derive the spatially resolved pore connectivity of the rock from BIB-SEM on Wood's Metal (WM) injected subsamples. In the second step, subsamples were saturated with the oil analog n-hexadecane, and then flooded with NaCl brine and MgSO4 brine, respectively, to study the effect of the brine chemistry on the microscopic fluid distribution. Cryo-BIB-SEM in combination with high-resolution energy-dispersive-spectroscopy (EDS) imaging and automated image analysis on the saturated samples allowed for a quantification of the oil-droplet size, the lengths of carbonate/oil interfaces, and the 2D contact angle of carbonate with brine and oil. Our results show that these features (e.g., interface length, contact angles, effect of asperities) 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.
Lixhe limestone showed relatively less carbonate/oil contacts despite a larger oil fraction in the MgSO4-brine-flooded sample compared with the sample flooded with NaCl brine, indicating a more-hydrophilic nature of the carbonate surface in this experiment. This feasibility study showed that the technique permits the testing of predictions on the morphology and dynamics of contact lines in relation to the mineral properties, which is not possible with other imaging methods, such as X-ray microcomputed tomography (µ-CT), because of limits in resolution.