Nanoscale imaging of pore-scale fluid-fluid-solid contacts in sandstone

Direct observations of oil-water-rock contacts are key for improving our understanding of multiphase flow phenomena in mixed-wet reservoir rocks. In this study we imaged pore-scale fluid-fluid-solid contacts in sandstone with nanometer resolution using cryogenic broad ion-beam polishing in combination with scanning electron microscopy and phase identification by energy-dispersive X-ray analysis. We observed, as expected, the nonwetting oil phase separated from quartz surfaces by a thin brine film, but also direct contacts between oil and rock at asperities and clay aggregates, which act as pinning points and cause discontinuous motion of the oil-water-solid contact line. For the rare classical configuration of a three-phase contact the microscopic contact angle has been determined by serial sectioning. Our results call for improvements in models of multiphase pore-scale flow in digital rocks.