In Situ Wettability, Capillary Pressure, and Matrix–Fracture Interactions in Fractured Oil-Wet Carbonate: A Microscale Investigation

Naturally fractured oil-wet carbonate reservoirs host a considerable fraction of oil reserves worldwide. However, the recovery factor in these reservoirs is typically low due to the inherent oil-wet characteristics of the carbonate rock and the limited interactions usually established between the fracture and matrix domains during enhanced oil recovery processes. This study was conceived to develop an improved fundamental understanding of the pore-scale displacement mechanisms responsible for oil recovery from naturally fractured oil-wet carbonates during secondary waterflooding and tertiary surfactant injection. To achieve this objective, a state-of-the-art multiphase core-flooding system integrated with a high-resolution micro-CT imaging platform was utilized to perform a series of flow experiments on a miniature fractured oil-wet carbonate sample at elevated pressure and temperature conditions. The fluid occupancy maps generated during the flow experiments were used to characterize the in situ wettability and capillary pressure, and track fracture displacement events, fracture–matrix interactions, and oil mobilization patterns at the pore scale. The results revealed that the surfactant solution reversed the wettability of both rough fracture walls and pore surfaces from an initially oil-wet state to a neutral-wet condition. This was accompanied by an order-of-magnitude reduction in the oil–water interfacial tension (IFT). The synergistic effects of wettability reversal and IFT reduction induced by surfactant injection were pivotal to decreasing the threshold pressure required for additional oil displacement from the rock matrix adjacent to the fracture. Additionally, notable evidence of fracture storage and displacement events at the pore scale was obtained in the form of wetting oil pockets and bridging events. Fracture oil pockets and bridges formed on the rough fracture walls and narrow aperture regions may enhance fracture–matrix interactions during base waterflooding by restricting brine flow through the higher-conductivity fracture. The collapse of such fracture oil storage features during the succeeding surfactant injection contributed considerably to the ultimate oil recovery from the combined media.