Oil recovery from fractured carbonates by surfactant-aided gravity drainage: Laboratory experiments and mechanistic simulations

Waterflooding recovers little oil from naturally fractured carbonate reservoirs if the matrix is oil-wet and fracture intensity is high. Laboratory experiments and mechanistic simulations have been conducted to understand the injection of dilute anionic surfactant solutions into oil-wet, fractured reservoirs. In this process, surfactant diffuses into the matrix, lowering the interfacial tension (IFT) and contact angle, which decreases the capillary pressure and increases oil relative permeability, enabling gravity to drain up the oil. The rate of oil recovery increases with an increase in matrix permeability, a decrease in initial gas saturation, a decrease of fracture height or spacing, and an increase in the wettability-altering capabilities of the surfactant. Increasing the surfactant concentration does not necessarily enhance the oil recovery rate, because IFT and wettability alterations are not linearly related to surfactant concentration. Adsorption of anionic surfactants on calcite can be suppressed with an increase in pH and a decrease in salinity.