Experiments and numerical simulations of chemical enhanced oil recovery in fractured carbonate rocks

Secondary oil recovery from naturally fractured, oil wet carbonate reservoirs is challenging due to high permeability fractures and unfavorable capillary forces. A series of new tertiary surfactant coreflood experiments in fractured oil wet limestone cores were performed in which surfactants lowered the IFT and formed viscous microemulsions upon mixing with crude oil and brine. The viscous microemulsions cause favorable transverse pressure gradients that result in increased oil recovery. The effects of surfactant slug size, salinity and injection rate were investigated experimentally. In all cases, the injected surfactant solutions recovered significantly more oil than waterflooding. Numerical simulations were used to interpret the experimental results. The experimental oil recovery and pressure drop data were matched by reducing the fluid mobility in the fracture. A new experiment verified that a viscous phase formed in the fracture, which resulted in a significant improvement in oil recovery.