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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.

Recent research indicates that even small viscous transverse pressure gradients can increase the rate of surfactant imbibition into the matrix of a fractured reservoir. However, surfactants are commonly tested using static imbibition cells without any imposed pressure gradient. Therefore, the effect of viscous pressure gradients was investigated by conducting a series of surfactant corefloods using fractured Silurian Dolomite and Texas Cream Limestone cores. The viscosity of the surfactant solution was increased by adding polymer or by changing the salinity of the aqueous surfactant solution, which affects the in-situ microemulsion viscosity. The fractured cores had a permeability contrast between the fracture and the matrix ranging from 2500 to 90,000. Non-fractured corefloods were also performed and compared with the fractured corefloods. The more viscous surfactant solutions achieved greater oil recovery from the fractured carbonate cores. These results indicate that a viscous microemulsion can serve as a mobility control agent in naturally fractured reservoirs (NFRs) that is analogous to mobility control with foams and polymers, but with less complexity and cost.