Although field data reports that adding surfactants into hydraulic fracturing fluid increases flowback efficiency, there is a lack of studies that investigate whether the higher flowback efficiency necessarily results in a higher oil recovery. This study aims to propose a relationship between flowback efficiency and oil recovery using a numerical modeling approach. The surfactants properties (interfacial tension/IFT reduction, wettability alteration, adsorption, and partitioning) are calibrated based on laboratory experimental data, while the reservoir model is validated based on field production data from the Middle Bakken shale reservoir. The reservoir flow model is also coupled with geomechanics to model the reservoir stress-dependent permeability. Our model successfully reproduces the field observations that adding surfactants into the fracturing fluid can increase the flowback efficiency. However, the higher flowback efficiency does not necessarily lead to a higher oil recovery. Sensitivity analysis is performed on the wettability alteration, IFT value, and surfactant concentration. Our model shows that to maximize the oil recovery, the surfactants should strongly alter the wettability to water-wet while maintaining a high oil-water IFT, as high as 20 mN/m. The high IFT will help maintain the high capillary pressure required to suck the water into the matrix while expelling the oil from the matrix. However, to maximize the flowback efficiency, the surfactants should achieve a low IFT (as low as 0.001 mN/m) with more flexible wettability-alteration criteria. This study quantitatively suggests that when surfactant is added into the fracturing fluid, a higher flowback efficiency does not necessarily result in a higher oil recovery. This is because the primary driving mechanism of oil recovery from unconventional reservoirs, which are generally mixed-to oil-wet, seems to be spontaneous imbibition, in which water-wet rock is then favorable.
- Coupled flow-geomechanical modeling
- Enhanced oil recovery
- Fracturing fluid