Ultralow-IFT Foam for Enhanced Oil Recovery in Naturally Fractured Oil-wet Carbonate Reservoirs
Hirasaki, George J.
Doctor of Philosophy
Oil recovery in heterogeneous carbonate reservoirs is typically inefficient because of the existence of high-permeability fracture networks and unfavorable capillary forces within the oil-wet matrix. This thesis demonstrates the feasibility and investigates the injection strategy of an ultralow-interfacial-tension (ultralow-IFT) foam process to improve oil recovery in high-temperature, high-salinity fractured oil-wet carbonate reservoirs. The three-component ultralow-IFT foaming formulation consists of two anionic surfactants, internal olefin sulfonate (IOS) and ethoxylated carboxylate (L38), and one zwitterionic surfactant, lauryl betaine (LB). The synergistic effect helps to simultaneously achieve good injectivity in porous media, ultra-low IFT (10^-2-10^-3 mN/m magnitude) with crude oils, and good foamability at under-optimum conditions. In fractured oil-wet dolomite cores, the pre-generated low-IFT foam achieved approximately 70% original oil in place (OOIP) incremental oil recovery compared with waterflood and lowered the remaining oil saturation in the matrix by over 20% compared with a foaming formulation lacking the low-IFT property. Meanwhile, despite the challenges associated with limestone geochemical instability, the low-IFT foam process achieved 64% OOIP incremental oil recovery compared with waterflood in fractured oil-wet limestone cores with matrix permeability as low as 5 mD. It remarkably extended chemical EOR process in fractured tight formations. In the presence of a salinity gradient between injected and connate brine, the crossflowed oil and high-salinity connate brine from the matrix may cause severe Winsor II, i.e., over-optimum, condition. The Winsor II condition significantly destabilized the pre-generated foam and introduced viscous macroemulsions. A constant slightly-under-optimal salinity or a salinity gradient with a slightly-over-optimal connate salinity is preferred to achieve ultralow-IFT with oil but avoid severe Winsor II condition in the mixing zone to ensure a sufficient foam strength. The unique properties of foam, i.e., a higher apparent viscosity in high-permeability regions and selective diversion of primarily aqueous solution into low-permeability zones, were identified to contribute to the more favorable mobility control and the better oil recovery efficiency. These properties, combined with the remaining oil mobilization by reducing unfavorable capillary forces, make the ultralow-IFT foam a promising EOR process in heterogenous/fractured oil-wet reservoirs. In addition, an alternative two-component ultralow-IFT foaming formulation was developed based on two anionic surfactants, ethoxylated sulfonate (Avanel S70) and internal olefin sulfonate (IOS). The formulation produced IFT of 10^-3-10^-4 mN/m magnitude with oil and relatively weak foamability. Corefloods reveal a relatively slow oil recovery resulted from slow foam propagation. The results indicate a sufficient foam strength is required to effectively divert fluids to achieve the success of ultralow-IFT foam process. This dissertation provides fundamental understanding of ultralow-IFT foam behavior in fractured reservoirs. The investigation of ultralow-IFT foam process will contribute to the success of enhanced oil recovery from abundant fractured oil-wet carbonate reservoirs.
low-IFT foam; enhanced oil recovery; fractured reservoir