IFT Vs. Viscoelasticity: What is the Most Potent Microscopic Light Oil Recovery Mechanism in Water-Wet High-Permeable Formations?

Although reducing the residual oil saturation (S_or) is challenging for enhanced oil recovery (EOR), a substantial reduction in its value, especially at low flux, means a higher incremental recovery factor. Historically, interfacial tension (IFT) reduction has been regarded as the key microscopic recovery mechanism at least in permeable water wet formation for reducing S_or. Recent studies have highlighted that the synthetic polymer's viscoelasticity can influence S_or reduction, especially at higher fluxes. Would the polymer's viscoelasticity be potent when compared to the surfactant's IFT for reducing the S_or, and how the flux rate would influence their ability to reduce S_or? The objective of current study is to answer these important questions first by formulating two solutions with similar viscosities but distinct IFT and viscoelastic properties and then subjecting the two solutions to spontaneous imbibition and forced imbibition (multi-rate core flooding) tests. A cationic surfactant-glycerol solution, designed to reduce IFT without exhibiting viscoelasticity and a synthetic HPAM polymer solution with significant viscoelasticity without IFT reduction capability were chosen for the experiments. A high salinity brine was used for solutions preparation. A series of IFT and steady-shear rheology experiments were conducted to formulate the concentration of two different solutions, ensuring appropriate properties (with similar viscosity but contrasting IFT and viscoelasticity) for studying their effects on microscopic oil recovery. To elucidate the role of other mechanisms, the Bentheimer sandstone rocks with similar permeability were selected for porous media experiments. The forced imbibition experiments were conducted using the cores that were water flooded at bump rates and these experiments are aimed to generate capillary desaturation curves. The spontaneous imbibition experiments were conducted with the cores that were subjected to prior water imbibition. The core flood results demonstrated that viscoelastic polymer flooding led to a higher incremental recovery factor of 55% in contrast to 42% obtained with viscous surfactant-glycerol flooding. Further, the viscoelastic polymer showed an early onset of rapid oil mobilization (desaturation) at the flux rate of 2 ft/day (N_C ~ 2.79×10⁻⁵) compared to 3 ft/day (N_C ~ 1.82×10⁻⁴) in the surfactant-glycerol flood indicating the viscoelasticity played a dominant role over IFT during forced flow at the studied conditions. The spontaneous imbibition experiments results revealed the higher imbibition potential and incremental oil recovery (over water imbibition) for surfactant-glycerol system over the HPAM system indicating the dominance of IFT over viscoelasticity in flux-free mode. This is the first work in existing knowledge that compares the potential of IFT and viscoelasticity on microscopic oil recovery at different fluxes. During forced flooding, the viscoelasticity appears to provide an advantage at relatively lower fluxes. However, IFT reduction contributes to higher imbibition oil recovery at no flux in the high permeable formation signifying its potential for imbibition-related recovery process.