Viscoelasticity vs. viscosity: what dominates the Sor reduction at shear thinning flux in low permeable limestone cores at low salinity polymer flood conditions?

Polymer flood is one of the most applied enhanced oil recovery (EOR) methods across the globe. Hydrolyzed polyacrylamide (HPAM) is the most commonly used EOR polymer that exhibits both viscous and viscoelastic characteristics. Although several polymer flood studies conducted in sandstones using viscous and viscoelastic polymers revealed that the viscoelastic polymer can lead to higher reduction of residual oil saturation (Sor) than viscous polymer at relatively lesser pressure gradient, the research question that remains unexplored is whether this will still hold in low-permeable carbonate rocks with the oil wetting tendency. Such a question carries a fundamental significance in the EOR perspective as the inherent oil-wetting tendency traps the oil not only by the usual interfacial trapping force but also by an adhesion force. The paper addresses this important question by adopting a systematic methodology that comprises series of experiments, including nuclear magnetic resonance (NMR) characterization, permeability measurement, bulk rheology, in-situ rheology, and two-phase core flooding experiments using viscous and viscoelastic polymers in low-permeable limestone cores across varying fluxes from shear thinning to shear thickening. This work includes the two polymer systems -2500 ppm AN 125 SH (lesser viscosity and higher viscoelasticity) and 2000 ppm xanthan gum (higher viscosity and lesser viscoelasticity) prepared in 5957 ppm total dissolved solids (TDS) low salinity brine. NMR characterizations on the cores showed similar permeabilities in the range of 82 to 92 md. In-situ rheological experiments are then performed to choose the flux rates to be used in the two-phase core flood experiments. Two-phase core flooding conducted on the aged limestone cores subjected to prior bump water flooding at the shear-thinning fluxes of 0.1 ft./day indicates that viscoelastic 2500 ppm AN 125 SH polymer yields an incremental recovery factor of 2.4% at the pressure gradient of 16 psi/ft. On the other hand, 2000 ppm xanthan gum failed to recover any incremental oil at the same flux rate in spite of generating a higher-pressure gradient of 31.75 psi/ft. These results showed that even in the oil-wet limestone cores, the viscoelastic polymer can yield higher Sor reduction than the viscous polymer despite generating the relatively low-pressure gradient. However, at higher fluxes of 1 ft./day and 10 ft/day, xanthan gum yielded an additional oil recovery of 5.3% and 3.2% associated with greater-pressure gradients of 91.2 and 285 psi/ft, indicating that a relatively high-pressure gradient is required to mobilize the residual oil from limestone cores in the absence of viscoelastic effects.