Increasing flooding solution viscosity with polymers provides a favourable mobility ratio, compared to brine flooding, and hence improved volumetric sweep efficiency. Flooding with a polymer solution exhibiting elastic properties has been reported to increase displacement efficiency, resulting in a sustained doubling of the recovery enhancement compared to that with conventional viscous polymer flooding (SPE-144294 (Wang, et al., 2011)). Flooding with viscoelastic polymer solutions is claimed to increase recovery more than expected from changes in capillary number alone (SPE-127453 (Wang, et al., 2010)). This increase in displacement efficiency by viscoelastic polymers is reported to occur due to changes in the steady state flow profile and enhancements in oil stripping and thread formation. However, within the industry there are doubts that a genuine effect is observed, or that improvements in displacement efficiency occur with field-applicable flow regimes (SPE-169681 (Vermolen, et al., 2014)).

In this work, we show a different mechanism by which viscoelastic polymer solutions provide improvements in displacement efficiency, and demonstrate that these effects may be obtained at field-relevant flow rates. Our observations implicate an effect previously unrecognised in this context. Furthermore, this underlying mechanism explains both the enhanced capillary desaturation curves and the observation of apparent flow thickening (SPE-129200 (Seright, et al., 2011), SPE-113620 (Delshad, et al., 2008)) for these viscoelastic solutions in porous media. The work contrasts experiments on flow and recovery using viscous and viscoelastic polymer solutions. The circumstances under which viscoelasticity is beneficial are demonstrated. The findings are applicable to the design of formulations for enhanced oil recovery by polymer flooding.

A combination of core flooding, micromodel flow, and rheometric studies are presented. The results include single-phase and multi-phase floods in sandstone cores. Polymer solutions are viscoelastic (partially hydrolysed polyacrylamide, HPAM) or viscous (xanthan). The effect of molecular weight, flow rate, and concentration of the HPAMs are described. The data lead us to suggest an alternative mechanism that may be used to explain the observations of improved displacement efficiency and why the improvement is not seen for all viscoelastic polymer floods.

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