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How Elastic Flow Instabilities Increase Oil Recovery

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In the production of crude oil, most of the oil recovery occurs during the process of water (brine) flooding. In this process, brine is pumped into the reservoir at injection wells so as to displace oil towards production wells, where the oil is “produced”. In situations where oil has a higher viscosity than water the flood front becomes unstable, which leads to a fingering / flow non-uniformity such that large oil-containing reservoir volumes are bypassed. Water-soluble polymers are often added to the “flooding” solution to increase viscosity and the increased production is explicable in terms of the uniformity of the flow profile (“sweep efficiency”) and the pressure gradient (flow capillary number). Recently, flooding with a polymer solution exhibiting elastic properties has been reported to increase “microscopic displacement efficiency” resulting in a sustained doubling of the recovery enhancement compared to that with conventional viscous polymer flooding. Flooding with viscoelastic polymer solutions is claimed also to increase recovery more than expected from changes in capillary number alone. This increase in displacement efficiency by viscoelastic polymers has been attributed to changes in the steady state flow profile and enhancements in oil stripping and thread formation. However, researchers in universities and in the oilfield industry have expressed significant doubts that a genuine effect is observed, or that improvements in displacement efficiency occur with field-applicable flow regimes. In this talk, I will demonstrate that flooding with viscoelastic polymer solutions can indeed increase recovery more than expected from changes in capillary number. The improvement in displacement efficiency arises from fluctuations in flow at low Reynolds Number. This behaviour, known as elastic turbulence, an elastic flow instability, is an effect previously unrecognised in oil recovery. The effect may be obtained at field-relevant flow rates and provides an underlying mechanism explaining both the enhanced capillary desaturation curves and the observation of apparent flow thickening for these viscoelastic solutions in porous media. , I will describe a combination of core flooding, micromodel flow, and rheometric studies, contrasting flow and recovery using viscous and viscoelastic polymer solutions. The circumstances under which viscoelasticity is beneficial will be demonstrated. The findings are applicable to the design of formulations for enhanced oil recovery by polymer flooding. The data lead to a 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. The studies were carried out in Schlumberger Research, Cambridge UK between 2012 and 2015 and particular acknowledgements are owed to Drs Andrew Clarke and Jonathan Mitchell

This talk is part of the BPI Seminar Series series.

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