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Designing viscoelastic solutions to enhance 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. In situations where the oil viscosity is greater than that of water the flood front becomes unstable, leading 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 hence uniformity of the flow profile (“sweep efficiency”) leading to increased production. The production from the swept regions (“displacement efficiency”) is rationalised in terms of the flow capillary number describing the balance between the pressure gradient and the interfacial forces that “trap” the oil in the rock pores. Recently, flooding with a polymer solution that exhibits elastic properties has been reported to increase displacement efficiency, resulting in a sustained doubling of the recovery enhancement over that from conventional viscous polymer flooding [1]. Flooding with viscoelastic polymer solutions is claimed also to increase recovery more than expected from changes in capillary number alone [2]. This increase in displacement efficiency by viscoelastic polymers was attributed to changes in the steady state flow profile and enhancements in oil stripping and oil-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 within field-applicable flow regimes [3]. 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 [4], 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 [5,6]. 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 reveal an unexpected dependence on solution composition. The data lead to a mechanism that may be used to explain the observations of improved displacement efficiency, are applicable to the design of formulations for enhanced oil recovery by polymer flooding and reveal 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 [7-9].

References: [1] Wang, D., Wang, G. & Xia, H. Large Scale High Visco-Elastic Fluid Flooding in the Field Achieves High Recoveries. SPE Conference, pp. SPE 144294 -MS, 2011. [2] Wang, D., Xia, H., Yang, S. & Wang, G. The Influence of Visco-elasticity on Micro Forces and Displacement Efficiency in Pores,Cores and in the field. SPE Conference, pp. SPE 127453 -MS, 2010. [3] Vermolen, E. C., Haasterecht, M. J. T. & Masalmeh, S. K. A systematic study of the polymer visco-elastic effect on residual oil saturation by core flooding. SPE Conference, pp. SPE169681 -MS, 2014. [4] A. Groisman and V. Steinberg, “Elastic turbulence in a polymer solution,” Nature, 405, 53, 2000. [5] Seright, R. S., Fan, T., Wavrik, K. & Balaban, R. d. C. New insights into polymer rheology in porous media. SPE Journal, pp. SPE129200 -PA, 2011. [6] Delshad, M. et al. Mechanistic Interpretation and Utilization of Viscoelastic Behaviour of Polymer Solutions for Improved Polymer-Flood Efficiency. SPE Conference, pp. SPE 113620 -MS, 2008. [7] Clarke, A., Howe, A.M, J.Mitchell, J.Staniland, L.A.Hawkes. Mechanism of anomalously increased oil displacement with aqueous viscoelastic polymer solutions. Soft Matter, 11, 3536, 2015. [8] Howe, A. M., Clarke, A. & Giernalczyk, D. Flow of concentrated viscoelastic polymer solutions in porous media: effect of MW and concentration on elastic turbulence onset in various geometries. Soft Matter, 11, 6419, 2015. [9] A.Clarke, A.M.Howe, J.Mitchell, J.Staniland, L.A.Hawkes. How Viscoelastic Polymer Flooding Enhances Displacement Efficiency, SPE Journal, in press.

This talk is part of the Chemical Engineering and Biotechnology Departmental Seminars series.

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