A Mechanistic Pore-Scale Analysis of the Low-Salinity Effect in Heterogeneously Wetted Porous Media
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A Mechanistic Pore‑Scale Analysis of the Low‑Salinity Effect in Heterogeneously Wetted Porous Media Michael G. Watson1 · Steven R. McDougall2 Received: 27 January 2020 / Accepted: 24 September 2020 © Springer Nature B.V. 2020
Abstract Low-salinity (LS) waterflooding has been a topic of substantial recent interest in the petroleum industry. Studies have shown that LS brine injection can increase oil production relative to high-salinity (HS) brine injection, but contradictory results have also been reported and a mechanistic explanation of these findings remains elusive. We have recently developed a pore-scale model of LS brine injection in uniformly wetted networks (Watson et al. in Transp Porous Med 118:201–223, 2017), and we extend this approach here to investigate the low-salinity effect (LSE) in heterogeneously wetted media. We couple a steadystate fluid displacement model to an innovative tracer algorithm and track the evolving salinity front as oil and HS brine are displaced from the network. The wettability of the pore structure is locally modified where water salinity falls below a critical threshold, and simulations show that this can have significant consequences for oil recovery. Our results demonstrate that, for heterogeneously wetted networks, the oil-wet (OW) pores are the only viable source of incremental oil by LS brine injection. Moreover, we show that a LSinduced increase in the displaced OW pore fraction is a necessary, but not sufficient, condition to guarantee additional oil production. Simulations further suggest that the initial OW pore fraction, the average network connectivity and the initial HS brine saturation are factors that can determine the extent of incremental oil recovery following LS brine injection. This study clearly highlights that the mechanisms of the LSE can be markedly different in uniformly wetted and in non-uniformly wetted porous media. Keywords Pore network modelling · Two-phase flow · Low-salinity waterflooding · Wettability modification · Enhanced oil recovery
1 Introduction As the global demand for energy continues to grow, new and innovative enhanced oil recovery (EOR) methods are required to maximise oil production from hydrocarbon reservoirs. One method that has attracted increasing interest over the last decade * Michael G. Watson [email protected] 1
School of Mathematics and Statistics, University of Sydney, Camperdown, Australia
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Institute of GeoEnergy Engineering, Heriot-Watt University, Edinburgh, UK
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M. G. Watson, S. R. McDougall
is low-salinity (LS) waterflooding, which has been shown to improve oil recovery at the core scale and the field scale in both secondary and tertiary modes (Webb et al. 2004; Gamage and Thyne 2011; Mahani et al. 2011). The improvement in oil recovery associated with LS brine injection—the so-called low-salinity effect (LSE)—is widely believed to arise from a favourable shift in the wettability of the crude oil/brine/ rock (COBR) system (Berg et al. 2010). However, LS brine injectio
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