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The enhanced oil recovery technique of low-salinity (LS) water flooding is atopic of substantial interest in the petroleum industry. Studies have shownthat LS brine injection can increase oil production relative to conventionalhigh-salinity (HS) brine injection, but contradictory results have also beenreported and an understanding of the underlying mechanisms remains elusive. Wehave recently developed a steady-state pore network model to simulate oilrecovery by LS brine injection in uniformly wetted pore structures (Watson etal., Transp. Porous Med. 118, 201-223, 2017). We extend this approach here toinvestigate the low-salinity effect (LSE) in heterogeneously wetted media. Wecouple a model of capillary force-driven fluid displacement to a novel traceralgorithm and track the salinity front in the pore network as oil and HS brineare displaced by injected LS brine. The wettability of the pore structure ismodified in regions where water salinity falls below a critical threshold, andsimulations show that this can have significant consequences for oil recovery.For networks that contain spanning clusters of both water-wet and oil-wet (OW)pores prior to flooding, our results demonstrate that the OW pores contain theonly viable source of incremental oil recovery by LS brine injection. Moreover,we show that a LS-induced increase in microscopic sweep efficiency in the OWpore fraction is a necessary, but not sufficient, condition to guaranteeadditional oil production. Simulations suggest that the fraction of OW pores inthe network, the average network connectivity and the initial HS brinesaturation are key factors that can determine the extent of any improvement inoil recovery in heterogeneously wetted networks following LS brine injection.This study highlights that the mechanisms of the LSE can be markedly differentin uniformly wetted and non-uniformly wetted porous media.

A Mechanistic Pore-Scale Analysis of the Low-Salinity Effect in Heterogeneously Wetted Porous Media