A proposed pore-scale mechanism for how low salinity waterflooding works

In this paper, we propose a detailed, semi-quantitative, theory of how the low salinity waterflooding effect works based upon pore-scale theoretical considerations. This theory follows on from detailed core flooding work performed in our laboratories demonstrating the importance of multicomponent ion exchange (MIE) as the underlying mechanism of low salinity waterflooding (Lager et al, 2006). Whilst this earlier work highlighted the importance of MIE, it did not explain the precise consequences of the theory in terms of quantifying the incremental oil recovery nor the precise impact on pore-scale physics. For example, with MIE occurring, the changes in the divalent cation concentrations (Ca²⁺ and Mg²⁺) leads to the development of a "self freshening" zone within the waterflooded region within which certain changes in the surface chemistry of the pore walls may occur. Such effects include expansion of the electrical double layers, changes in the adsorption of polar organic species, and resulting changes in wetting. The theory presented here attempts to show the consequences of these changes which are supported by some plausible pore-scale model calculations. These calculations i) indicate the pore-scale origins of the low salinity oil recovery mechanism, ii) show the magnitude of the effect semi-quantitatively, and iii) allow some clear predictions to be made which can be tested experimentally. This work also follows Lager et al (2006) in further explaining why both crude oil and clay-bearing reservoir rock are required for the low salinity effect to occur, and why the effect is not seen in strongly water-wet, clay-free porous media with mineral oils. Thus, this proposed pore-scale physical model of the low salinity waterflooding effect both complements and extends previous mechanistic explanations based upon multicomponent ion exchange (MIE). Although this proposed mechanism is consistent with all the available observations, further experimental studies are required to definitively confirm it. Copyright 2010, Society of Petroleum Engineers.