Enhanced oil recovery (EOR) techniques are aimed at improving the recovery efficiency of mature oil fields in secondary and tertiary recovery modes. In particular, chemically tuned waterflooding (CTWF) has been a rising EOR technique toward improving oil recovery from rocks that are difficult to produce due to their initial wetting state, e.g., oil-wet and intermediate-wet. With an increasing oil-wetting affinity of a reservoir rock, extraction of oil becomes more challenging. As such, wettability alteration has been identified as the primary mechanism for oil recovery from oil-wet and intermediate-wet rock types. Recently, researchers have attempted to categorize the factors and mechanisms governing wettability alteration by CTWF. Multiple studies have identified the importance of brine salinity and ion composition on promoting wettability alteration to a more favorable water-wet state. Reservoir temperature, the surface charge of the rock, and the surface active components of crude oil are also reported to influence wettability alteration and therefore oil recovery from waterflooding. In this paper, we present an extensive literature review on the subject of wettability alteration, with an emphasis on experimental work conducted on carbonate and sandstone rocks, as they constitute the majority of the oil reserves in the world. The purpose of this review paper is to synthesize the current state of knowledge regarding the factors and mechanisms that govern wettability alteration by CTWF and, through this exercise, set the platform to pose new research questions.

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Summary Reliable simulation of enhanced oil recovery (EOR) processes depends on an accurate description of fluid transport in the subsurface. Current empirical transport models of rock-fluid interactions are fit to limited experimental data for specific rock types, fluids, and boundary conditions. In this paper, a general equation-of-state (EoS) approach is developed for relative permeability (kr) based on a set of geometric state parameters: normalized Euler characteristic (connectivity) and saturation. Literature data and pore network modeling (PNM) simulations are used to examine the functional form of the EoS. Our results show that the new kr-EoS matches experimental data better than the conventional Corey form, especially for highly nonlinear relative permeabilities at low saturations. Using hundreds of PNM simulations, relative permeability scanning curves show a locus of residual saturation and connectivity that defines an important limit for the physical kr region. The change of this locus is also considered for two contact angles. PNM data further allow for the estimation of the relative permeability partial derivatives that are used as inputs in the EoS. Linear functions of these partials in the connectivity-saturation space renders a quadratic response of kr, which shows excellent predictions. Unlike current empirical models that are based on only one residual saturation, the state function approach allows for dynamic residual conditions critical for capturing hysteresis in relative permeability.