Relationship Between Residual Saturations and Wettability Using Pore-Network Modeling

The relationship between residual saturation and wettability is critical for modeling multiphase processes like enhanced oil recovery, CO₂ sequestration, and geologic storage of hydrogen. The wetting state of a core is often quantified through Amott indices, which are estimated from the ratio of the saturation fraction that flows spontaneously to the total saturation change that occurs due to spontaneous flow and forced injection. Observations from traditional coreflooding experiments show a minimum in the trends of residual oil saturation (S_or) around mixed-wet conditions. Amott indices, however, provide an average measure of wettability because of their intrinsic dependence on a variety of factors such as the initial oil saturation, aging conditions, rock heterogeneity, etc. Thus, the use of Amott indices could potentially cloud the observed trends of residual saturation with wettability.

Using pore-network modeling (PNM), we show that S_or varies monotonically with the contact angle, which is a direct measure of wettability. That is, for fixed initial oil saturation, the S_or decreases monotonically as the reservoir becomes more water-wet (decreasing contact angle). Further, the calculation of Amott indices for the PNM data sets shows that a plot of the S_or vs. Amott indices also shows this monotonic trend, but only if the initial oil saturation is kept fixed. Thus, for the cases presented here, we show that there is no minimum residual saturation at mixed-wet conditions as wettability changes.

In this research, we employ a numerical approach to quantify trends of S_or against the traditional definition of wettability. Through the analysis of our numerical work and literature experiments, we find that under isolated conditions (constant initial saturation), linear trends exist between S_or and wettability. This can have important implications for low salinity waterflooding, water-alternating-gas enhanced oil recovery, or CO₂ sequestration where the effects of wettability are critical to understand phase trapping.