Nonmonotonic Effects of Salinity on Wettability Alteration and Two-Phase Flow Dynamics in PDMS Micromodels

The interaction between fluid-fluid and solid-fluid interfacial forces and surface roughness controls the wettability. The ionic strength is the most important factor that controls electrostatic forces. Thus, a modification of the ionic strength can potentially lead to a change of the wettability, as shown in recent experimental works related to low-salinity waterflooding, which is an enhanced oil recovery technology. Despite the significant research published on this topic, for the first time, we present how a change of the ionic strength alters the wettability in a pore network micromodel made of silanized polydimethylsiloxane (PDMS). We visualized the invasion of brine in an elongated hydrophobic PDMS micromodel, initially saturated with Fluorinert. Under different injection rates and ionic strengths and using image processing, we quantified the contact angle distribution in the flow network, the recovery curve with time, the brine breakthrough time, and the temporal change of resident saturation. The results imply that there is an optimal range of salinity at which saturation change accelerates and the breakthrough saturation maximizes, which highlights the concept of optimal salinity in wettability alteration. Also, we observed a shift of the contact angle distribution toward a more water-wet state. Given the nonmonotonic trend of the breakthrough saturation with brine salinity, as well as recovery time versus the ionic strength, we conclude that the induced surface roughness is not the primary drive behind the accelerated saturation change. Therefore, the recovery time difference can be primarily attributed to the local alterations of the wetting properties of the porous medium due to the change of the ionic strength.