Dynamic Dewetting and Detachment of Crude Oil Droplet by Surface-Active Nanoparticles in Low-Salinity Brines: An Influence of Disjoining Pressure on Wettability Alteration

Nanoparticles and low-salinity brines are of interest in oil recovery applications, although the main interfacial mechanisms governing their behavior remain controversial. While lowering the oil–water interfacial tension is desirable for EOR, many studies report the onset of a disjoining pressure between the oil–water and water–solid interfaces to significantly contribute to oil recovery via changing the apparent contact angle. In this study, wettability alteration was quantitatively examined by measuring the dynamic contact angle of a heavy crude oil droplet that was initially spread thinly on a glass substrate. The oil film was flooded with different displacing fluids: (i) synthesized surface-active nanoparticles (N-isopropylacrylamide, pNIPAM, 5 × 10–4 wt%); (ii) low-salinity brines (2000 ppm monovalent NaCl and divalent CaCl2); and (iii) blends of the nanoparticles and brines. For wettability alteration, the brines led to greater oil dewetting than nanoparticles, allowing lower steady-state contact angles (NaCl = 18.1°; CaCl2 = 27.2°) than the nanofluid (37.0°). These results from the hydration force generated by the hydrated salt ions which enhanced the oil displacement more than the nanoparticles structural force. Used alone, the polymeric nanoparticles bridged the oil droplet interface and substrate resulting in a dewetting hindrance. In the blend system, divalent salt ions were able to bind with the crude oil acidic species and solid substrate leading to reduced oil droplet dewetting (pNIPAM in CaCl2: 58.7°), regardless of the low oil–water interfacial tension (11.2 mN/m). On the contrary, for the monovalent salt (pNIPAM in NaCl), the combination of particle structural forces and brine hydration forces successfully displaced the oil droplet with fast receding rates which led to the gradual detachment (liberation) of the oil droplet from the substrate. These findings emphasized the minor role of the oil–water interfacial tension in both systems. However, they enhanced the fact that the disjoining pressure through both hydration and structural forces is a significant mechanism for an apparent wettability alteration.