The research considers the dewetting dynamics of heavy crude oil on solid substrates in the presence of chemical additives and at elevated temperatures and pressures. Increasing the temperature from 40 to 80 °C was found to increase the initial receding rate of an oil droplet on substrate from 0.07 to 3.73 °/s, a consequence of the reduced oil viscosity. Higher temperatures also induced release of natural surfactants and thus decreased the oil-water interfacial tension (σ_OW), promoting increased oil droplet dewetting (equilibrium contact angle (θ) decreased from 63.7° to 51.3°), with the effect described by the Young’s equation. At high pressure (200 bar at 140 °C), asphaltenes partition less at the oil-water interface leading to slightly higher σ_OW (14.4 mN/m at 10 bar to 17.6 mN/m at 200 bar). Increase in σ_OW led to less oil dewetting as the θ increased from 17.9° to 23.1°. Adding a surfactant demonstrated the benefit of reducing σ_OW and increasing oil-substrate electrostatic repulsion. At high surfactant concentration the oil droplet attained low θ, and eventually pinched-off from the surface when the ultra-low oil-surface adhesion was exceeded by droplet buoyancy.
Oil droplet dewetting was studied in brine fluids at 60 °C where the addition of salt was shown to change σ_OW depending on the synergistic interfacial adsorption of salt ions and native surface-active species (i.e. naphthenic acids and asphaltenes). However, the oil droplet contact angles in brines were more influenced by the disjoining pressure and not σ_OW as described by the Young’s equation. Increased oil droplet dewetting (θ: 43.2° in water → 18.1°) was observed in low-salinity NaCl fluid (2,000 ppm), with hydration forces strongly influencing repulsion between the oil droplet and substrate. In contrast, attractive hydrophobic forces, as measured in CaCl2 brines, acting between the oil droplet and hydrophobised substrate (via divalent cation bridging), reduced the oil droplet dewetting rate and increased θ (≥ 27.2° at 60 °C). Initial droplet receding rates were increased by a strong oil-substrate repulsion and low steady-state θ, without the influence of changing σ_OW.
Surface-active nanoparticles, poly(N-isopropylacrylamide) (PNIPAM), were synthesised to study their effect on lowering σ_OW and enhancing the dewetting dynamics in the presence of surfactant. Blends of PNIPAM and SDS (1:1 mass ratio) were considered at different bulk concentrations. At low concentration (5 × 10-4 wt%), SDS interfacial adsorption was greater than PNIPAM with the dominance reversed at high concentration (5 × 10-3 wt%). The difference in interfacial activity was shown to influence the oil dewetting process, but is not fully described by the Young’s equation. Increased oil dewetting by nanoparticles was shown at low concentration, with the PNIPAM and SDS blend displacing the oil droplet at 5.66 °/s and attaining low θ (37.0°), while σ_OW remained relatively high (25.3 mN/m). This was due to PNIPAM particles remaining in the bulk fluid and self-assembling in the oil-water-substrate “wedge”, thus inducing a structural disjoining pressure, which promoted oil dewetting. In the presence of NaCl (2,000 ppm), contributions from PNIPAM induced further structural forces that led to a gradual liberation of oil from the substrate, even though the droplet buoyant force was less than the oil-substrate adhesion force.
