Experimental Investigation of Water-in-Oil Emulsions on Viscosity and Gas Exsolution Behavior of Heavy Oil Under CCEC Conditions

Water-in-oil (W/O) emulsions are commonly encountered during heavy oil production, yet their effects on fluid rheology and gas exsolution behavior remain poorly characterized. This study systematically investigates how the water content of W/O emulsions (10%, 20%, and 30 vol.%) influences the viscosity, interfacial tension (IFT), and pressure–volume behavior of heavy oils under conditions relevant to cyclic solvent injection. Viscosity measurements show that water content is the dominant rheological factor, with the 30% emulsion exhibiting approximately three times the viscosity of the 10% emulsion at 25 °C. In contrast, IFT between emulsified oils and methane is insensitive to water content, indicating that rheological differences across emulsions are structurally rather than thermodynamically driven. Constant-composition expansion and compression (CCEC) experiments reveal that pseudo-bubble point pressures decrease with increasing water content during fast depressurization, consistent with enhanced foamy-oil stabilization by the more viscous emulsion network. A hysteresis inversion phenomenon is identified, in which the compression path falls below the expansion path at higher water contents. Quantitative analysis using an integrated hysteresis index confirms a monotonic transition from positive to negative hysteresis with increasing water content at 15 °C, which is suppressed at elevated temperature and during slow depressurization. These findings demonstrate that conventional PVT data obtained with clean oil will systematically mischaracterize foamy-oil behavior in emulsified systems.