High-Pressure Rheology of Waxy Oils: Pressure and Gas Dissolution Effects

Abstract Wax precipitation and gelation in high-pressure oil and gas flowlines pose major flow-assurance challenges, but the effects of pressure and gas dissolution on the thermodynamics and rheology of waxy oils remain largely unexplored. In this study, high-pressure calorimetry and rheometry were used to investigate a model waxy oil under two contrasting gas environments: inert helium to isolate the intrinsic effect of pressurization, and oil-soluble methane to capture the combined effects of pressurization and gas dissolution. Under helium pressurization, wax appearance temperature (WAT) and the wax precipitation curve shifted to higher temperatures, indicating enhanced wax crystallization. This, together with pressure-induced liquid-phase compression, increased oil viscosity and gel yield stress. Under methane pressurization, dissolved methane shifted the WAT and wax precipitation curve to lower temperatures, while also weakening liquid-phase intermolecular interactions, thereby reducing oil viscosity and markedly decreasing gel yield stress. Stepwise shear-rate tests revealed time-dependent structural breakdown and pronounced hysteresis under all conditions, with pressure and dissolved methane altering the magnitude of shear stress but not the fundamental breakdown mechanism. Herschel–Bulkley (HB) modeling showed that helium pressurization modestly increased HB-extrapolated dynamic yield stress and consistency index, whereas methane primarily reduced the consistency index with only minor changes in HB-extrapolated dynamic yield stress and shear-thinning behavior. These findings demonstrate that dissolved gas, not merely pressurization, governs live waxy oil rheology and must be explicitly considered in flow assurance design.