Liquid to Liquid-Liquid Transitions: Impact on the Apparent Viscosity of Model Hydrocarbon + "Solvent" Mixtures

Abstract

Injection of carbon dioxide or low molar mass hydrocarbons to improve the productivity of oil wells has become a common practice. Such injectants are not fully miscible in reservoir fluids and oils leading to liquid-liquid phase behaviour in reservoirs, in production systems and surface facilities at high pressures. Well-known and relevant binary mixture examples include: CO2 + n-tridecane, propane + phenanthrene, methane + hexane. In this work, a setup was put together and validated to study such mixtures. Single-phase liquid mixtures mimicking this behaviour were fed to a rheometer. The temperature was then lowered (2.5°C/minute) to a final temperature at fixed pressure. The mixtures transitioned to liquid-liquid phase behaviour in the rheometer. Rheological responses were measured at constant shear rate. For these proof of concept measurements, the rheological responses of glycerol + 1-pentanol binary mixtures, which exhibit liquid-liquid phase behaviour below 63.6°C, was evaluated at atmospheric pressure. The mixtures were fed from a heated reservoir at 75°C to a rheometer (Anton-Paar 301). The matrix of experiments included shear rate, mixture composition, and final temperature realized in separate experiments (20°C, 40°C and 60°C). The miscible glycerol + water binary mixture provided a negative (single phase) control. Supporting data including phase densities and volumes of mixing are reported. Measurement reproducibility, the impacts of the proximity of the liquid-liquid critical point, and the relative volumes of the glycerol-rich and pentanol-rich phases on rheological responses are discussed. The key findings include the uncertainty in apparent viscosity at 0.5 phase volume fraction and saturated phase viscosities acting as limits for apparent viscosities. The implication of these findings include applicability of present equations used in industry to calculate blend viscosity of oil + low molar mass hydrocarbon/non-hydrocarbon mixtures. More detailed study of the near critical region for two-phase mixtures is warranted.