An experimental investigation into the influence of diffusion and dispersion on heavy oil recovery by vapex

This paper investigates the role of convective dispersion on oil recovery by VAPEX using an analogue fluid system of ethanol and glycerol in well-characterized glass bead packs. Laboratory studies of VAPEX in porous media result in significantly high production rates than predicted either by analytic models derived from Hele-Shaw experiments or numerical simulations. Previous workers have obtained an improved match between experiment and simulation by artificially increasing the diffusion coefficient of the injected vapour into the oil. Justifications for this increase include convective dispersion, an increased surface area due to the formation of oil films on sand grains, imbibition of oil into those films and a greater dependence on drainage height. Convective dispersion seems to be the most plausible mechanism. A first contact miscible liquid-liquid system was used in these experiments so that all mechanisms contributing to increased-mixing apart from convective dispersion were eliminated. Improved onfidence and prediction of VAPEX oil drainage rates will increase the likelihood of field scale application of VAPEX. This has been limited, due to difficulties in predicting the outcome on the laboratory scale and upscaling the results. Longitudinal and transverse dispersion coefficients were measured experimentally as a function of flow-rate and viscosity ratio, with and without gravity. Vapex drainage experiments were performed over a range of injection rates. More than 80% of oil in place recovered after one pore volume of solvent injection. The oil drainage rates were compared with those predicted by the Butler-Mokrys analytical model using either molecular diffusion or convective dispersion. Using measured convective dispersion improved prediction of oil drainage rate by over 50%. Nonetheless experimental oil dranage rates were still slightly higher than predicted. These results indicate that convective dispersion needs to be included in mixing calculations in order to better predict oil drainage rates during VAPEX.