Experimental Investigations into the Effect of Heterogeneities on the Recovery of Heavy Oil by VAPEX

Vapour extraction (VAPEX) of heavy oil is a low energy alternative to Steam Assisted Gravity Drainage (SAGD). Nearly miscible vapour is injected into the formation instead of steam. It dissolves in the heavy oil, reducing its viscosity. The diluted oil is then produced via gravity drainage (in the same way that heated oil drains during SAGD). Although there are many reported numerical and experimental studies into VAPEX, very few have investigated the impact of heterogeneities on this process. None have compared the predictions of simulation with experiment. We report a series of laboratory studies into the effect of layering and discontinuous shales on oil rate and recovery from VAPEX. These were performed in well characterized glass bead packs using glycerol and ethanol as analogues of oil and solvent respectively. The permeability, porosity and fluid properties (including diffusion and dispersion) were measured independently. Numerical predictions of the experiments were performed, using these data as input, to determine whether conventional reservoir simulation can capture the flow processes observed in the experiments. We found that the observed oil drainage rates from the layered packs were between the rates observed in homogeneous packs formed of the lowest permeability beads (lower bound) and the highest permeability beads (upper bound). The numerical simulations tended to underpredict oil drainage rate although they correctly predicted the pattern of solvent and oil distribution seen in the experiments. The performance of the layered system could be predicted by a homogeneous system using the arithmetic average of the layer permeabilities. A single discontinuous shale had little impact on recovery unless it was directly between the injection and production wells or its length approached that of the reservoir. These results suggest that these types of heterogeneity have little impact on VAPEX performance except through their influence on effective permeability of the reservoir.