EXPERIMENTAL STUDIES ON EFFECTS OF RESERVOIR HETEROGENEITY ON THE VAPEX PROCESS

In thermal recovery processes, heat can penetrate by conduction throug h solids into low-permeability layers and shales. In the Vapex process , vapour is confined to the pore spaces and its ingress into fine pore s can be limited by capillarity. In steam processes, heat can penetrat e relatively rapidly beyond the interface and mobilize the oil at dept h; in Vapex, the diffusion of vapour is much slower and the thickness of the mobilized oil is much less. Therefore, reservoir heterogeneity, such as that created by low-permeability layers and shales, becomes m ore important in affecting the performance of the Vapex process than w ith SAGD because it is more difficult for the gas phase to penetrate t he low-permeability regions. In this study, experiments were carried o ut using a 2-D packed model to investigate the effects of low-permeabi lity layers and lenses on Vapex. The model was packed in horizontal la yers with two different sized sands. Crude from the Tangleflags North field, Lloydminster, was extracted with butane. The effects of both co ntinuous, low-permeability layers and discontinuous, low-permeability lenses were investigated using different well configurations. The resu lts using these models have been compared with those from uniformly pa cked models. All experiments involving low-permeability layers gave lo wer production rates than those involving uniformly high-permeability packing. When the horizontal low permeability layers were continuous, the production rate was even lower than that for a model uniformly pac ked with the same low-permeability sand. A higher oil production rate was found with discontinuous low-permeability lenses than with continu ous layers. This is because the vapour chamber could rise around these lenses to penetrate further into the high-permeability material above ; thus the oil could be drained concurrently from different heights. C apillarity, due to interfacial tension between the oil and solvent vap our phases, has a significant influence on the Vapex process. Its effe cts are both beneficial, because of the increased contact area between solvent vapour and oil, and detrimental, because of the resistance to the rising of vapour fingers, on a pore scale, from a coarse sand lay er into a fine sand layer. This latter effect changes the shape and gr owth rate of the solvent chamber. A much higher residual oil saturatio n was found in the fine sands than in the adjacent coarse sands.