Mass transport of solvent into heavy oil during Vapex was studied using techniques of Magnetic Resonance Imaging (MRI) and visual glass micro-models aided by advanced image analyses. Whereas experiments involving MRI images of sand-pack provided detailed description of vapour chamber propagation and phenomena such as asphaltene deposition, diffusion and dilution of heavy oil occurring in the transition zone (on scale of several centimetres), micro-models revealed phenomena at pore level. These included episodic nature of de-saturation within transition region leading to mobilization and entrapment of heavy oil in the vapour chamber.
It was seen that bulk of diluted/de-asphalted oil drains via sides of the vapour chamber. Episodes of capillary desaturation within the transition zone cause not only a relatively long residence time of solvent into diluted oil within the reservoir but also provide large surface area for expediting such transport. Consequently, solvent content in the diluted oil within the transition zone could exceed the amount needed for onset of de-asphalting. De-asphalting also seems to be promoted by the presence of connate water. The diluted and de-asphalted oil, in turn, drains near the edge of the vapour chamber as continuous oil column or, within the transition region as discontinuous oil ganglia. With continued solvent transport, oil viscosity as well as, interfacial tension between diluted oil and vapour decrease. As a result, occasionally, interfacial forces holding up diluted oil ganglia are reduced below the prevailing hydrostatic head and episodes of desaturation are initiated. These episodes may culminate in mobilized oil ganglia coalescing with other ganglia, or with continuous oil column to the production well. Alternately, upon encountering finer sized pores, they could break-up and some oil may again get held up.
Propagation of vapour chamber and oil rates during Vapex, therefore, depends upon properties of diluted and de-asphalted oil within the transition region (between the vapour chamber and oil region unaffected by solvent transport). This paper presents evidence for these mechanistic insights and discusses practical implications to operation and design of Vapex projects.