The Vapour Extraction Process(VAPEX)for heavy oil recovery is targeted for reservoirs having oil viscosity greater than 10 Pa.s.The pore scale events of heavy oil recovery using the VAPEX process are not yet well developed to the extent of incorporating pore level physics into mathematical models to the point that the observed phenomena can be described mathematically.The primary objective of this paper is to elucidate the pore scale events of gas absorption type of mass transfer during the gravity drainage of live oil in heavy oil reservoirs when employing the VAPEX process by way of documenting pore scale events seen in micromodels of porous media.VAPEX experiments were carried out in micromodels of capillary networks etched on glass saturated with heavy oil,as well as in thin slabs of porous media consisting of glass beads sintered between glass plates and were vapour extracted using butane.The observed phenomena of vapour extraction at the pore scale demonstrate that gravity drainage of heavy oil from a vertically standing heavy oil/VAPEX interface occur in a very thin layer comprised of a monolayer of pores in the direction perpendicular to the interface.This leads to a continuously draining and renewed surface where gas absorption takes place at capillary interfaces in pore bodies and pore throats.The diluted bitumen drains from pores in a hierarchical manner dictated by drainage displacement mechanisms and the interplay of gravity and capillary forces.As oil filled pores with vapour-diluted bitumen drain at the interface,the corresponding volume of oil expelled flows downwards in a film flow regime along the first row of pores adjacent to the VAPEX interface.The periodic nature of draining pores with diluted bitumen and diffusion limiting the gravity drainage,result in a continuously evolving VAPEX interface having a constant shape.As a result,oil production takes place at constant rate that depends on the permeability and height of the porous medium.Observed phenomena in VAPEX demonstrate that oil film flow,snap-off of the liquid films and localized trapping of vapour enhance the mass transfer rate.
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