Abstract

The solvent based recovery process "Vapex" has a great potential for therecovery of heavy oil and bitumen resources, due to the low energy intensityand reduced GHG emission associated with the process. The process has beenextensively investigated in the laboratory models and through numericalsimulation studies. This has indicated the technical viability of the processas an alternative to thermal recovery processes, viz. SAGD. In the last fewyears several proprietary pilot has been developed to establish the commercialviability of this concept.

The most important uncertainty about this process is the rate of mixing ofsolvent molecules with high viscosity hydrocarbons. Due to low moleculardiffusivity the theoretical predictions of extraction rates are significantlylower than the SAGD process. However, the results of physical modelexperimental carried out by this author showed a considerably higher masstransfer rate at the solvent bitumen interface. The process, whether in thelaboratory model or in the reservoir porous media, takes place in a microscopiclevel. An experimental evidence of this phenomenon will be presented in thepaper.

Transferring this microscopic phenomenon into a macroscopic simulation modelpresents a serious challenge. Artificially higher diffusion or dispersioncoefficients are used to match the experimental data. Even with that both theproduction rate and the solvent saturation profile can not be matchedsimultaneously. For example the higher dispersion coefficient results in deeppenetration of the solvent, resulting in a diffusion zone, thicker than theexperimentally determined value. Lower dispersion coefficient results in alower production rate. Some of these simulation results are presented in thispaper. A recent development in the simulation model, Dynamic Grid Refinement, improves the simulation match by allowing the use of smaller grid blocks at thediffusion boundary layer.

Introduction

The Vapex technology has been started from the laboratory of late professorRoger Butler[1] at the University of Calgary in late eighties. Over the pastfifteen years the process has moved from the concept to field scale pilots. Ifsuccessful, this energy efficient, environmentally friendly recovery processwill revolutionize the recovery of viscous oil. So far laboratory investigationhas shown great promise for the process. Results of the field pilots areproprietary. However, the progress seems to be slow.

The initial study focused on the improvement in mass transfer mechanism suchthat the recovery rates become comparable or within order of magnitude of thethermal recovery processes. This author's research[2] indicated that in theporous media the mass transfer rate is enhanced by orders of magnitude due tothe combination of several mechanisms inside the sand matrix. At the bitumensolvent vapor interface the solvent dissolves in the bitumen and the solventconcentration at that point would be the solubility of the solvent under thecorresponding pressure and temperature. This creates a layer of mobile dilutedbitumen. Due to the thermodynamics of the interface, this diluted oil mayspread on the thin film of the connate water inside the extracted sand matrixand provide increased area for mass transfer. Once this diluted oil drains outfrom the area, fresh bitumen is exposed to the high solvent concentration. Repetition of this cycle creates a pseudo steady solvent flux, much higher thanthat estimated through Fician diffusion mechanism[3]. Therefore, the simulationmodels that use diffusion and dispersion may not be able to capture thismechanism of mass transfer unless very fine grids are used to model theinterface where most of these microscopic processes take place.

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