Mass Transfer Coefficients in Vapour Extraction (VAPEX)

Abstract The in-situ extraction of heavy oil and bitumen requires enhanced oil production techniques to sufficiently reduce the viscosity of the oil to produce it at economical rates. Vapour extraction (VAPEX) and warm VAPEX (where the solvent condenses in-situ) are two possibilities for producing highly viscous in-situ oil from Canada's vast reserves. It has been recognized, by now, that the mass transfer involved in the vapour extraction of heavy oil includes both diffusion and convection mechanisms. However, to date, researchers have not been able yet to quantify the role of diffusion and convection in the VAPEX process. Before accurate field scale predictions can be made, the mass transfer and fluid mechanics aspects need to be understood. Laboratory scale experiments were conducted using systems of different length and permeability as well as different solvents and solvent flow rates to examine the effect on sweep efficiency and production. Using a mass transfer coefficient approach, results from the experiments are presented and the mass transfer coefficients compared for different systems. Introduction Vapour extraction (VAPEX) is an enhanced oil recovery technique for recovering in-situ heavy oil and bitumen from Canada's vast reserves. The governing mechanisms are mass transfer and gravity drainage. Solvent that is injected into a horizontal injection well where it diffuses and mixes into the heavy oil/bitumen phase and exponentially reduces the oil's viscosity. The live oil (reduced viscosity) oil can then drain via gravity to a production well. VAPEX laboratory results look promising. But, oil companies have yet to embrace VAPEX; there is no public field trial data, the mass transfer mechanisms are not quantified and there is no reliable model to predict field scale production rates from laboratory data. The goal of this work is to experimentally investigate the mass transfer involved in the VAPEX process using a mass transfer coefficient approach. Previous VAPEX experiments performed in glass etched micromodels showed bitumen dissolving into the liquid butane phase. This was observed under conditions of warm VAPEX, where the butane condensed inside the glass micromodel. In comparison, bitumen dissolution was not observed (due to the opacity of the live oil) in the normal mode of VAPEX operation when solvent vapour diffuses directly into the bitumen (James, L.A. and Chatzis, I., 2004) and (Chatzis, I., 2002). The dissolution of bitumen into draining live oil would still be valid and thus, the concept of using mass transfer coefficients was formulated. Mass transfer coefficients have been widely used in chemical processes such as absorption towers, strippers, packed columns, etc. More recently, mass transfer coefficients of non-aqueous phase liquid (NAPL) dissolution in groundwater were found and used in contaminant transport models (Kim, T-J et. al., 1999). Mass transfer coefficients are a measure of the amount of mass transferred from one phase to another through an effective area based on the concentration gradient driving force. Equation (1) shows the mass transfer coefficient (k) as a function of mass flux (NA) and species A mass fraction. In terms of finding the mass transfer coefficient of bitumen (kB) into solvent, the relationship is given by the rate of mass transferred (mB, g/s).

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