Investigation of Interplay of Capillarity, Drainage Height, and Aqueous Phase Saturation on Mass Transfer Phenomena in Heavy Oil Recovery by Vapex Process

Abstract

The vapor extraction (Vapex) process has emerged as a promising recovery technique in low-pressure and shallow heavy oil and bitumen reservoirs where conventional recovery processes are not technically, environmentally, or economically feasible. The Vapex process utilizes horizontal wells for injection of a vaporized hydrocarbon solvent (e.g., propane and butane) at pressures close to their dew point pressures into the reservoir, dissolution of solvent in the oil, and production of in-situ upgraded oil. The realistic approximation of diffusion and convective dispersion occurring on the edge of the vapor chamber is required for reliable prediction of production rates in this process. This research was aimed at investigating the interplay of capillarity, drainage height, and aqueous phase saturation on efficiency of mass transfer rate in permeability range similar to those found in western Canadian heavy oil and bitumen reservoirs. For this purpose, a combination of experimental and numerical studies was conducted in this research. Experimental study of this research included the design and manufacturing of a new experimental set up and approach for investigating the mass transfer phenomenon at the edge of vapor chamber realistically. The rock and fluid chracteristics were invetisgated by measuring the capillary pressure and pore size distribution of the sand packs, as well as phase behavior measurements to collect adequate PVT data for anlytical and numerical modeling of conducted experiments. Numerical simulation of the laboratory experiments were carried out using a commercial simulator (i.e. CMG-STARSTM) in order to history match the experimental results and determine the effect of studied parameters on mass transfer rate. Conducted analytical and numerical modelings showed that the effective diffusion coefficient was in the range of 4.91×10-8-1.10×10-5 cm2/s. The effective diffusion of the butane vapor into the heavy oil saturated sand pack in absence of immobile water saturation was in the range of 4.91×10-8-7.89×10-6 cm2/s. In presence of immobile of aqueous phase saturation, the effective diffusion coeffiecient varied between 4.91×10-8 and 7.89×10-6 cm2/s. The comparison between the calculated effective diffusion coefficients and reported molecular diffusion in literature by different researchers confirms that the velocity-dependent term in convective dispersion plays only a minor role at higher capillarities, i.e. lower permeability media. This means molecular diffusion becomes the major driver for mass transfer phenomena under these conditions. This study showed that the selection of a realistic mass transfer coefficient is required for the simulation of the performance of the Vapex process. On this basis, the proposed effective diffusion/dispersion coefficients up to 4 orders of magnitude higher that the molecular diffusion coefficients in the literature could not be used for simulation of the Vapex process at reservoir condition similar to western Canadian heavy oil reservoirs.