Measurements of Molecular Diffusion Coefficients of Carbon Dioxide and Methane in Heavy Oil

摘要

Heavy oil and bitumen reserves in Western Canada are of great economic importance. An efficient heavy oil recovery from these reserves requires significant reduction in heavy oil viscosity. Recent studies focus on vapor extraction (VAPEX) and other solvent-based processes, which are considered as promising heavy oil recovery techniques. In order to design a solvent-based EOR operation, one needs to know the rate and extent of oil mobilization by solvent. This requires the knowledge of molecular diffusion coefficient of solvent gas in heavy oil. Among the existing experimental methods for measuring the diffusivity, in this paper, the so-called pressure decay method is employed to measure the molecular diffusivity of a gas solvent in heavy oil by monitoring the decaying pressure. In this non-intrusive method, physical contact of gas solvent and heavy oil is made and thereby the pressure vs. time data are measured until the heavy oil reaches its complete saturation. With the measured data and a mathematical model for diffusion process, the diffusion coefficient is determined. Specifically, the molecular diffusion coefficients of carbon dioxide-heavy oil and methane-heavy oil systems are measured. The experiment is conducted in a closed high-pressure cell at constant reservoir temperature. Furthermore, the mathematical modeling is undertaken to obtain the analytical solution to the bulk diffusion equation subjected to appropriate boundary conditions at the solvent-heavy oil interface for each solvent. In terms of conservation law of mass and the equation of state for a real gas, the pressure in the gas phase is predicted from the analytical solution. Thus, solvent diffusivity is determined by finding the best match of the numerically predicted pressures with the experimentally measured pressures. The simultaneous search method is adopted in the numerical scheme. Finally, such obtained diffusion coefficients are compared with the limited data available in the literature.