Determination of Individual Diffusion Coefficients of Solvent/CO2 Mixture in Heavy Oil With Pressure-Decay Method

Abstract

Summary A novel methodology was developed to determine the molecular-diffusion coefficient for each component of the solvent/CO2 mixture in heavy oil under reservoir conditions on the basis of the pressure-decay theory. Experimentally, molecular-diffusion tests for the solvent/CO2/heavy-oil systems (i.e., pure-CO2/heavy-oil system, C3H8/CO2/heavy-oil system, and n-C4H10/CO2/heavy-oil system) are performed with a DBR pressure/volume/temperature system at constant temperature and decayed pressure. Theoretically, the Peng-Robinson equation of state combined with a 1D diffusion model is developed to describe the diffusion process of solvent/CO2 mixture in heavy oil. The composition analysis in the beginning and the end of pressure-decay experiments for the solvent/CO2/heavy-oil system indicate that the gas-phase solvent fraction decreases as diffusion proceeds, whereas the gas-phase CO2 fraction increases during the tests. One can determine the individual molecular-diffusion coefficient for each component in the mixture by minimizing the discrepancy between the measured composition change and the calculated composition change with the diffusion model. The newly developed methodology is successfully validated with the diffusion tests on the two solvent/CO2 mixtures: C3H8/CO2/heavy-oil system and n-C4H10/CO2/heavy-oil system. As for the solvent/CO2 mixtures tested, the molecular-diffusion coefficient of solvent in heavy oil is found to be significantly larger than that of CO2 in heavy oil. At similar test conditions, the C3H8/CO2/heavy-oil system ends up with a swelling factor of 1.058 after 168 hours of diffusion test, in comparison to 1.031 for the CO2/heavy-oil system.