Quantification of preferential and mutual mass transfer of gases-light oil systems at high pressures and elevated temperatures by dynamic volume analysis

Abstract

• Dynamic liquid volume due to gas dissolution and light-component extraction is measured and recorded continuously. • Gas fractions at the beginning and end of each test is respectively measured with GC. • Light-component extraction from oil phase to gas phase results in a shrinkage of the oil phase with pressure. • Both individual diffusion coefficients for each gas component of a gas mixture in the oil phase and oil components extracted from oil phase to gas phase are increased with pressure. • By isolating natural convection from the molecular diffusion, larger diffusivities are determined. In this study, a robust and pragmatic technique has been developed for the first time to quantify preferential and mutual mass transfer of gases-light oil systems at high pressures and elevated temperatures by performing dynamic volume analysis. Experimentally, diffusion tests have been conducted for gases-light oil systems at high pressures and a constant reservoir temperature with a PVT setup. During experiments, the dynamic liquid volume due to gas dissolution and light-component extraction has been measured and recorded continuously, while gas fractions at the beginning and end of each test have been measured with gas chromatography (GC) analysis, respectively. Theoretically, the preferential and mutual diffusion between gas and oil phases can be quantified by minimizing the discrepancies between the measured and calculated swelling factor and gas composition. According to the measured swelling factors, oil swelling effect slows down during the late stage of a diffusion experiment, indicating that CO 2 dissolution is transited from the natural convection-dominated period to the diffusion-dominated period. Compared to the swelling effect at a higher pressure, the effect of light-component extraction on the determined diffusion coefficients of gas components becomes weaker. In addition, both individual diffusion coefficients for each gas component of a gas mixture in an oil phase and oil component extracted from the oil phase to the gas phase are increased with pressure. The diffusion-convection model allows us to isolate the natural convection from the molecular diffusion, leading to larger diffusivities compared with the ones determined from the conventional method.