Quantification of Mutual Mass Transfer of Gas-Light Oil Systems at High Pressures and Elevated Temperatures

Abstract

Numerous tight oil resources that are characterized by both low porosity and permeability have been found in North America during past decades. Due to the extremely low permeability, water injection has found its limitation with its relatively low injectivity. Alternatively, gas injection, such as CO2, N2, hydrocarbon gas, and flue gas, has been made physically possible for enhancing oil recovery under certain conditions, during which molecular diffusion is of great importance. Due to the affordability and sustainability of CO2, N2 and flue gas have been found to be costeffective for enhancing hydrocarbon recovery to a certain extent. Physically, there exists two-way mass transfer between the injected gas and light oil, though the light component extraction has been theoretically neglected. Therefore, it is essential to quantify the mutual mass transfer of gas-light oil systems under reservoir conditions. In this study, a novel and pragmatic technique has been developed to quantify mutual mass transfer between a gas and light oil by dynamic volume analysis. Experimentally, diffusion tests for a CO2-light oil system, a N2-light oil system, and two flue gas-light oil systems, have been conducted at a constant temperature and pressure with a pressure/volume/temperature (PVT) system, while the dynamic swelling factors of oil phase are measured and recorded continuously during the experiments. Gas samples have been collected at end of each diffusion experiment to measure gas compositions by performing gas chromatography (GC) analysis. Theoretically, by combining Fick’s second law and Peng-Robinson equation of state, the diffusion coefficients of both gas components and oil phase can be determined once the discrepancies between the measured and calculated dynamic swelling factors and gas compositions have been minimized simultaneously. At end of diffusion experiments, the swelling factor measured for the CO2-light oil system is 1.029, which is higher than that of N2-light oil system (i.e., 1.005). For the two flue gas-light oil systems, the enriched flue gas, which has a higher CO2 concentration, results in a higher swelling factor (i.e., 1.013) at end of diffusion experiment, comparing with that of flue gas-light oil system (i.e., 1.009). Besides, based on the GC analysis results, light components have been found in the gas phase, which proves that there exists two-way mass transfer between gas and oil phases. For the CO2-light oil system and N2-light oil system, at temperature of 336.15 K, the diffusion coefficients of CO2 and N2 are determined to be 12.87×10-9 m2/s at pressure of 2170 kPa and 1.35×10-9 m2/s at pressure of 5275 kPa, respectively. The diffusion coefficients of light oil in gas phase are determined to be 6.04×10-11 m2/s for the CO2- light oil system and 0.26×10-11 m2/s for the N2-light oil system under the corresponding conditions. Similarly, for the enriched flue gas-light oil system, the individual diffusion coefficients determined for CO2 and N2 are 8.35×10-9 m2/s and 1.52×10-9 m2/s at temperature of 336.15 K and pressure of 5275 kPa, respectively, while that of oil in gas phase is 0.07×10-11 m2/s. For the flue gas-light oil system, at the same condition, the individual diffusion coefficients calculated for CO2 and N2 are 6.42×10-9 m2/s and 2.19×10-9 m2/s, respectively, while that of oil in gas phase is 0.08×10-11 m2/s.