Effective diffusion coefficients of gas mixture in heavy oil under constant-pressure conditions

摘要

We develop a method to determine the effective diffusion coefficient for each individual component of a gas mixture in a non-volatile liquid (e.g., heavy oil) at high pressures with compositional analysis. Theoretically, a multi-component one-way diffusion model is coupled with the volume-translated Peng-Robinson equation of state to quantify the mass transfer between gas and liquid (e.g., heavy oil). Experimentally, the diffusion tests have been conducted with a PVT setup for one pure CO_2-heavy oil system and one C_3H_8-CO_2-heavy oil system under constant temperature and pressure, respectively. Both the gas-phase volume and liquid-phase swelling effect are simultaneously recorded during the measurement. As for the C_3H_8-CO_2-heavy oil system, the gas chromatography method is employed to measure compositions of the gas phase at the beginning and end of the diffusion measurement, respectively. The effective diffusion coefficients are then determined by minimizing the discrepancy between the measured and calculated gas-phase composition at the end of diffusion measurement. The newly developed technique can quantify the contributions of each component of mixture to the bulk mass transfer from gas into liquid. The effective diffusion coefficient of C_3H_8 in the C_3H_8-CO_2 mixture at 3945 ± 20 kPa and 293.85 K, i.e., 18.19 x 10~(-10)m~2/s, is found to be much higher than CO_2 at 3950 ± 18 kPa and 293.85 K, i.e., 8.68 x 10~(-10)m~2/s. In comparison with pure CO_2, the presence of C_3H_8 in the C_3H_8-CO_2 mixture contributes to a faster diffusion of CO_2 from the gas phase into heavy oil and consequently a larger swelling factor of heavy oil.