Determination of effective diffusivity of each component of a binary gas-mixture in porous media saturated with heavy oil

Abstract

In this work, the conventional pressure-decay technique has been improved and extended to estimate the effective diffusivity of each component in a binary gas mixture in an unconsolidated porous medium saturated with heavy oil. Experimentally, a workflow has been well devised to prepare a heavy oil-saturated sandpack, making it possible for evaluating the preferential contribution of each gas in a mixture to dissolving into heavy oil. Firstly, temperature is increased to reduce viscosity of a heavy oil sample contained in a diffusion cell to construct the heavy oil-saturated silica sand column above which a void space was created. Then, such a well-prepared diffusion cell is integrated with the pressure-decay system to continuously monitor and record the pressure history throughout the diffusion test. At the beginning and end of the test for a binary gas mixture diffusing into heavy oil, the gas-phase samples were collected to have their compositions analysed so that the amount of each gas component dissolved in heavy oil can be determined. Theoretically, mass transfer models are integrated with the volume-translated Peng-Robinson equation of state (PR EOS) to determine the effective gas diffusivity once the discrepancy between the measured and computed composition of each gas component is minimized as well as the deviation between the experimentally measured and computed pressure-decay profiles. Under 299.65 K, the effective diffusivity of CO2 is estimated to be 2.93×10–10 m2/s at 855–817 kPa for pure CO2–heavy oil systems, and those of CO2 and C3H8 are calculated to be 2.78×10–10 m2/s and 11.41×10–10 m2/s, respectively, at 854–783 kPa for the binary gas mixture-heavy oil systems. Furthermore, the reproduced pressure profiles are found to be generally in decent agreements with the experimentally measured ones. Due to the tortuous paths in porous media, the effective diffusivity of each component in the binary gas mixture is found to be smaller than its corresponding molecular diffusivity in the bulk phase, while the cross-term contribution on the mass transfer can be neglected under certain conditions.