Determination of the Diffusion Coefficient of Supercritical CO₂ in Low-Permeability Formation Cores

Abstract

CO2 diffusion in the oil phase in the presence of porous media is of great importance to forecast the performance of enhanced oil recovery and carbon dioxide capture and storage. When CO2 is brought into contact with an oil phase, diffusion occurs. The indirect pressure-decay methodology has been applied successfully to determine the mutual diffusion coefficients of supercritical CO2 and crude oil at temperature of 65 °C. Theoretically, a one-dimensional double-way mass transfer model incorporating the capillary and adsorption effects of porous media has been constructed to quantify the mutual dynamic diffusion coefficients. The crude oil has been characterized into five pseudo-components for the accurate description of the phase behavior of CO2 gas and oil phases. An optimization method along with the PR equation of state is employed to determine each component’s diffusion coefficients in the gas and oil phases for the purpose of minimization of the difference between the experimental data and calculated results. Satisfactory agreements are obtained with a merely 0.97% pressure difference when a variable diffusion coefficient is implemented. The short-time experimental test and a long-time-duration calculation are recommended for further diffusion investigation. The capillary analysis gave a regressed fractal dimension of 2.7049 with the average absolute deviation of 0.602%. The adsorption phenomenon can be greatly influenced by the existence of heavy components. The heavier the components, the greater their impact on adsorption. Furthermore, a faster and enhanced diffusion process is observed at high pressure and temperature. The combination of several complex mechanisms leads to the initial gradual increase of the diffusion coefficient with time, and then, it arrives at a plateau, which is slightly different from the previous diffusion coefficient changing tendency mentioned in the literature. In the presence of a low-permeability formation core, the determined diffusion coefficients of CO2 in the gas and liquid phases are increased from 4.38 × 10–9 to 6.21 × 10–9 and 6.16 × 10–10 to 9.21 × 10–10 m2/s when the system pressure is decreased from 18.2 to 12.11 MPa, respectively. In addition, the capillary effect can lead to a reduction of the diffusion coefficient in the oil phase but has little effect on the diffusion coefficient in the gas phase.

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