In this paper, CO2 diffusion
coefficients in a carbonate
water–oil system are determined by measuring the pressure buildup
in the closed water–oil system experimentally and modeling
the pressure change mathematically. The mathematical method of investigating
one-dimensional, time-dependent heat conduction in a composite medium
is adopted to solve the mass transfer problem between two liquid phases.
The model is combined with well-designed trial-and-error method to
determine diffusion coefficients of CO2 in both water and
oil phases at the same time. The model considers a moving interface
between carbonated water and oil as well as variations of interface
concentrations of CO2 in these two phases, which more effectively
conforms to reality. Results show that the pressure buildup during
the diffusion process resulted from the increased density and swelling
of the oil phase. The diffusion coefficient of CO2 in the
water phase plays a major role in the interphase mass transfer process.
Diffusion of carbon dioxide (CO2) in a carbonated water–oil
system is of great importance for proper design of CO2-based
enhanced oil recovery (EOR) processes. We study the effects of operational
parameters such as saturation pressure, temperature, and phase volumes
on diffusion coefficients of CO2 in a carbonated water–oil
system. Results show that diffusion coefficients of CO2 in both phases are susceptible to saturation pressure. The greater
the saturation pressure, the larger the diffusion coefficients. At
a given saturation pressure, diffusion coefficients of CO2 in two phases increase by increasing temperature. Values of the
coefficients determined at 40 °C are about twice those determined
at 20 °C. The equilibration of the system was found to be much
faster at the higher temperature. The results indicate that the predicted
diffusion coefficients are insensitive to phase volumes, indicating
applicability of the determined diffusion coefficients to simulate
the mass transfer in large-scale reservoirs.
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