Abstract

The injection of CO2 underground has been investigated by many scientists, be it for sequestration or enhanced oil recovery. When CO2 enters the reservoir, the main questions, into which phase the CO2 concentrates first (oil or water) and in which amounts it is going into solution, will be answered. Both, numerical and experimental studies in the literature report two different diffusion parameters, differing by more than one order of magnitude. These findings are confirmed in our experiments. In the literature, rapid dissolution is attributed to the appearance of natural convection due to density differences of the CO2-enriched liquid, entrailing a faster mixing, sometimes being called super-diffusion. This paper reviews the physical effects appearing during CO2 injection and investigates the mathematical basis for the determination of solubility and diffusivity together with the assumptions made in this case. We come to the conclusion that a parabolic diffusion law cannot explain the two different diffusion constants, but a hyperbolic law can. In a detailed dimensional analysis of the Navier-Stokes equation dimensionless groups are formed, the magnitude of which determines the importance of the different terms, in our case the importance of convection. Using the parameters of our physical setup we find that the convection is only prevalent in a thin layer close to the interface of gas and liquid. This is in contrast to present belief. Stability considerations for the onset of convection complete the scope of this work.

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