A study of pressure decay in a closed CO2-water system

Master's thesis in Petroleum engineeringAnalytical and semi-analytical solutions to the diffusion equation have been obtained in an effort to model pressure decay in a closed CO2-water system. Various boundary conditions that include different physical effects and simplifications have been investigated. Experimental data have been interpreted qualitatively and quantitatively by making use of the analytical solutions. Numerical modelling of the system in question has also been explored. It has been concluded that the pressure decay can not exclusively be described by a diffusion process, and that advection currents increase the rate of mass transfer between the gas and liquid phase. It has been found that advection becomes less dominant compared to diffusion in late times of the pressure decay experiment. The experimental data has been interpreted in terms of a time-dependent effective diffusion coefficient that initially is two orders of magnitude greater than the diffusivity of CO2 in water, and that gradually decreases towards the literature value. The diffusion-only model that has been put forward is found to accurately predict pressure decay in CO2 -bitumen, and methane-pentane systems