A new method for experimental measurement of diffusion coefficients in reservoir fluids

Abstract

A simple method is presented for determining diffusion coefficients of dense gases in liquids using a PVT cell. When a non-equilibrium gas is brought into contact with a liquid in a sealed container at a constant temperature, the final state is determined by thermodynamic equilibrium. However, the time which is required to reach the final state is determined from the diffusion process in each phase. At the gas-liquid interface, thermodynamic equilibrium exists between the two phases at all times, but the position of the interface as well as the pressure may change with time. The rate of change of pressure and the interface position as a function of time depends on the rate of diffusion in each phase and therefore on the diffusion coefficients. No compositional measurements are necessary for this method of measuring diffusion coefficients; hence, it is less expensive than conventional methods. Results obtained by this method for the binary system of methane and n-pentane at 311 K and 70 bar yielded diffusion coefficients within ± 5% of literature values. The technique can be easily applied to multicomponent systems for measurement of effective diffusion coefficients in reservoir fluids.