Abstract Physically and mathematically rigorous improved models, considering equilibrium and nonequilibrium diffusion gas transport in the liquid phase and resistance of the gas-liquid interface to gas dissolution in liquids, are developed. This allows the accurate determination of the gas diffusion coefficient from experimental measurements of the gas pressure decline in a closed tank by dissolution of gas in the liquid. The short- and long-time solutions of these models are derived analytically under various conditions. These solutions are reformulated for direct determination of the best estimate of the diffusion coefficient by regression of the resultant analytical expressions to experimental data. Procedures are presented and demonstrated for accurate estimation of the gas diffusivity coefficient by conforming the present models to experimental data.
The results developed in this paper can be utilized for determination of the gas diffusivity and the rate of dissolution of the injection gases used for secondary recovery and the rate of separation of light gases from reservoir oil and brine. The present analytic expressions can be facilitated to establish the significance of the equilibrium vs. nonequilibrium conditions under in situ conditions for gas injection, including carbondioxide, nitrogen, and methane. The gas diffusivity in drilling muds and completion fluids can also be determined using the present analytical interpretation methods.
Introduction Gas diffusivity is an important parameter determining the rate of dissolution of the injection gases used for secondary recovery and the rate of separation of light gases from the reservoir oil and brine. The frequently used equilibrium assumption in reservoir simulation may lead to significant errors in the prediction of oil recovery by miscible flooding and the miscibility can be optimized for best recovery by developing proper gas injection strategies.
Laboratory measurement of gas diffusivity in liquids is usually accomplished via the measurement of the pressure of gas in contact with certain liquids, such as oil, brine, drilling muds, and completion fluids, in a closed PVT-cell (see Fig. 1) during gas dissolution in the liquid phase. The accuracy of the available models, including by Riazi, Sachs, Zhang et al. , are limited by their inherent simplifying assumptions involved in their analytic treatise used for interpretation of the experimental data. Zhang et al. have shown that there is no consensus amongst the available simplified models used for diffusivity measurement.