This paper presents a physically and mathematically rigorous transient-state non-equilibrium diffusion model for accurate determination of the gas diffusion coefficient from experimental measurements of the gas pressure decline in a closed tank by dissolution of gas in a liquid (oil and brine). The short- and long-time solutions of this model for equilibrium and nonequilibrium transfer processes are derived analytically. These solutions are reformulated to enable direct determination of the best estimate of the diffusion coefficient by regression of the resultant analytical expressions to experimental data. These analytic expressions are also facilitated to develop a set of charts, similar to the type-curves used in well testing for routine applications in the laboratory. The experimental data given by previous researchers are analyzed by means of the present improved methods and the diffusion coefficients obtained are compared with the previous studies. The accuracy of the present results are confirmed by comparing their values to those measured and reported at the gas saturation conditions of the liquid phase, which are accurately known, because the saturation is a well defined state, at which state the amount of dissolved gas attains a maximum. This model can be used for determining gas diffusivity in oil, brine, as well as drilling mud and completion fluids.

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