Accurate Measurement of Gas Diffusivity in Oil and Brine Under Reservoir Conditions

摘要

This paper presents a physically and mathematically rigoroustransient-state non-equilibrium diffusion model for accuratedetermination of the gas diffusion coefficient from experimentalmeasurements of the gas pressure decline in a closed tankby dissolution of gas in a liquid (oil and brine). The short- andlong-time solutions of this model for equilibrium and nonequilibriumtransfer processes are derived analytically. These solutionsare reformulated to enable direct determination of thebest estimate of the diffusion coefficient by regression of theresultant analytical expressions to experimental data. Theseanalytic expressions are also facilitated to develop a set ofcharts, similar to the type-curves used in well testing for routineapplications in the laboratory. The experimental datagiven by previous researchers are analyzed by means of thepresent improved methods and the diffusion coefficients obtainedare compared with the previous studies. The accuracyof the present results are confirmed by comparing their valuesto those measured and reported at the gas saturation conditionsof the liquid phase, which are accurately known, because thesaturation is a well defined state, at which state the amount ofdissolved gas attains a maximum. This model can be used fordetermining gas diffusivity in oil, brine, as well as drillingmud and completion fluids.