Evaluation of Analytical Methods for Measuring the Molecular Diffusion Coefficient in Gas-Oil Systems by the Pressure Decay Method

Gas is injected into reservoirs for pressure maintenance, enhanced oil recovery and greenhouse gas storage. The molecular diffusion coefficient is one of the most important mechanisms in describing the mass transfer of gas during injection. The molecular diffusion coefficient is determined using indirect methods like pressure decay method associated with appropriate models and analyzing the experimental results. In this study, analytical methods for analyzing the experimental data from the pressure decay method were evaluated. For this purpose, three analytical solution methods of diffusivity equation were introduced using equilibrium, quasi-equilibrium and non-equilibrium boundary conditions at the interface of two phases in contact with the diffusion cell. Then, the models were applied for determining diffusivity in different fluid systems of heavy and light oil. The evaluation of the proposed models was based on the difference between the production pressure by these models and the experimental pressure. The results showed that for heavy oil systems, the boundary conditions on the surface is dependent on the type of the injection gas and the experimental conditions. Equilibrium boundary conditions are used for light oil systems because of their lower viscosity and continuity at the interface of two phases.