Analysis and Interpretation of Gas Diffusion in Quiescent Reservoir, Drilling, and Completion Fluids: Equilibrium vs. Non-equilibrium Models

Abstract

Abstract This paper presents physically and mathematically rigorous transient-state equilibrium and non-equilibrium diffusion models for practical determination of the gas diffusion coefficient from experimental measurements of the gas pressure decline in a closed tank by dissolution of gas in a quiescent liquid (oil, brine, drilling mud, and completion fluids). The short- and long-time solutions of this model for equilibrium and nonequilibrium transfer processes are developed 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 used to establish the significance of the equilibrium vs. nonequilibrium gas dissolution in liquids under in situ conditions. The experimental data given in previous studies are analyzed by means of the present improved methods and the diffusion coefficients obtained are compared with the reported values. The results developed in this paper can be utilized for determination of the gas diffusivity, the interface mass-transfer coefficient, and the rate of dissolution of the injection gases in oil during secondary recovery, and the rate of dissolution and separation of light gases in reservoir oil and brine, drilling mud, and completion fluids.