Quantification of gas exsolution and preferential diffusion for alkane solvent(s)–CO2–heavy oil systems under nonequilibrium conditions

Abstract

The in-situ formation of foamy oil has been found to be a crucial mechanism accounting for the better-than-expected production performance in heavy oil reservoirs under solution gas drive. Not only have the physical laws dominating gas exsolution in foamy oil not yet been well understood, but also the different contributions of each component in a gas mixture to the generation of foamy oil associated with an extremely complicated dynamic process has not been quantified. In this study, a novel and pragmatic technique has been proposed and validated to quantify the gas exsolution and preferential diffusion for alkane solvent(s)–CO2–heavy oil systems under nonequilibrium conditions by taking gas bubble size distribution and preferential mass transfer of each gas component into account. Experimentally, constant-composition expansion (CCE) tests with various constant-pressure decline rates are utilized to describe gas exsolution behaviour of alkane solvent(s)–CO2–heavy oil systems under nonequilibrium conditions, during which not only pressure and volume are simultaneously monitored and measured, but also gas samples were respectively collected at the beginning and end of experiments to perform compositional analysis. Theoretically, a mathematical model has been formulated to quantify gas exsolution and preferential mass transfer between each gas component and the liquid phase in alkane solvent(s)–CO2–heavy oil systems under nonequilibrium conditions. More specifically, quasi-equilibrium boundary conditions, real gas equation, and Rayleigh distribution function are combined with the classical equation of motion, continuity equation, and mass transfer equation to form a novel equation matrix for quantifying gas bubble growth in foamy oil. Considering gas bubble size distribution and preferential diffusion of each component in a gas mixture, the total number of gas bubbles and individual diffusion coefficient of each gas component are determined by minimizing the discrepancy between the measured and calculated volume of alkane solvent(s)–CO2–heavy oil systems. More importantly, the dynamic composition of the gas phase and the amounts of both entrained gas and evolved gas can also be obtained simultaneously during the gas exsolution processes. Excellent agreements between the experimentally measured parameters (i.e., volume of foamy oil, composition of evolved gas, and volume of free gas) and the calculated ones have been respectively achieved. Compared with the individual diffusion coefficient for each component in a gas mixture determined under the traditional equilibrium conditions, a relatively large value has been found during mass transfer processes in a supersaturated oleic phase. Moreover, pseudo-bubblepoint pressure and gas exsolution rate are found to be two main mechanisms dominating the volume-growth rate of the evolved gas.