Nonequilibrium Phase Behavior Plays a Role in Solvent-Aided Processes

Abstract Several studies focused on experimental results of nonequilibrium phase behavior and its numerical implementation for solvent-aided recovery processes. However, there is a clear discrepancy between optimistic simulation results and the less than-favorable performance of pilot projects. The intent of the present study is to demonstrate the importance of nonequilibrium phenomena in terms of oil production, steam-oil ratio (SOR), and solvent-oil ratio. This evaluation is based on solvent type, process type, operational conditions, and reservoir properties. A pore-scale simulator was developed and used to investigate solvent propagation in porous media based on solvent properties (density, viscosity, and diffusion coefficient). Several models were used to investigate the effect of petrophysical properties, e.g. tortuosity, on the upscaled diffusion coefficient, dispersion tensor, and interfacial mass exchange coefficient. In the next step, an in-house thermal reservoir simulator, incorporating nonequilibrium phase behavior, was used to investigate the effect of the particular recovery process. The butane expanding-solvent steam-assisted gravity drainage (ES-SAGD) process with nonequilibrium assumption leads to a 50% reduction in incremental bitumen production in early stages compared with the result for equilibrium assumption. This number reduces to 20% when the chamber becomes mature. It is shown that lighter and more volatile solvents, such as propane show significant deviation from equilibrium behavior compared to heavier and less volatile solvents. The results for ES-SAGD show that close to the chamber edge or in the cold part of the reservoir, the ratio of nonequilibrium constants to standard K-values is very large which means that solvent/oil system is not at equilibrium. Inside the chamber, the temperature is higher which increases the diffusion coefficient of the solvent. In addition, the contact time for oil and solvent for gridblocks inside the chambers is high enough, so that the nonequilibrium constants are close to the K-values. It was found that nonequilibrium behavior can be diminished by increasing solvent-oil contact time in huff and puff processes. This study presents a systematic approach to evaluate a given solvent-aided recovery process from the pore to the reservoir scale and provides the guidelines to assess the importance of nonequilibrium phase behavior in both experiments and numerical studies. Based on the findings of this work, the performance of solvent-aided recovery processes can be predicted more realistically, and the process can be optimized for given operating and reservoir conditions and solvent type.