Modeling of Non-Equilibrium Interphase Mass Transfer during Solvent-Aided Thermal Recovery Processes
Abstract
As most of the heavy oil reserves in the world are too viscous to be exploited conventionally, enhanced oil recovery (EOR) methods are applied mainly through utilizing heat or dilution. Two thermal recovery methods stand out to be the most viable and commercially practical for exploiting extra-heavy and highly viscous oil reservoirs are Steam-Assisted Gravity Drainage (SAGD), and Cyclic Steam Stimulation (CSS) processes. Both processes apply heat to the reservoir using steam to reduce the viscosity of the bitumen rendering it mobile. Despite the commercial success of these thermal recovery processes, solvent-aided thermal recovery processes recently gained increased industrial interest for their potential to achieve higher energy efficiency, reduced environmental impact, and increased economic viability. In solvent-aided thermal processes, solvent is coinjected with steam to further aid in reducing bitumen viscosity through mass and heat transfer and diffusion of solvent into bitumen.
Several field trials of solvent-based recovery processes have been carried out and field results were mixed or inconclusive, and that can be attributed to the lack of knowledge of the physics and interrelated mechanisms involved with interphase-mass transfer and solvent dissolution into bitumen. The first part of this thesis aims to address the mechanisms of solvent dissolution into bitumen due to solvent diffusion and defines the key parameter of diffusive dominant interphase-mass transfer coefficient for several solvent/bitumen binary mixtures. The results show that the diffusion of lighter solvents into bitumen is lower than heavier solvents particularly at low temperatures. Also, it was found that the diffusion dominant interphase mass transfer coefficient is relatively higher for lighter solvents such as methane, ethane, and propane. Therefore, modelling of the non-equilibrium interphase mass transfer phenomena is relatively more important for lighter solvents for designing and implementing a successful solvent-aided thermal recovery process.
One of the most important mechanisms involved in solvent-aided thermal recovery processes is interphase-mass transfer phenomena which involves a variation of a system property due to a non-equilibrium state. However, in current reservoir simulation models a local equilibrium is assumed such that a simulation grid block is at instantaneous equilibrium. In reality, local equilibrium assumption often fails at larger scales or in situations where flow velocities are large compared to that of mass or heat transfer. In the second part of this thesis, solvent-aided gravity drainage of bitumen was simulated with propane as a solvent using CMG-STARS. The effect of non-equilibrium mass transfer was included in the model to simulate the process using a kinetic approach. The results show that the assumption of the local instantaneous equilibrium result in 3% to 6% lower oil recovery for the typical field scale simulation models. This difference in oil recovery can be mitigated fairly through the inclusion of the non-equilibrium interphase mass transfer. Correlations for the non-equilibrium interphase mass transfer coefficients for propane/bitumen mixture were developed which can be used as guidelines for modelling the non-equilibrium interphase mass transfer for field scale simulations of solvent-based EOR processes.
