Modeling Displacement Efficiency Improvement During Solvent Aided-SAGD

Abstract Coinjection of small amounts of hydrocarbon solvents with steam has the potential to improve the oil recovery efficiency while reducing the energy intensity of conventional SAGD (steam assisted gravity drainage). A number of studies have reported lower residual oil saturations inside the coinjection chamber compared to SAGD. There is, however, no mathematical model available to predict the extent of such displacement efficiency improvement or to compare it during the coinjection of different solvents. This paper presents a mathematical procedure for the estimation of local displacement efficiency improvement in the coinjection process. Displacement efficiency is modeled as a function of local solvent accumulation, upon the arrival of coinjection chamber interface, and the temperature, as the region is swept by the chamber. The model is used to investigate the interaction of displacement efficiency improvement, ultimate bitumen recovery, and solvent retention inside the swept region, the last of which is of significant concern in large scale applications. The complex interaction of mass and energy flow is simplified without loss of the fundamental mechanisms and phase behavior details. Initially, phase equilibrium equations are solved to find the thermodynamic conditions inside and at the boundary of the coinjection chamber. Then, the saturation of phases as well as the retained amount of solvent are estimated along the temperature profile inside the chamber by making reasonable assumptions. The model is also used to investigate the impact of changing a solvent-steam coinjection ratio on the displacement efficiency improvement and/or solvent retention. Results indicate that coinjection can achieve improved displacement efficiency even without modifying the end point saturations of the relative permeability curves as a result of solvent coinjection. Eventually, the results are validated by using numerical simulations for the coinjection process. It is demonstrated that a robust understanding of phase behavior interaction with heat and solvent transport is critical to explaining the recovery mechanisms involved in solvent-aided SAGD.