Semi-analytical modeling of steam-solvent gravity drainage of heavy oil and bitumen: Steady state model with linear interface

Abstract

With increasing world demand for energy, greater attention has been given to the exploitation of the huge resources present in the form of heavy oil and bitumen. Although thermal methods such as steam assisted gravity drainage (SAGD) have been successful in recovering heavy oil and bitumen, the low thermal efficiency of the process and the high level of greenhouse gas emissions and water usage remain major concerns.Co-injection of solvent with steam has shown to be promising in enhancing oil rates as well as in reduction of energy and water consumption with lower environmental impacts. In hybrid steam-solvent methods, there is a balance between the solubility of the solvent and its ability to reduce bitumen viscosity, and the viscosity reduction due to temperature increase. Therefore, proper selection of the solvent for the operating conditions is key to improving the overall efficiency of the steam-solvent process over the steam-only method.A steady state semi-analytical model is developed to predict the oil flow rate during spreading and depletion phases of steam chamber development in the solvent-aided SAGD (SA-SAGD) process. The model assumes steady state temperature and unsteady state concentration distribution ahead of the linear steam-bitumen interface. It also accounts for transverse dispersion and concentration-dependent molecular diffusion for solvent distribution employing the Integral Method.The model is validated against CMG-STARS® thermal simulator and also SAGD experimental results for hexane co-injected with steam. It is shown that by adjusting a few parameters using SAGD results, the model can fairly predict the oil production and cumulative steam-to-oil ratio for the solvent-aided process with average absolute deviations up to 7% and 20%, respectively. The results suggest that the steady state model can be used as a screening tool for SA-SAGD. Also, it may be employed to find the optimum solvent candidate and the operational variables to maximize the flow rate of the SA-SAGD process. The model can also be applied for a mixture of solvents provided that the equilibrium experimental phase behavior data are available for a given solvent-bitumen system.