Solvent co-injection with steam-assisted gravity drainage process performance evaluation–New analytical treatment in relation to constant injection rate

Abstract

Steam-based enhanced oil recovery methods like Steam-Assisted Gravity Drainage (SAGD) for extracting extremely heavy oils and bitumen are energy intensive and require high steam volumes with steam generation resulting in significant greenhouse gas emission. The oil industry has resorted to adding solvent to steam aiming to overcome these limitations. A new analytical model is developed for describing solvent co-injection with steam (solvent-SAGD) process in relation to constant injection rate, for the first time, based on the combination of an overall solvent mass balance, heat balance and volumetric oil displacement and Darcy's oil rate using a mixture viscosity model as a function of temperature and solvent concentration ahead of a moving vapor-oil front which satisfy the equilibrium in the system. The objectives of this work are to evaluate the performance of the solvent-SAGD process by predicting; vapor chamber growth, oil production rate, solvent production rate, solvent loss (or solvent retention) rate, and the effect of solvent type and concentration on the process. The mechanism of oil recovery by steam with added solvent is not clear. Given the controversy regarding the use of solvents with steam, by developing the new mathematical approach, this study is intended to explain whether solvent-SAGD process increases oil recovery compared to SAGD or not. If any, which solvent results in better recovery. The results of this approach give a better understanding of the oil production mechanism during the solvent-SAGD process by interconnecting vapor chamber conditions and the conditions of heated and diluted oil ahead of the moving vapor-oil interface. The results show that mass transfer occurs over a scale of centimeters, while heat transfer scale is of the order of meters. The rate of solvent retention increases over time, while the production rates of solvent and bitumen decrease. Also, it is shown that the solvent-SAGD process which is a combination of heat and mass transfer would yield a higher oil recovery, if viscosity reduction from dilution at a lower temperature is greater than viscosity reduction by temperature in the SAGD process, for the same total injection rate.