Solvent-aided Steam-Assisted Gravity Drainage in thin oil sand reservoirs

Abstract

About one-quarter of the 1.7 trillion barrels of bitumen resource located in Alberta, Canada is hosted in thin reservoirs with thickness less than about 10 m and is at this time considered inaccessible by current commercial recovery processes such as Steam-Assisted Gravity Drainage (SAGD) and Cyclic Steam Stimulation. SAGD is effective at recovering heavy oil and bitumen in reservoirs greater than about 15 m thickness but under this thickness, heat losses from the steam depletion chamber are too large and the energy efficiency of the process suffers. Since a large amount of steam is used per oil unit volume produced, this implies large amounts of natural gas combustion which also implies large amounts of carbon dioxide emitted per unit oil volume produced. Thus, thermal processes in thin pay are not energy efficient and produce excessive amounts of flue gases per unit oil volume produced. Solvent additives injected with steam, as done in Expanding-Solvent SAGD (ES-SAGD), can improve oil production rates or at least maintain similar oil production rates with reduced steam injection. The central idea is that steam plus solvent is better than steam alone to mobilize heavy oil in the reservoir. This implies that ES-SAGD can potentially use less water and require smaller water handling and treatment facilities than that in SAGD. Also, the operating temperature is lower than in steam-only processes so that heat losses are reduced and potentially ES-SAGD could be productive in thin reservoirs. In this research, the solvent injection pressure and volume fraction are designed for a single wellpair ES-SAGD operation by optimizing the net energy injected to oil ratio in a detailed and realistic, three-dimensional, heavy oil reservoir. The results show that the operating pressure and injection strategy can be controlled in a sequence that provides high energy efficiency and solvent recovery.