Thermal recovery methods involving steam injection have long been considered as an effective means of extracting heavy oil resources. In addition, the high recovery performance of SAGD makes it a popular option for these non-conventional oil resources. Steam processes are energy intensive and result in generation of emissions which are detrimental to humankind and the environment. The use of non-thermal processes involving CO2 as a miscible or immiscible gas phase in combination with steam for heavy oil recovery is considered as a viable alternative to limit the drawbacks of steam generation. These processes have the capability to enhance oil recovery through CO2 utilization during production and also provide an avenue to dispose CO2 after production. Numerical simulation studies have been carried out utilizing STARS (a three phase, multi-components reservoir simulator) to optimize a baseline SAGD process and wind-down process with CO2 Injection. The baseline process was operated until maturity then CO2 injection was used to initiate wind-down after 4, 6 and 8 years of a 12 year production operation. Following each of the wind-down processes, CO2 disposal was undertaken for 25 years and the storage potential evaluated. The baseline SAGD process had a recovery factor of 76%. The SAGD wind-down processes with CO2 injection after 4, 6 and 8 years had recovery factors of 54%, 77% and 79% respectively. This and other parameters proved the feasibility of SAGD wind-down process with CO2 injection and its potential to enhance oil recovery.
Heavy oil, extra heavy oil and bitumen resources (generally referred to as heavy crude oil) account for most of the world's oil-in-place. A vast amount of these non-conventional oil resources are deposited in Venezuela, Canada, USA and China. Some estimates expect Alberta, Canada's heavy oil sands production to exceed 1.2 million bbl/day in the near future. Recoverable bitumen reserves in Alberta are estimated at 300 billion bbl. In Venezuela, Orinoco tar sands could be producing 600,000 bbl/day and it contains about 300 billion bbl of recoverable heavy oil and EOR reserves. In China, CNPC produces about 150,000 bbl/day of heavy crude and it has about 8 billion bbl of heavy oil reserves. USA has 20–25 billion barrels on the North Slope of Alaska. As global energy demand continues to rise and production of conventional oil declines, further development of heavy oil and oil sands (bitumen, sand, clay and water) recovery processes and technologies is integral to meeting future energy requirements. While conventional oil is easily extracted from the ground by drilling wells into formations bearing light and medium density oil which flow under natural reservoir pressures, heavy oils and oil sands require surface mining or in-situ techniques which reduce oil viscosity and increase its mobility using thermal or non-thermal processes.
Thermal heavy oil recovery processes include Steam Flooding, Cyclic Steam Stimulation (CSS), Steam-Assisted Gravity Drainage (SAGD) and Toe-to-Heel Air Injection (THAI). Examples of non-thermal methods are Cold Heavy Oil Production with Sand (CHOPS) and Vapour Extraction Processes (VAPEX). Thermal methods achieve high oil recovery but are characterized by excessive energy consumption and CO2 emissions. Non-thermal methods on the other hand, do not achieve as much recovery as thermal processes but are usually less energy intensive and result in lower CO2 emissions. Improved recovery efficiency can be achieved by combining thermal and non-thermal processes. The resulting processes are referred to as hybrid processes.