Steam-Assisted Gravity Drainage (SAGD) is a mature commercial heavy oil and bitumen recovery technology operated by over ten operators in Athabasca and Cold Lake reservoirs in Alberta. Initially, in Athabasca reservoirs, the in situ oil phase viscosity typically exceeds 1 million cP yet with heating to over 200 °C, the viscosity drops to between 1 and 20 cP. The process consists of two parallel horizontal wells: the top one injects steam into the reservoir while the bottom one collects mobilized oil and produces it to the surface. The process is made more thermally efficient by maintaining a liquid pool that surrounds the bottom production well and prevents escape of steam from the steam chamber. This is often referred to as steam-trap control. In field practice, the continued existence of the liquid pool is monitored by examining the temperature difference, called the interwell subcool, between the injected steam and produced fluids. Typically, the subcool is maintained between 15 and 30 °C. Given that there are non-uniformities of the reservoir properties in the downwell direction, there is potential for loss of steam-trap control at one or more locations along the wellbore. In this study, the impact of subcool on the thermal efficiency and performance of SAGD is examined by detailed, 3D reservoir simulation.
Heavy oil and bitumen production in Central and Northern Alberta is growing due in part of the technical success of the Steam-Assisted Gravity Drainage (SAGD) process. Western Canada's reserves of heavy oil and bitumen exceeds 175 billion barrels (AED, 2005) and with uncertainties associated with geopolitical struggles in other parts of the world, Canada is becoming an attractive supplier of petroleum products. SAGD has undergone extensive pilot testing and is now becoming a mature commercial heavy oil and bitumen recovery technology just over twenty-six years after it was invented by Butler (1982). The process is operated. commercially by greater than ten operators in Athabasca and Cold Lake reservoirs in Alberta. At original conditions, in Athabasca reservoirs, the in situ oil viscosity typically exceeds 1 million cP which means it is practically immobile under gravity drainage. After heating to over 200 °C, the viscosity falls to less than 10 to 20 cP and can readily flow under gravity drainage to a production wellbore.
The SAGD process, displayed in cross-section in Figure 1, consists of two horizontal wells that are aligned parallel to each other. The top well injects steam into the reservoir and supplies heat to the formation whereas the bottom one collects mobilized oil and steam condensate and produces it to the surface. The injected steam flows convectively away from the injection well and releases its latent heat at or near the edges of the steam depletion chamber. The temperature of the oil sand at the chamber edge rises and consequently the mobility of the bitumen there increases and under the action of gravity, it drains along the edges of the chamber to the production well.
