Steam-Assisted Gravity Drainage (SAGD) is becoming on of the key in situ recovery processes being used today to extract oil from heavy oil and bitumen reservoirs especially those with low solution gas content. In this process, steam, injected through a horizontal well, flows convectively to the outer edges of a steam depletion chamber. At the edges of the chamber, the steam releases its latent heat to the cool oil sand and raises its temperature leading to a reduction of the oil's viscosity to between 2 and 20 cP. The oil flows under the action of gravity to a horizontal production well located several meters below the injection well. It remains unclear what is the exact mechanism of chamber growth and the flow, energy, and phase dynamics at the edge of the chamber. Some have suggested it is through the formation of pointed steam fingers at the edges of the chamber which penetrate the oil sand. In theory published by Butler (1987), it was determined that steam fingers can be as long as 6 m Athabasca type bitumen reservoirs. In this research, an extension to this theory is derived and it is shown that the length of the fingers is smaller than that determined with Butler's theory. The new theory provides predictions of the rise rate which compares better to estimates derived from field thermocouple data and experimental observations than values obtained from Butler's theory. Detailed fine-scale thermal reservoir simulations of the edge of the steam chamber have been done to examine steam, water, oil, gas, and energy flow. The detailed simulations confirm the results of the theory.
An increasing number of heavy oil and bitumen reservoirs, not amenable to surface mining, are being recovered by the Steam-Assisted Gravity Drainage (SAGD) process; displayed schematically in verticalsection in Figure 1. In this process, steam injected into the top wellbore enters the reservoir and collects in a steam depletion chamber in the region above and surrounding the wellbores. The injected steam flows convectively to the outer edges of the chamber and releases its latent heat to cool oil sand at the edges warming up oil, rock, and connate water. The bitumen, now heated, becomes more mobile because its viscosity drops by several orders of magnitude. It then flows under gravity to the lower wellbore. The steam chamber grows by continuously stripping off layers of bitumen from the edges of the steam chamber. As the steam-bitumen interface moves into the oil sand, heat is conducted from the hot steam chamber to the cooler oil sand beyond. At the edges of the chamber, it has been suggested that there are steam fingers, shown as steam undulations, which penetrate the oil sand ahead of the chamber (Butler, 1987; Butler, 1994, Gotawala & Gates, 2008a & b).
