Steam assisted gravity drainage
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Steam Assisted Gravity Drainage (SAGD) is an enhanced oil recovery technology for heavy crude oil and bitumen. Two parallel horizontal oil wells are drilled in the formation. The upper well injects steam and the lower one collects the water that results from the condensation of the injected steam and the crude oil or bitumen. The injected steam heats the crude oil or bitumen and lowers its viscosity which allows it to flow down into the lower wellbore. The large density contrast between steam on one side and water / hot heavy crude oil on the other side ensures that steam is not produced at the lower production well. The water and crude oil or bitumen is recovered to the surface by several methods such as natural steam lift where some of the recovered hot water condensate flashes in the riser and lifts the column of fluid to the surface, by gas lift where a gas (usually natural gas) is injected into the riser to lift the column of fluid, or by pumps such as progressive cavity pumps that work well for moving high-viscosity fluids with suspended solids.
The gravity drainage idea was originally conceived by Dr. Roger Butler, an engineer for Imperial Oil around 1969. But it wasn't until 1975 when Imperial Oil moved him from Sarnia, Ontario to Calgary, Alberta to head their heavy oil research effort that he pursued the concept. The idea didn't work well on paper until he hit on the idea of using horizontal instead of vertical wells. Tests, first by Imperial Oil, and later by the Alberta Oil Sands Technology and Research Authority (AOSTRA), at their Underground Test Facility (UTF) in the Athabasca Oil Sands, proved the feasibility of the concept. After retiring from Imperial Oil, Dr. Butler was appointed to the University of Calgary's Endowed Chair in Petroleum Engineering, which allowed him to pursue the concept full-time and fine-tune the process.
The original SAGD wells were drilled horizontally from a tunnel in the limestone underburden, accessed with vertical mineshafts. The concept coincided with development of directional drilling techniques that allowed companies to drill horizontal wells accurately, cheaply and efficiently, to the point that it became hard to justify drilling a conventional vertical well any more. With the low cost of drilling horizontal well pairs, and the very high recovery rates of the SAGD process (up to 60% of the oil in place), SAGD is economically attractive to oil companies.
This technology is now being exploited due to increased oil prices. While traditional drilling methods were prevalent up until the 1990's, high crude prices of the 21st Century are encouraging more unconventional methods (such as SAGD) to extract crude oil. The Canadian oil sands have many SAGD projects in progress, since this region is home of one of the largest deposits of bitumen in the world (Canada and Venezuela have the world's largest deposits).
The SAGD process is not entirely without drawbacks however; it requires large amounts of fresh water and wastewater handling facilities, abundant and cheap natural gas or electricity in order to create the steam, and thus produces large amounts of greenhouses gases, including carbon dioxide. Relying upon gravity drainage, it also requires comparatively thick and homogeneous reservoirs.
More efficient processes are still being developed. Derivated processes are being developed including SAGP and ES-SAGD in which non condensable / condensable gas is mixed with the steam.
However, the SAGD process allowed the Alberta Energy and Utilities Board to increase its proven oil reserves to 179 billion barrels, which raised Canada's oil reserves to the second highest in the world after Saudi Arabia and approximately quadrupled North American oil reserves.
Alternative enhanced oil recovery mechanisms include VAPEX (for Vapor Extraction) and ISC (for In Situ Combustion). VAPEX uses solvents instead of steam to displace oil and reduce its viscosity. ISC uses oxygen to generate heat that diminishes oil viscosity; alongside carbon dioxide generated by heavy crude oil displace oil toward production wells.
