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.
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[edit] Description
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.
[edit] Invention and development
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. He tested the concept with Imperial Oil in 1980, in a pilot at Cold Lake which featured one of the first horizontal wells in the industry, with vertical injectors. The latter were established to be inefficient by research at the Alberta Oil Sands Technology and Research Authority (AOSTRA) in the early '80s. This resulted in the first test of twin (horizontal) well SAGD, at their Underground Test Facility (UTF) in the Athabasca Oil Sands, which proved the feasibility of the concept, briefly achieving positive cash flow in 1992 at a production rate of about 2000 bbl/day from 3 well pairs.
The original UTF 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.
[edit] Current applications
This technology is now being exploited due to increased oil prices. While traditional drilling methods were prevalent up until the 1990s, 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 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.
[edit] Disadvantages
The SAGD process is not entirely without drawbacks however; it requires small amounts of fresh water and large water re-cycling facilities and either natural gas or cheap coal fired electricity in order to create the steam. Relying upon gravity drainage, it also requires comparatively thick and homogeneous reservoirs.
Different processes are still being developed. Derivative processes are being developed including SAGP and ES-SAGD in which non condensable / condensable gas is mixed with the steam.
[edit] Alternative methods
Alternative enhanced oil recovery mechanisms include VAPEX (for Vapor Extraction), Electro-Thermal Dynamic Stripping Process, ET-DSPTM, and ISC (for In Situ Combustion). VAPEX uses solvents instead of steam to displace oil and reduce its viscosity. ET-DSPTM is a patented process that uses electricity to heat oil sands deposits to mobilize bitumen allowing production using simple vertical wells. ISC uses oxygen to generate heat that diminishes oil viscosity; alongside carbon dioxide generated by heavy crude oil displace oil toward production wells.
ET-DSPTM is the focus of a field test conducted by E-T Energy Ltd., in the Athabasca Oil Sands in Alberta Canada since January 2006.
A very promising ISC approach is called THAI for Toe Heel Air Injection. Being developed by the Canadian company Petrobank which owns the patents to the technique developed at Bath University in Scotland, the method has been in field testing now since June, 2006.
