Steam-assisted gravity drainage (SAGD; "Sag-D") is an enhanced oil recovery technology for producing heavy crude oil and bitumen. It is an advanced form of steam stimulation in which a pair of horizontal wells are drilled into the oil reservoir, one a few metres above the other. High pressure steam is continuously injected into the upper wellbore to heat the oil and reduce its viscosity, causing the heated oil to drain into the lower wellbore, where it is pumped out. Dr. Roger Butler, engineer at Imperial Oil from 1955 to 1982, invented the steam assisted gravity drainage (SAGD) process in the 1970s. Butler "developed the concept of using horizontal pairs of wells and injected steam to develop certain deposits of bitumen considered too deep for mining". In 1983 Butler became director of technical programs for the Alberta Oil Sands Technology and Research Authority (AOSTRA),AOSTRA is now known as the Alberta Energy Research Institute. a crown corporation created by Alberta Premier Lougheed to promote new technologies for oil sands and heavy crude oil production. AOSTRA quickly supported SAGD as a promising innovation in oil sands extraction technology.
Steam-assisted gravity drainage (SAGD) and cyclic steam stimulation (CSS) steam injection (oil industry) are two commercially applied primal thermal recovery processes used in the oil sands in Geological formation sub-units, such as Grand Rapids Formation, Clearwater Formation, McMurray Formation, General Petroleum Sand, Lloydminster Sand, of the Mannville Group, a stratigraphic range in the Western Canadian Sedimentary Basin.
Steam-assisted gravity drainage is one of the two primary extraction techniques in Alberta's oil sands, the other being strip-mining. While strip-mining is limited to deposits near the surface, steam-assisted gravity drainage technique (SAGD) is better suited to the larger deep deposits that surround the shallow ones. Much of the expected future growth of production in the Canadian oil sands is predicted to be from SAGD.
Steam Assisted Gravity Drainage emissions are equivalent to what is emitted by the steam flood projects which have long been used to produce heavy oil in California's Kern River Oil Field and elsewhere around the world.
Description The SAGD process of heavy oil or bitumen production is an enhancement on the steam injection techniques originally developed to produce heavy oil from the Kern River Oil Field of California. The key to all steam flooding processes is to deliver heat to the producing formation to reduce the viscosity of the heavy oil and enable it to move toward the producing well. The cyclic steam stimulation (CSS) process developed for the California heavy oil fields was able to produce oil from some portions of the Alberta oil sands, such as the Cold Lake oil sands, but did not work as well to produce bitumen from heavier and deeper deposits in the Athabasca oil sands and Peace River oil sands, where the majority of Alberta's oil sands reserves lie. To produce these much larger reserves, the SAGD process was developed, primarily by Dr. Roger Butler of Imperial Oil with the assistance of the Alberta Oil Sands Technology and Research Authority and industry partners. The SAGD process is estimated by the National Energy Board to be economic when oil prices are at least US$30 to $35 per barrel.
In the SAGD process, two parallel horizontal oil wells are drilled in the formation, one about 4 to 6 metres above the other. The upper well injects steam, and the lower one collects the heated crude oil or bitumen that flows down due to gravity, plus recovered water from the condensation of the injected steam. The basis of the SAGD process is that thermal communication is established with the reservoir so that the injected steam forms a "steam chamber". The heat from the steam reduces the viscosity of the heavy crude oil or bitumen which allows it to flow down into the lower wellbore. The steam and associated gas rise because of their low density compared to the heavy crude oil below, ensuring that steam is not produced at the lower production well, tend to rise in the steam chamber, filling the void space left by the oil. Associated gas forms, to a certain extent, an insulating heat blanket above (and around) the steam. Oil and water flow is by a countercurrent, gravity driven drainage into the lower well bore. The condensed water and crude oil or bitumen is recovered to the surface by pumps such as progressive cavity pumps that work well for moving high-viscosity fluids with suspended solids.
Sub-cool is the difference between the saturation temperature (boiling point) of water at the producer pressure and the actual temperature at the same place where the pressure is measured. The higher the liquid level above the producer the lower the temperature and higher is the sub-cool. However, real life reservoirs are invariably heterogeneous therefore it becomes extremely difficult to achieve a uniform sub-cool along the entire horizontal length of a well. As a consequence many operators, when faced with uneven stunted steam chamber development, allow a small quantity of steam to enter into the producer to keep the bitumen in the entire wellbore hot hence keeping its viscosity low with the added benefit of transferring heat to colder parts of the reservoir along the wellbore. Another variation sometimes called Partial SAGD is used when operators deliberately circulate steam in the producer following a long shut-in period or as a startup procedure. Though a high value of sub-cool is desirable from a thermal efficiency standpoint as it generally includes reduction of steam injection rates but it also results in slightly reduced production due to a corresponding higher viscosity and lower mobility of bitumen caused by lower temperature. Another drawback of very high sub-cool is the possibility of steam pressure eventually not being enough to sustain steam chamber development above the injector, sometimes resulting in collapsed steam chambers where condensed steam floods the injector and precludes further development of the chamber.
Continuous operation of the injection and production wells at approximately reservoir pressure eliminates the instability problems that plague all high-pressure and cyclic steam processes and SAGD produces a smooth, even production that can be as high as 70% to 80% of oil in place in suitable reservoirs. The process is relatively insensitive to shale streaks and other vertical barriers to steam and fluid flow because, as the rock is heated, differential thermal expansion allows steam and fluids to gravity flow through to the production well. This allows recovery rates of 60% to 70% of oil in place, even in formations with many thin shale barriers. Thermally, SAGD is generally twice as efficient as the older CSS process, and it results in far fewer wells being damaged by the high pressures associated with CSS. Combined with the higher oil recovery rates achieved, this means that SAGD is much more economic than cyclic steam processes where the reservoir is reasonably thick.
History
The gravity drainage idea was originally conceived by Dr. Roger Butler, an engineer for Imperial Oil in the 1970s In 1975 Imperial Oil transferred Butler from Sarnia, Ontario to Calgary, Alberta to head their heavy oil research effort. 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.
Alberta Oil Sands Technology and Research Authority (AOSTRA) 1974
In 1974, Premier of Alberta Peter Lougheed created the Alberta Oil Sands Technology and Research Authority (AOSTRA) as an Alberta crown corporation to promote the development and use of new technology for oil sands and heavy crude oil production, and enhanced recovery of conventional crude oil. Its first facility was owned and operated by ten industrial participants and received ample government support (Deutsch and McLennan 2005) including from the Alberta Heritage Savings Trust Fund. One of the main targets of AOSTRA finding of suitable technologies for that part of the Athabasca oil sands that could not be recovered using conventional surface mining technologies.
AOSTRA Underground Test Facility 1984
In 1984, AOSTRA initiated the Underground Test Facility in the Athabasca oil sands, located between the MacKay Rivers and the Devon River west of the Syncrude plant as an in-situ SAGD bitumen recovery facility. It was here that their first test of twin (horizontal) SAGD wells took place, proving the feasibility of the concept, briefly achieving positive cash flow in 1992 at a production rate of about from three well pairs.
Foster Creek
The Foster Creek plant in Alberta Canada, built in 1996 and operated by Cenovus Energy, was the first commercial Steam-assisted gravity drainage (SAGD) project and by 2010 Foster Creek "became the largest commercial SAGD project in Alberta to reach royalty payout status. "
The original UTF SAGD wells were drilled horizontally from a tunnel in the limestone underburden, accessed with vertical mine shafts. 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.
At Foster Creek Cenovus has employed its patented 'wedge well' technology to recover residual resources bypassed by regular SAGD operations, this improves the total recovery rate of the operation. The 'wedge well' technology works by accessing the residual bitumen that is bypassed in regular SAGD operations by drilling an infill well between two established operating SAGD well pairs once the SAGD steam chambers have matured to the point where they have merged and are in fluid communication and then what is left to recover in that reservoir area between the operating SAGD well pairs is a 'wedge' of residual, bypassed oil. Wedge well technology has been shown to improve overall recovery rates by 5%-10% at a reduced capital cost as less steam is required once the steam chambers mature to the point where they are in fluid communication and typically at this stage in the recovery process, also commonly known as the 'blow down' phase, the injected steam is replaced with a non-condensable gas such as methane, further reducing production costs.
Current applications
This technology was not at-first commercially viable. It became so during the increased oil prices during the 2000s. 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 Resources Conservation Board (ERCB) to increase its proven oil reserves to 179 billion barrels, which raised Canada's oil reserves to the third highest in the world after Venezuela and Saudi Arabia and approximately quadrupled North American oil reserves. As of 2011, the oil sands reserves stand at around 169 billion barrels.
Disadvantages
Oil and water nexus
SAGD, a thermal recovery process, consumes large quantities of water and natural gas.