Steam assisted gravity drainage
Steam assisted gravity drainage or SAGD is a method that is widely used to extract bitumen from underground oil sands deposits. This method involves forcing steam into sub-surface oil sands deposits to heat the bitumen locked in the sand, allowing it to flow well enough to be extracted. This technology is particularly relevant in Canada because it is the primary method of in situ extraction used in the oil sands. In Alberta alone, 80% (or 135 billion barrels) of the oil sands are located in these underground deposits and would be difficult to access without techniques like SAGD.
In recent history, SAGD operations have become more common as technology continues to advance. In 2000, there were 5 SAGD projects underway in Alberta, but by 2013 those numbers had jumped to 16. In 2012, the total in situ production of bitumen was 990 000 barrels a day, which is about 52% of the total crude bitumen production. By 2022 predictions put the in situ production at 2.2 million barrels a day. In total, there is an estimated 1 trillion barrels of oil that are potentially recoverable.
To extract bitumen from below ground, a pair of horizontal wells are drilled into the formation. In these horizontal wells, there are two parallel horizontal pipes with one situated about 4-6 meters above the other. The upper section of this configuration is known as a steam injection well whereas the bottom section is known as the production well. At a nearby plant, water is turned into steam and travels to the location where the drilling is taking place. The steam is then passed through the upper well and into the reservoir that contains the oil sand. Steam then exits the upper well, expanding out into the formation in all directions. The heat from the steam is transferred to the bitumen. The warming of this bitumen results in a reduction of its viscosity, allowing it to flow more easily. Since the viscosity was decreased so dramatically, it is able to flow downward under the force of gravity into the production well. This draining of the bitumen is known as gravity drainage. From the production well, the now more fluid bitumen is pumped to the surface. The steps of steam injection and bitumen production happen simultaneously and continuously. The resulting bitumen and condensed steam emulsion is piped to the plant where it is separated and treated. The water from this process is recycled for generating more steam.
After the extraction, water is injected into the deposit to maintain stability. 25-75% of the bitumen in the deposit can be extracted through SAGD operations.
Developing any sort of infrastructure to obtain a resource requires a certain amount of land use and SAGD is no exception. Although land must still be used when extracting bitumen through a SAGD operation, the surface impact is similar to that of conventional oil and gas extraction processes. All of these methods recover product from underground by making a small hole and having the petrochemical flow out. Estimates say that the well pad disturbs less than 10% of the total resource area being accessed. Although the land impact is small, surrounding ecosystems could be dramatically affected depending on the ecological significance of the land. The labelled, interactive map below can be used to explore the size difference in the land used by in situ (the SAGD) mining and traditional surface mining. Take a note of the scale on the bottom right-hand side of the map (this will change when zooming in and out, but gives an idea of the size of mines).
The extensive use of water in oil sands extraction is also an issue that is spoken of widely. Although up to 90% of the water used during extraction is recycled and used again, 10% is still quite a bit of water that needs to be disposed. This water can be fresh or brackish, and may contain a variety of pollutants. Depending on how this water is disposed of, it could contaminate clean groundwater deposits or harm plant and animal life around the deposit. Additionally, the volume of water creates concerns with how much is being taken for in situ use.
Emissions are also an issue when it comes to anything involving the production of fossil fuels. The emissions of carbon dioxide for bitumen development and pre-processing are about 110 kg per barrel, three times what it is for a barrel from crude oil. The SAGD process is responsible for roughly half of that (60 kg per barrel). SAGD is on par with other mining and upgrading operations for bitumen specifically (as opposed to crude oil). Additionally, the consumption of natural gas to produce the steam used in these operations causes further issues since this combustion releases emissions such carbon monoxide and sulfur compounds depending on the purity of the gas. Although the exact numbers for the consumption of natural gas vary, some reports say that it takes at least a gigajoule of natural gas (around 26 cubic meters) to heat water into the steam needed to obtain the bitumen and produce one barrel of oil.
Steam assisted gravity drainage is not the only way to extract bitumen from underground deposits, but it is the most widely used. Other options include Toe-to-Heel Air Injection, Vapour Extraction Process, and an Electro-Thermal Dynamic Stripping Process. The Toe-to-Heel Air Injection process utilizes pressurized air and a hot fluid injected into the formation instead of steam. This process has a higher recovery rate of bitumen, is less expensive, uses less natural gas and water, and produces fewer greenhouse gas emissions. Additionally, this process can work in deposits that are of lower quality, thinner, deeper, or contain more shale.
The Vapour Extraction Process uses a solvent instead of steam and thus reduces the thickness of the bitumen even more. Solvents such as ethane or propane are injected and this method has the benefits of having lower energy costs and the fact that the method can be used in thinner reservoirs. However, there are significant field challenges which make this hard to implement in practice. At the time of this writing field tests haven't yet been done.
The final Electro-Thermal Dynamic Stripping Process is one of the most advanced, and is supposed to allow for the recovery of bitumen buried too deeply to dig for but too shallowly for traditional in situ recovery. Electricity passes through steel electrodes in the deposit, electrically heating the bitumen to make it flow better. This process can potentially produce next to no greenhouse gas emissions and use minimal water (but there would still be greenhouse gas emissions from producing the electricity). This process is still highly experimental and at the time of writing hasn't been attempted.
For Further Reading
- Athabasca Oil Corporation. (June 2, 2015). SAGD [Online]. Available: http://www.atha.com/operations/technology/sagd.html
- Alberta Government. (June 9, 2015). Talk about SAGD [Online]. Available: http://www.energy.alberta.ca/OilSands/pdfs/FS_SAGD.pdf
- Government of Alberta. (June 9, 2015). Facts about Alberta's oil sands and its industry [Online]. Available: http://history.alberta.ca/oilsands/resources/docs/facts_sheets09.pdf
- Created internally by a member of the Energy Education team.
- Alberta Energy Regulator. (June 9, 2015). In Situ Impacts [Online]. Available: https://www.aer.ca/about-aer/spotlight-on/oil-sands/in-situ-impacts
- US Department of Energy, National Energy Technology Laboratory "An Evaluation of the Extraction, Transport and Refining of Imported Crude Oils and the Impact on Life Cycle Greenhouse Gas Emissions", March 27th 2009. DOE/NETL-2009/1362 available online: http://www.netl.doe.gov/File%20Library/Research/Energy%20Analysis/Publications/DOE-NETL-2009-1362-EvalExtTransRef-ImportCrudeOils-ImpactLCGHGEmis.pdf accessed June 22nd, 2015.