Oil sands

Figure 1. Oil sands production area in Fort McMurray, Alberta.

Oil sands are naturally occurring deposits of bitumen in sand, clay, and water. Bitumen is the fossil fuel component of oil sands. Bitumen is a highly viscous form of crude oil that must be processed and upgraded before it can be refined into fuels such as gasoline.

Oil sand deposits are found in several regions around the world, including Venezuela, Canada, the United States, Russia, and parts of the Middle East. The largest oil sands deposit in Canada and widely considered the largest in the world, is the Athabasca oil sands in northern Alberta. It is the most extensively developed oil sands deposit in the world, with the largest and most established oil sands mining and in situ extraction operations.

Formation

Like conventional crude oil, the bitumen found in oil sands originated from the remains of ancient marine organisms. Oil sands are believed to have originated from organic material deposited in ancient marine environments that covered parts of present-day Albertamillions of years ago. As microscopic marine organisms died, they settled on the seafloor and were partially decomposed by bacteria. During decomposition and burial, much of the oxygen and nitrogen was removed, leaving organic material enriched in carbon and hydrogen. Over millions of years, layers of sediment buried the organic material. Increasing heat and pressure transformed it into petroleum within the typical oil-generation temperature range of approximately 60 and 150°C. The formation of bitumen is thought to have initially followed the same process as conventional crude oil. During migration toward the surface, lighter hydrocarbons were lost through evaporation, biodegradation, and water washing, leaving behind the heavier bitumen. Oil sands deposits are generally found at relatively shallow depths, where groundwater and microbial activity are thought to have contributed to the biodegradation of the original crude oil. The most widely accepted explanation is that conventional crude oil migrated into the region and was subsequently altered by microbial activity and the loss of lighter hydrocarbons, leaving behind bitumen. A less widely accepted hypothesis suggests that bitumen formed directly from organic-rich source rocks without first existing as conventional crude oil. According to this hypothesis, bitumen was generated directly from kerogen-rich source rocks rather than the previously formed conventional crude oil.

Terminology

Figure 2. Oil sands deposit in Trinidad and Tobago.

Three terms that are commonly used to describe these deposits: oil sands, tar sands, and bituminous sands. Oils sands is the most widely used term. The term tar sands is often used by critics of oil sands development because it emphasizes the environmental impacts associated with extraction and production. Technically, bituminous sands is the most precise term because the deposits consist primarily of sand, clay, water, and bitumen rather than tar. The bitumen content of oil sands deposits typically ranges from about 1% to 18% by weight. In the mid-1990s, the Canadian oil industry and governments began promoting the use of the term "oil sands" in place of tar sands. Historically, these deposits were commonly referred to as tar sands, partly because tar and bitumen was used in early roofing and paving applications. Over time, the term became less common because tar and bitumen are chemically distinct substances. Tar is produced through the destructive distillation of materials such as coal, wood, petroleum, or peat, whereas bitumen is a naturally occurring form of petroleum that can be upgraded into a variety of petroleum products.

Extraction

Figure 3. Overview of the oil sands extraction process.

Oil sands must undergo several processing steps before they can be converted into useful petroleum products (Figure 3). First, the bitumen must be extracted through mining or in situ recovery, followed by mechanical and chemical processing to separate it from the surrounding sand, clay, and water. The extracted bitumen is then upgraded, refined, and converted into finished petroleum products. On average, about 2 tonnes of mined oil sands are required to produce one barrel of synthetic crude oil. Depending on the depth of the deposit, bitumen can be recovered either by surface mining or by in situ extraction methods (Figure 4). After extraction, the bitumen is separated from the sand, clay, and water before being upgraded. Since bitumen is highly viscous and has a relatively low hydrogen-to-carbon ratio, it must be upgraded by removing carbon or adding hydrogen to produce a higher-quality synthetic crude oil that is suitable for refining.

Figure 4. Difference between surface and in-situ mining.

Surface Mining

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Some oil sands deposits are shallow enough to be recovered by surface mining, in which large earth-moving equipment is used to excavate the oil sands for processing. Approximately 65 billion barrels of bitumen are estimated to be recoverable using surface mining methods. These deposits are typically located within about 75 metres of the surface to be economically recovered by surface mining. Currently, surface mining occurs on only a small portion of Alberta's oil sands deposits, approximately 3% of the total oil sands area, with the remainder recovered primarily in situ extraction methods.

To excavate the oil sands, large shovels are used to dig out the oil sands and deposit them into the back of haul trucks. The trucks transport the oil sands to mechanical processors, which include rock crushers and sorting mechanisms. Crushers break up large chunks of rock and sorters process the crushed rock, sending any pieces still too large back through the crusher. The crushed material is then sent for chemical processing. The oil sand is immersed in a mixture of hot water and a caustic diluting chemical. This forces the bitumen to separate from the sand. The non-bitumen component that remains is composed of sand, water, fine clays, and minerals. These leftover components are known as tailings and are sent to tailings ponds to allow the sand to settle out. The separated, diluted bitumen is then sent for upgrading to be turned into more useful products.

In-Situ

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In-situ is a Latin term meaning "in place" and refers to oil sands deposits that are recovered without surface mining. Approximately 80% of Canada's oil sands reserves are recoverable using in situ methods. These deposits are located deeper then approximately 75 metres below the surface, making surface mining impractical or uneconomical. Many of these deposits occur at depths ranging from 350 to 600 metres below the surface. Several in situ recovery methods have been developed to extract bitumen from these deeper deposits.

The two most common methods of in in-situ recovery are steam-assisted gravity drainage (SAGD) and cyclic steam stimulation (CSS). Both methods use heat to reduce the viscosity of the bitumen so that it can flow to a production well and be pumped to the surface. Some newer technologies also incorporate solvents to improve efficiency. One advantage of in situ recovery is that the sands remains underground, eliminating the need for tailings pond. In situ operations generally disturb less surface land than mining operations, although they still require water and energy to generate steam. Modern in situ operations typically recycle much of the water used to generate steam, reducing the need for fresh water, although water requirements vary between projects. New technologies, including solvent-assisted recovery methods and vapour extraction (VAPEX), continue to be researched and developed to improve recovery efficiency and reduce environmental impacts.

Environmental Impacts

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One of the primary challenges associated with oil sands development is the environmental impact of extracting, processing, and upgrading bitumen. Some of the key environmental concerns include:

• Climate and Air: Oil sands production generally produces more greenhouse gas emissions per barrel than conventional oil production because additional energy is required to extract and upgrade the bitumen. As production increases, managing these emissions remains an ongoing environmental challenge.
• Water: Oil sands extraction, particularly surface mining, requires significant amounts of water. Although much of the water is recycled, fresh water is still needed for many operations.
• Tailings: Surface mining produces large volumes of tailings that are stored in tailings ponds. These ponds can contain residual bitumen, fine sediments, and other substances that require careful management to reduce the risk of seepage and impacts on aquatic ecosystems and wildlife.
• Land and Wildlife: Mining operations disturb boreal forests and wetlands, affecting wildlife habitat. Although reclamation is required by government regulations, restoring ecosystems to their original condition can be difficult, particularly for wetlands. Habitat disturbance associated with oil sands development can affect birds and other wildlife.