Oil sands are naturally occurring deposits composed primarily of sand, clay, and water, with bitumen filling the spaces between the grains. Bitumen is a thick, sticky, and highly viscous form of petroleum, distinct from conventional crude oil. These deposits represent a significant global energy resource, with vast reserves found in locations like the Athabasca region of Alberta, Canada. The extraction and processing of this bitumen are complex, involving specialized techniques to separate it from the sand and water.
Characteristics of Oil Sand Deposits
Oil sand deposits consist of approximately 80% sand and clay, 10% water, and 10-12% bitumen. Each grain of sand is enveloped by a thin film of water, then covered by the bitumen. The depth and consistency of these deposits are primary factors determining the extraction method. Shallow deposits, generally less than 75 meters below the surface, are accessible through surface mining, while deeper formations require in-situ techniques. Bitumen’s inherent viscosity, which can be nearly solid at ambient temperatures, necessitates heating or dilution for it to flow.
Surface Mining Techniques
For oil sand deposits located close to the surface, typically within 75 meters, surface mining is employed. This process begins with removing the “overburden” of soil, vegetation, and rock. Large-scale machinery then excavates the exposed oil sand.
The excavated oil sand is transported to a primary processing facility. Here, the material is fed into crushers to break down large clumps. Hot water is added to create a slurry, which is essential for separating the bitumen from the sand and clay. This method is currently used to recover about 20% of Alberta’s oil sands reserves.
In-Situ Extraction Methods
For oil sand deposits buried deeper than 75 meters, which account for about 80% of reserves, in-situ extraction methods are necessary. These techniques involve heating the bitumen underground to reduce its viscosity, allowing it to flow and be pumped to the surface. The two most common in-situ methods are Steam-Assisted Gravity Drainage (SAGD) and Cyclic Steam Stimulation (CSS).
SAGD
SAGD involves drilling two horizontal wells, one positioned a few meters directly above the other, into the oil sands reservoir. High-pressure steam is injected into the upper well, heating the bitumen and reducing its viscosity. This heated, more fluid bitumen then drains by gravity into the lower production well, from which it is pumped to the surface. Steam injection and bitumen production occur simultaneously, with the resulting mixture piped to a plant for separation.
Cyclic Steam Stimulation (CSS)
Cyclic Steam Stimulation (CSS), also known as the “huff and puff” method, uses a single well for both steam injection and bitumen production. Steam is injected into the well for a period to heat the surrounding formation. After a soaking period, the well is then used to produce the heated, less viscous bitumen. Once the production rate declines, the cycle is repeated, with steam re-injected into the same well.
Processing the Extracted Bitumen
After extraction, the raw bitumen requires further processing. For surface-mined oil sands, the excavated material undergoes a hot water extraction process. This mixture is agitated and sent to large separation vessels where the bitumen, being less dense, floats to the surface as a froth, while sand and other solids settle. The bitumen froth is collected and treated to remove residual water and fine solids.
Bitumen extracted via in-situ methods is partially diluted by injected steam, but still contains water and impurities. Following initial separation, the heavy, viscous bitumen often undergoes an “upgrading” process. Upgrading transforms the bitumen into synthetic crude oil (SCO), a lighter, less viscous product suitable for transport via pipelines and processing in conventional refineries. This involves processes like thermal conversion (coking) or hydrocracking, which break down large hydrocarbon molecules and remove impurities. Diluents are also added to the bitumen to facilitate its transportation through pipelines.