Oil sands are a naturally occurring geological formation, typically an unconsolidated sandstone, that contains a mixture of sand, clay, water, and bitumen, an extremely thick, heavy form of petroleum. This bitumen is highly viscous, possessing a consistency similar to cold molasses, which prevents it from flowing naturally within the reservoir. Conventional oil drilling methods cannot be used, requiring specialized recovery techniques that either physically excavate the material or heat it underground to mobilize the bitumen.
Why Depth Matters for Recovery Methods
The depth of the oil sands deposit is the primary factor determining the recovery method used. Shallow deposits, generally less than 75 meters (about 250 feet) deep, are accessed through surface mining operations and account for approximately 20% of the total recoverable reserves.
The vast majority of the resource (about 80%) is buried too deep for mining. When the bitumen is at these greater depths, operators must use in-situ extraction techniques. These methods involve injecting heat or solvents into the reservoir to allow the bitumen to be recovered through wells.
Surface Mining Operations
Surface mining begins with the removal of the overburden (soil, rock, and muskeg) that lies above the deposit. Once the oil sand layer is exposed, large-scale extraction uses massive equipment, such as hydraulic shovels and heavy haul trucks, to move the ore. The mined oil sand is transported to a primary crusher where large clumps are broken down into smaller pieces.
After crushing, the material is sent to an extraction facility. The separation process begins with the addition of hot water and steam, often mixed with a small amount of diluent. This mixture is agitated and moves through a pipeline, a conditioning step that helps release the bitumen from the sand grains. The combination of heat and mechanical agitation causes the bitumen to separate from the sand and water.
The conditioned slurry is then introduced into large separation vessels. Here, the components settle by density: heavier sand and mineral solids sink to the bottom. The lighter bitumen, coated with tiny air bubbles, floats to the surface as a layer of bitumen froth. This froth is skimmed off the top for further treatment, while the sand and remaining water are sent to tailings ponds.
In-Situ Extraction Techniques
For deep oil sands deposits, the most widely adopted method is Steam Assisted Gravity Drainage (SAGD). This process involves drilling two parallel horizontal wells into the reservoir, with one positioned four to six meters above the other. High-pressure steam is continuously injected into the upper wellbore, creating a steam chamber that heats the surrounding oil sand.
As the temperature rises, the heat significantly reduces the bitumen’s viscosity, causing it to thin and become mobile. This mobilized bitumen, along with condensed steam water, flows downward due to gravity into the lower production well. The fluid is then continuously pumped to the surface from this lower well, allowing for sustained production.
A less common but related in-situ method is Cyclic Steam Stimulation (CSS), sometimes called “huff and puff.” In CSS, steam is injected into a single wellbore, allowed to soak to heat the bitumen, and then the same well is used for production. SAGD is favored because its continuous injection and production cycle results in higher, more sustained production rates and achieves greater ultimate recovery compared to CSS.
Preparing Raw Bitumen for Transport
The raw bitumen recovered from either surface mining or in-situ methods cannot be transported directly through pipelines due to its highly viscous nature. Immediately after extraction, the bitumen is subjected to initial treatment to remove residual water, sand, and fine clay particles. This cleaning process purifies the raw product before transportation.
To meet pipeline specifications, the treated bitumen must be blended with lighter hydrocarbons, known as a diluent, such as natural gas condensate. The resulting mixture, often called “dilbit” (diluted bitumen), is fluid enough to be pumped through long-distance pipelines to upgraders or refineries. This preparation step is a physical modification for transport.