February 19, 2013
The oil sands of Alberta are unconsolidated, held together by the pore-filling bitumen. The bitumen is a natural, tar-like mixture of hydrocarbons, that when heated has a consistency of molasses. In its natural state, bitumen (density range of 8° to 12° API; at room temperature viscosity >50,000 centipoises) will not flow to a wellbore. In Alberta other heavy oil in sand is also considered as 'oil sands' if located within the oil sands application areas. However because the pore-fluid is heavy oil and will flow to a well, these deposits are referred to as 'primary in-situ crude bitumen.' The major challenge of recovering bitumen from depth is to overcome its high viscosity to allow it to flow to the wellbore. To do this, thermal (or other non-primary) in-situ methods are used, most commonly Cyclic Steam Stimulation (CSS) and Steam Assisted Gravity Drainage (SAGD).
Canada's largest in-situ bitumen recovery project uses CSS at Cold Lake. Steam injected down the wellbore into the reservoir heats the bitumen, followed by a soak time, and then the same wellbore is used to pump up fluids (Fig. 3A). At Cold Lake, about 3200 wells are currently operating from multiple pads, with two above ground pipelines, one to deliver steam and the other to transport fluids back to the processing plant. At Athabasca, the SAGD technology is used. Horizontal well pairs (700 metres long with 5-metre vertical separation) are drilled from surface pads to intersect bitumen pay. Steam from the upper injector well expands, reducing the viscosity of the bitumen, allowing the bitumen to flow. A shell forms at the cold interface with the unheated reservoir, along which heated bitumen/condensate drain by gravity to the lower producing well. Locally electrical submersible pumps (ESPs) may assist in lift.
Continuing challenges for economic in-situ bitumen recovery involve water and gas requirements for steam generation, reclamation and emission controls of greenhouse gases. Generally, it takes 28 m3 (1000 ft3 ) of natural gas and from 2.5 to 4 barrels of water to produce one barrel of bitumen. Reclamation of mining sites is done to a standard to at least the equivalent of their previous biological productivity. Beginning in the mid 1970s, the North American energy crises have made the Canadian oil sands a more strategic resource for North American energy needs, accelerating industry's interest and efforts to tap these vast bitumen reserves.