Among the world’s largest reserves of oil, Canada’s oil sands are also among the most difficult sources of oil to tap. The 20 per cent or so of the oil sands that are relatively easy to mine require the clearing of huge tracts of land and the construction of vast tailings ponds. The remaining 80 per cent of the resource is locked up deep underground and accessible only by drilling. These in situ deposits are so technically challenging to develop that until recently, few considered it worth the cost or effort.
Due to a combination of several factors that include the advancement of technology and the favourability of oil prices, expensive but efficient methods like steam-assisted gravity drainage (SAGD) are now being used to extract the oil from in situ deposits. The challenges, however, remain daunting.
Canadian-based Norwest Corporation provides technical consultation services, including for SAGD, to resource industry projects around the world. Integrating competencies in mining, oil and gas, hydrology and environmental processes, Norwest provides comprehensive services aimed at optimizing every stage of project development in four principal areas: geology; engineering; water resources; and environmental and management consulting.
Horizontal SAGD wells
Full steam below
SAGD was conceived by Roger Butler, an engineer for Imperial Oil, around 1969 and finally developed in 1975. As John Campanella, president, oil and gas services at Norwest described it: “SAGD is an enhanced oil recovery technology for producing heavy crude oil and bitumen. It is an advanced form of steam stimulation in which a pair of horizontal wells is drilled into the oil reservoir, one a few metres above the other. Steam is continuously injected into the upper wellbore to heat the oil and reduce its viscosity, causing the heated oil to drain into the lower wellbore, where it is lifted out.”
As seen in the figures, two parallel horizontal oil wells are drilled in the formation, one about four to six metres above the other. The upper well injects steam, possibly mixed with solvents, and the lower one collects the heated crude oil or bitumen that flows out of the formation, along with any water from the condensation of injected steam. “The injected steam forms a steam chamber that grows vertically and horizontally in the formation,” Campanella elaborated. The heat from the steam reduces the viscosity of the heavy crude oil or bitumen and allows it to flow down into the lower wellbore. The steam and gases rise because of their low density compared to the heavy crude oil below, ensuring that steam is not produced at the lower production well.
Campanella went on to explain that the steam and gases released (usually methane, carbon dioxide and some hydrogen sulphide) tend to rise in the steam chamber and fill the empty space left by the oil. Oil and water flow by a counter current with gravity-driven drainage running into the lower well bore. Finally, said Campanella, “the condensed water and bitumen are transported to the surface using a variety of production methods, such as submersible pumps specifically designed to handle the high-temperature fluids.”