Heavy oil is characterized by a higher viscosity and density than that of conventional crude oil (reservoir viscosity of around 10,000 Mpa-s).  For this reason, extracting and refining heavy oil is much more difficult and costly than lighter conventional oil.  However, the existing reserves of heavy oil far outnumber the existing reserves of conventional oil at 5.6 trillion to 1.02 trillion barrels.  Over 80% of these heavy oil resources are found in Canada, Venezuela, and the United States.  Canada and Venezuela have heavy oil supplies of around 2.5 and 1.5 trillion barrels respectively. Only a 20% recovery rate of these reserves would match the current reserves of conventional oil in the Middle East. Therefore, these supplies could decrease dependence on foreign nations.
Though heavy oil has the potential to greatly extend our oil supply and reduce dependence on foreign countries, the production numbers currently do not match the potential.  Most of this discrepancy between available reserves and actual production is the difficult recovery of the highly viscous and dense heavy oil. Even the heavy oil that flows well at its reservoir temperature would require heated pipelines once brought above the ground for transport, which is only economical at short distances.  The other type of heavy oil that flows little or does not flow at all requires catalytic cracking into smaller hydrocarbons or dilution with smaller hydrocarbons. Without these additional measures, recovery of heavy oil is difficult. For example, in the Venezuelan Orinoco Oil Belt, there exist an estimated 1.7 trillion barrels of heavy oil, where only an estimated 250 billion barrels are recoverable via traditional recovery techniques. Therefore, because of the relatively low recovery rates, much emphasis has been placed on more efficient recovery practices.
Several techniques have been used for the recovery of heavy oil. One of the more recent technologies for heavy oil recovery is called Steam Assisted Gravity Drainage (SAGD).  SAGD works by drilling two parallel, horizontal wells. The upper well is for steam injection and the lower is for oil recovery. The steam heats the heavy oil and allows it to flow by gravity into the lower well, where it is then pumped back up for processing. The counter-current system allows less heat loss and can achieve a recovery of up to 60%.  This is much better than the previously used steam flooding method, where much of the steam fails to reach the oil once it was pumped into a vertical well. Though it has greatly improved the viability of heavy oil usage, SAGD is problematic, like steam flooding, in that it requires a huge energy input. For every barrel of heavy oil produced, 30 cubic meters of natural gas is required for the injected superheated steam and another 15 cubic meters for the addition of hydrogen and removal of impurities necessary for refining. For recovery of conventional oil, the upstream process requires 6% of the energy content extracted. For heavy oil, the number can climb to 25%. 
Other less energy-intensive methods are being explored today. One method is called Vapor Assisted Petroleum Extraction (Vapex).  Vapex operates with a similar setup as SAGD with the horizontal wells, but instead of injecting superheated steam into the upper well, a hydrocarbon vapor at its dew point is injected. When the vapor meets the heavy oil in the reservoir, it condenses, mixes with the oil, where then the liquefied heavy oil and solvent mixture can gravitate toward the lower well and then be pumped to the surface. Mokrys and Butler found, using a propane vapor, that this method is much less energy-intensive than SAGD, with 0.2-0.5 kg propane/kg oil as compared to 3 kg steam/kg oil.  Also, the relative temperature increase of the reservoir with this method ranges from 5-10°C, which is small compared to the 200°C increase with the steam method.] This poses less of risk of reservoir damage and eliminates the environmental effects of water treatment and consumption. Though this process is promising, it still has not achieved economic viability. 
Other methods are currently being researched, but none have been proven economically viable as of today. These include in-situ combustion, which burns oil in the well to provide heat necessary for flow, and microbial degradation, which uses microbes to break down heavy oil into smaller, less viscous molecules.  Nevertheless, heavy oil, as demonstrated by its large reserve numbers, has the potential to sustain the oil supply and reduce dependence on foreign nations until new, economically viable alternative energy technologies are developed and refined in the years to come.
© Benjamin Reinhardt. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 I. J. Mokrys and R. M. Butler, "In Situ Upgrading of Heavy Oils and Bitumen by Propane Deasphalting: The Vapex Process," Society of Petroleum Engineers, 25452-MS, 21 Mar 93.
 Resources to Reserves: Oil and Gas Technologies for the Energy Markets of the Future, International Energy Agency, 2005.
 F. J. Hein, "Heavy Oil and Oil (Tar) Sands in North America: An Overview & Summary of Contributions," Natural Resources Research, 15, 67 (2006).