Heavy crude oil or extra heavy crude oil is any type of crude oil which does not flow easily. It is referred to as "heavy" because its density or specific gravity is higher than that of light crude oil. Heavy crude oil has been defined as any liquid petroleum with an API gravity less than 20°.[1] Extra heavy oil is defined with API gravity below 10.0 °API (i.e. with density greater than 1000 kg/m3 or, equivalently, a specific gravity greater than 1). With a specific gravity of greater than 1, extra heavy crude is present as a dense non-aqueous phase liquid in ambient conditions.
Production, transportation, and refining of heavy crude oil present special challenges compared to light crude oil. The largest reserves of heavy oil in the world are located north of the Orinoco river in Venezuela,[2] the same amount as the conventional oil reserves of Saudi Arabia,[3] but 30 or more countries are known to have reserves. Heavy crude oil is closely related to oil sands, the main difference being that oil sands generally do not flow at all. Canada has large reserves of oil sands, located north and northeast of Edmonton, Alberta.
Physical properties that differ between heavy crudes lighter grades include higher viscosity and specific gravity, as well as heavier molecular composition. Extra heavy oil from the Orinoco region has a viscosity of over 10,000 centipoise (10 Pa·s) and 10 °API.[4] Generally, a diluent is added at regular distances in a pipeline carrying heavy crude to facilitate its flow.
Some petroleum geologists categorize bitumen from oil sands as extra heavy crude oil although due to the density of less than 10 °API. Other classifications label this as bitumen. Often, bitumen is present as a solid and does not flow at ambient conditions.
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Heavy crude oils provide an interesting situation for the economics of petroleum development. The resources of heavy oil in the world are more than twice those of conventional light crude oil. In October 2009, the USGS updated the Orinoco tar sands (Venezuela) recoverable value to 513 billion barrels (8.16×1010 m3),[5] making this area the world's largest recoverable oil deposit, ahead of Saudi Arabia and Canada.
On one hand, due to increased refining costs and high sulfur content for some sources, heavy crudes are often priced at a discount to lighter ones. The increased viscosity and density also makes production more difficult (see reservoir engineering). On the other hand, large quantities of heavy crudes have been discovered in the Americas including Canada, Venezuela and California. The relatively shallow depth of heavy oil fields[6] (often less than 3000 feet) can contribute to lower production costs; however, these are offset by the difficulties of production and transport that render conventional production methods ineffective.[6] Specialized techniques are being developed for exploration and production of heavy oil.
Production of heavy oil is becoming more common in many countries, with 2008 production led by Canada and Venezuela.[6] Methods for extraction include Cold heavy oil production with sand, steam assisted gravity drainage, cyclic steam stimulation, vapor extraction, Toe-to-Heel Air Injection (THAI), and open-pit mining for extremely sandy and oil-rich deposits.
With current production and transportation methods, heavy crudes have a more severe environmental impact than light ones. With more difficult production comes the employment of a variety of enhanced oil recovery techniques, including steam flooding and tighter well spacing, often as close as one well per acre. Heavy crudes also carry contaminants. For example, Orinoco extra heavy oil contains 4.5% sulfur as well as vanadium and nickel.[7] Heavy crude oils contain more carbon in relation to hydrogen, thus releasing more carbon dioxide (a greenhouse gas) per amount of usable energy when burned. However, because crude oil is refined before use, generating specific alkanes via cracking and fractional distillation, this comparison is not valid in a practical sense. Heavy crude refining techniques may require more energy input though, so its environmental impact is presently much more significant than that of lighter crude.
With present technology, the extraction and refining of heavy oils and oil sands generates as much as three times the total CO2 emissions compared to conventional oil,[8] primarily driven by the extra energy consumption of the extraction process (which may include burning natural gas to heat and pressurize the reservoir to stimulate flow). Current research in to better production methods seek to reduce this environmental impact.
In a 2009 report, the National Toxics Network, citing data provided by the Carbon Dioxide Information Analysis Center of the government of the United States and the Canadian Association of Petroleum Producers (CAPP), found that heavy oils can have higher CO2 emissions per ton than coal. Emissions were lower than coal on a "per unit of energy produced" basis, at about 84% of those for coal (0.078/0.093) and thus higher on this basis of CO2 emissions, than conventional oil.[9]
Environmental Research Web has reported that "because of the energy needed for extraction and processing, petroleum from Canadian oil tar sands has higher life cycle emission" versus conventional fossil fuels; "up to 25% more."[10]
Most geologists agree that crude becomes "heavy" as a result of biodegradation, in which lighter ends are preferentially consumed by bacterial activity in the reservoir, leaving heavier hydrocarbons behind. This hypothesis leans heavily on the techniques of petroleum geochemistry. Poor geologic reservoir sealing exposes the hydrocarbon to surface contaminants, including organic life (such as bacteria) and contributes to this process.
Heavy oils can be found in shallow, young reservoirs, with rocks from the Pleistocene, Pliocene, and Miocene[6] (younger than 25 million years). In some cases, it can also be found in older Cretaceous, Mississippian, and Devonian reservoirs. These reservoirs tend to be poorly sealed, resulting in heavy oil and oil-sands.
Heavy oil is asphaltic and contains asphaltenes and resins. It is "heavy" (dense and viscous) due to the high ratio of aromatics and naphthenes to paraffins (linear alkanes) and high amounts of NSO's (nitrogen, sulfur, oxygen and heavy metals). Heavy oil has a higher percentage of compounds with over 60 carbon atoms and hence a high boiling point and molecular weight. For example, the viscosity of Venezuela's Orinoco extra-heavy crude oil lies in the range 1000–5000 cP (1–5 Pa·s), while Canadian extra-heavy crude has a viscosity in the range 5000–10,000 cP (5–10 Pa·s), about the same as molasses, and higher (up to 100,000 cP or 100 Pa·s for the most viscous commercially exploitable deposits).[1] A definition from the Chevron Phillips Chemical company is as follows:
The "heaviness" of heavy oil is primarily the result of a relatively high proportion of a mixed bag of complex, high molecular weight, non-paraffinic compounds and a low proportion of volatile, low molecular weight compounds. Heavy oils typically contain very little paraffin and may or may not contain high levels of asphaltenes.[11]
There are two main types of heavy crude oil: