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] meaning that its specific gravity is greater than 0.933. This mostly results from crude oil getting degraded by being exposed to bacteria, water or air resulting in the loss of its lighter fractions while leaving behind its heavier fractions.
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 distinguish heavy crudes from lighter ones 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)[citation needed] and 10° API gravity [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 oil although bitumen does not flow at ambient conditions.
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Economics
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. On one hand, due to increased refining costs and high sulfur content, 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[5] (often less than 3000 feet) contributes to lower production costs; however, these are offset by the difficulties of production and transport that render conventional production methods ineffective[5]. Specialized techniques are being developed for exploration and production of heavy oil.
Extraction
Production of heavy oil is becoming more common in many countries, with 2008 production led by Canada and Venezuala[5]. 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 tar-rich deposits.
Environmental impact
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 3.5% sulfur as well as vanadium and nickel. [4] Heavy crude oils contain more carbon in relation to hydrogen[dubious ], thus releasing more carbon dioxide (a greenhouse gas) per amount of usable energy when burned[dubious ]. 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 tar sands generates as much as three times the total CO2 emissions compared to conventional oil[6], 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.
Geological Origin
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[5] (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.
Chemical properties
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 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.[7]
There are two main types of heavy crude oil:
1) Those that have over 1% sulfur (high sulfur crude oils) with aromatics and asphaltenes and these are mostly found in North America (Canada: Alberta, Saskatchewan; United States: California; Mexico), South America: (Venezuela, Colombia and Ecuador) and the Middle East: Kuwait, Saudi Arabia.
2) Those that have less than 1% sulfur (low sulfur crude oils) with aromatics, naphthenes and resins and these are mostly found in Western Africa: Chad, Central Africa: Angola and East Africa: Madagascar.
References
- ^ a b Dusseault, M.B. (June 12-14, 2001). "Comparing Venezuelan and Canadian Heavy Oil and Tar Sands" (PDF). Calgary, Canada: Canadian International Petroleum Conference.
- ^ Energy Information Administration (2001) "Venezuela Offers Full Market Value to Encourage Foreign Investment in Oil" [1]
- ^ M. Talwani, "The Orinoco heavy oil belt in Venezuela (Or heavy oil to the rescue?)" [2]
- ^ Rodriguez H. A., Vaca P., Gonzalez O., and De Mirabal M. C., "Integrated study of a heavy oil reservoir in the Orinoco Belt : A field case simulation" [3]
- ^ a b c d The Leading Edge, Special Section - Heavy Oil. Vol. 27, No. 8. September, 2008. Society of Exploration Geophysics.
- ^ J.R. Century. Tar Sands: Key geologic risks and opportunities. The Leading Edge, Vol. 27, No. 9, Pg. 1202-1204. September 2008.
- ^ "What causes heavy oil if they don't have asphaltene or paraffin problems?" - Chevron Phillips Chemical
See also
External links
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