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A barrel is one of several units of volume applied in various contexts; there are dry barrels, fluid barrels (such as the UK beer barrel and US beer barrel), oil barrels and so on. For historical reasons the volumes of some barrel units are roughly double the volumes of others; volumes in in common usage range from about 100 litres (22 imp gal; 26 US gal) to 200 litres (44 imp gal; 53 US gal). In many connections the term "drum" is used almost interchangeably with "barrel".
Since medieval times the term barrel as a unit of measure has had various meanings throughout Europe, ranging from about 100 litres to 1000 litres, or more in special cases. The name was derived in medieval times from the French baril, of unknown origin, but still in use, both in French and as derivations in many other languages such as Italian, Polish and Spanish. In most countries such usage is obsolescent, increasingly superseded by SI units. As a result the meaning of corresponding words and related concepts (vat, cask, keg etc) in other languages often refers to a physical container rather than a known measure.
In the international oil market context, however, prices in USD per barrel are commonly used, and the term is variously translated, often to derivations of the Latin/Teutonic root fat (for example vat or Fass).
In other commercial connections barrel sizes such as beer keg volumes also are standardised in many countries.
Some products have a standard weight or volume that constitutes a barrel:
Fluid barrels vary depending on what is being measured and where. In the UK a beer barrel is 36 imperial gallons (43 US gal; 164 L). In the US most fluid barrels (apart from oil) are 31.5 US gallons (26 imp gal; 119 L) (half a hogshead), but a beer barrel is 31 US gallons (26 imp gal; 117 L). The size of beer kegs in the US is based loosely on fractions of the US beer barrel. When referring to beer barrels or kegs in many countries, the term may be used for the commercial package units independent of actual volume, where common range for professional use is 20-60 L, typically a DIN or Euro keg of 50 L.
An oil barrel (abbreviated as bbl) is a unit of volume whose definition has not been universally standardized. In the United States and Canada, an oil barrel is defined as 42 US gallons, which is about 159 liters  or 35 imperial gallons, and it can also be defined in those units, depending on the context. Oil companies that are listed on American stock exchanges typically report their production in terms of volume and use the units of bbl, Mbbl (one thousand barrels), or MMbbl (one million barrels).
Outside of the United States and Canada, volumes of oil are usually reported in cubic metres (m3) instead of oil barrels. More commonly, companies on the European stock exchanges report the mass of oil in metric tonnes. Since different varieties of petroleum have different densities, however, there isn't a single conversion between mass and volume. For example, one tonne of heavy distillates might occupy a volume 256 US gallons (6.1 bbl). In contrast, one tonne of crude oil might occupy 272 gallons (6.5 bbl) and one tonne of gasoline will require 333 gallons (7.9 bbl). Overall, the conversion is usually between 6 and 8 bbl per tonne.
The measurement of an "oil barrel" originated in the early Pennsylvania oil fields. In the early 1860s, when oil production began, there was no standard container for oil, so oil and petroleum products were stored and transported in barrels of different shapes and sizes. Some of these barrels would originally have been used for other products, such as beer, fish, molasses or turpentine. Both the 42-US-gallon barrels (based on the old English wine measure), the tierce (159 litres) and the 40-US-gallon (151.4-litre) whiskey barrels were used. 45-gallon barrels were also in common use. The 40-gallon whiskey barrel was the most common size used by early oil producers, since they were readily available at the time.
The origins of the 42-gallon oil barrel are obscure, but some historical documents indicate that around 1866, early oil producers in Pennsylvania came to the conclusion that shipping oil in a variety of different containers was causing buyer distrust. They decided they needed a standard unit of measure to convince buyers that they were getting a fair volume for their money. They agreed to base this measure on the more-or-less standard 40-gallon whiskey barrel, but, as an additional way of assuring buyer confidence, they added an additional two gallons to ensure that any measurement errors would always be in the buyer's favor, on the same principle as that underlying the baker's dozen and some other long units of measure. By 1872, the standard oil barrel was firmly established as 42 US gallons.
In modern times many different types of oil, chemicals, and other products are transported in steel barrels. In the United States these commonly have a capacity of 55 US gallons and are referred to as such. They are called 210 litre or 200 kg drums outside the United States. In the United Kingdom and its former dependencies a 44 imperial gallon drum is used, even though all those countries now officially use the metric system and the drums are filled to 200 litres. Thus, the 42 US gallon oil barrel is a unit of measure, and is no longer a physical container used to transport crude oil, as most petroleum is moved in pipelines or oil tankers. In the United States, the 55-US-gallon size of barrel as a unit of measure is largely confined to the oil industry, while different sizes of barrel are used in other industries. Nearly all other countries use the metric system. Many oil-producing countries still use the American oil barrel.
The abbreviations Mbbl and MMbbl refer to one thousand and one million barrels, respectively. It is noteworthy that these are derived from the Latin "mille" meaning "thousand". This is distinctly different from the SI convention where "M" stands for the Greek "mega", meaning "million". Outside of the oil industry, the unit Mbbl (megabarrel) can sometimes stand for one million barrels. The "b" may have been doubled originally to indicate the plural (1 bl, 2 bbl), or possibly it was doubled to eliminate any confusion with bl as a symbol for the bale. Some sources assert that "bbl" originated as a symbol for "blue barrels" delivered by Standard Oil in its early days.
Oil wells recover not just oil from the ground, but also natural gas and water. The term barrels of liquids per day (BLPD) refers to the total volume of liquid that is recovered. Similarly, barrels of oil equivalent or BOE is a value that accounts for both oil and natural gas while ignoring any water that is recovered.
Other terms are used when the discussing only oil. These terms can refer to either the production of crude oil at an oil well, the conversion of crude oil to other products at an oil refinery, or the overall consumption of oil by a region or country. One common term is barrels per day (BPD, BOPD, bbl/d, bpd, bd, or b/d) where 1 BPD is equivalent to 0.0292 gallons per minute. One BPD also becomes 49.8 tonnes per year. At an oil refinery, production is sometimes reported as barrels per calendar day (bc/d or bcd), which is total production in a year divided by the days in that year. Likewise, barrels per stream day (BSD or BPSD) is the quantity of oil product produced by a single refining unit during continuous operation for 24 hours. Lastly, the terms mbd and mmbd are sometimes used to denote one thousand or one million barrels per day, respectively. These abbreviations use the Roman numeral "M", which means one thousand, but write it as a lower-cased "m". This is in direct contrast with SI prefixes, where "m" means one-thousandth, "k" means one thousand, and "M" means one million.
The density of oil changes with temperature, so the above conversion is not exactly correct. Since some countries use imperial units while others use SI units, the American Petroleum Institute adopted two different methods for reporting the volume of oil. If volume is to be reported in bbl, then the volume will be measured at 14.696 psi and 60 °F. Likewise, the conditions are 101.325 kPa and 15 °C (or in some cases 20 °C) if the volume will be reported in m3. However, it is noteworthy that bbl and m3 are not exactly comparable. While the pressures of 14.696 psi and 101.325 kPa are exactly equivalent, the temperature 60 °F is equivalent to 15.56 °C. Since the measurement for m3 uses 15.00 °C instead of 15.56 °C, this difference will lead to a small error when converting between bbl and m3.
In addition, the magnitude of this error also depends on the type of oil. For a light oil with an API gravity of 35, warming the oil from 15.00 °C to 60.00 °F (which is 15.56 °C) might increase its volume by about 0.047%. Conversely, a heavy oil with an API gravity of 20 might only increase in volume by 0.039%. If physically measuring the density at a new temperature is not possible, then tables of empirical data can be used to accurately predict the change in density. In turn, this allows maximum accuracy when converting between bbl and m3.
A barrel can technically be used to specify any volume. Since the actual nature of the fluids being measured varies along the stream, sometimes qualifiers are used to clarify what is being specified. In the oil field, it is often important to differentiate between rates of production of fluids, which may be a mix of oil and water, and rates of production of the oil itself. If a well is producing 10mbd of fluids with a 20% water cut, then the well would also be said to be producing 8 thousand barrels of oil a day (mbod).
In other circumstances, it can be important to include gas in production and consumption figures. Normally, gas amount is measured in standard cubic feet or cubic metres for volume (as well as in kg or Btu which don't depend on pressure or temperature). But when necessary, such volume is converted to a volume of oil of equivalent enthalpy of combustion. Production and consumption using this analogue is stated in barrels of oil equivalent per day (boed).
In the case of water injection wells, it is common to refer to the injectivity rate in barrels of water per day (bwd).
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