{{Distinguish|text=[[Oil in place|Stock tank original oil-in-place]] or [[Oil in place|Stock tank oil-initially-in-place]]}}

{{more footnotes|date=February 2016}}

'''Stock tank oil''' is the volume of oil after flashing to nominal atmospheric (or other stated) storage pressure and temperature (as opposed to reservoir conditions).

== Background == Crude oil is a complex mixture of many individual compounds which influence the physical properties of the oil, such as its density ([[API gravity]]), [[viscosity]], [[Permeability (Earth sciences)|Permeability]], [[Dew point]], Formation Volume, etc.. The properties also influence how the oil is classified and its value.<ref name=":0">{{Cite web |title=Stock Tank Oil |url=https://www.sciencedirect.com/topics/engineering/stock-tank-oil |access-date=29 November 2023}}</ref>

The quantity of oil originally in place in a reservoir is usually expressed as Stock Tank Oil Originally In Place (STOOIP), this is usually given in millions of stock tank barrels (MMstb).<ref>{{Cite web |title=Reservoir engineering for geologists |url=http://discoverygeo.com/Papers/Reservoir%20Eng%20for%20Geos%203.pdf |access-date=2023-12-13 |archive-date=2013-12-28 |archive-url=https://web.archive.org/web/20131228154637/http://discoverygeo.com/Papers/Reservoir%20Eng%20for%20Geos%203.pdf |url-status=bot: unknown }}</ref>

== Processing == Crude oil from subterranean reservoirs is usually produced with associated gas, produced water and solids such as sand. The crude oil must be stabilised to prevent excessive gassing during storage and to remove water and solids.<ref name=":1">{{Cite web |title=Crude oil stabilization Process |url=https://upcommons.upc.edu/bitstream/handle/2117/179600/TFG%20-%20AMIRUL%20ASYRAFF%20BIN%20OTHMAN.pdf?sequence=1&isAllowed=y |access-date=29 November 2023}}</ref> Stabilisation involves passing the wellhead fluids through one, two, or three stages of separation operating at successively lower pressure to flash off lighter [[hydrocarbons]] and to remove water and solids. For a three stage train the operating pressures of the high, intermediate and low pressure separators might typically be:<ref name=":2">{{Cite book |last=Arnold |first=K. |title=Surface Production Operations Vol. 2 chapter 6 |last2=Stewart, Mauice |publisher=Elsevier |year=1999}}</ref>

* HP separator: 1200 psig * IP separator: 200 psig * LP separator: 50 psig * Stock tank: 2 psig

The vapor pressure and relative volatility of constituent compounds is:<ref name=":2" /> {| class="wikitable" !Component !Vapor pressure at 100&nbsp;°F, psia !Relative volatitity |- |C1 (methane) |5000 |96.9 |- |C2 (ethane) |800 |15.5 |- |C3 |190 |3.68 |- |iC4 |72.2 |1.40 |- |nC4 |51.6 |1.00 |- |iC5 |20.4 |0.40 |- |nC5 |15.6 |0.30 |- |C6 |5.0 |0.10 |- |C7+ |0.1 |0.00 |- |CO<sub>2</sub> |<nowiki>-</nowiki> |infinite |- |N<sub>2</sub> |<nowiki>-</nowiki> |infinite |- |H<sub>2</sub>S |394 |7.64 |}

Alternatively, plant may be arranged with each stage operating at successively higher temperature.<ref name=":2" />

The product is dead crude which is stored in stock tanks operating at just above atmospheric pressure.<ref name=":1" /> Offshore production also produces a dead crude but is often spiked with [[Natural-gas condensate|natural gas liquids]] (NGL) as a convenient transport route for these liquids. This is known as live crude. The NGL is flashed off and recovered in the onshore separation plant. The operating pressure and temperature of the separation plant may be specified such that the oil meets a required [[vapor pressure]], such as [[Reid Vapor Pressure]].<ref name=":1" />

== Properties == Density and viscosity are important property inputs into correlations for pipe-flow calculations. Stack Tank Oil (STO) density (or API) may also be used by regulatory bodies to classify oil and oil products. Other properties such as [[Molecular mass|Molecular Weight]], saturated aromatic resin and asphaltene (SARA), [[refractive index]], wax appearance temperature, [[asphaltene]] precipitation, and [[Acid value|acid number]], are also specified at Stock Tank conditions.<ref name=":0" /> Typically stock tank conditions are 14.7 psia and 60&nbsp;°F (101,325 Pa and 16&nbsp;°C).Flowrates of oil are commonly specified at Stock Tank conditions, e.g: Stock Tank barrels of oil per day STBOPD.

==See also== * [[Oil in place]]

==References== {{reflist}} *[http://www.oilgasglossary.com/stock_tank_oil.html www.oilgasglossary.com/stock_tank_oil.html]

[[Category:Petroleum industry]] [[Category:Petroleum production]]

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