Page 22 - Oil and Gas Production Handbook An Introduction to Oil and Gas Production

thinner) the crude. This means, put simply, that the lower the degrees API,
the denser (heavier, thicker) the crude. Crude from different fields and from
different formations within a field can be similar in composition or be
significantly different.

In addition to API grade and hydrocarbons, crude is characterized for other
undesired elements like sulfur etc, which is regulated and needs to be
removed.

Crude oil API gravities typically range from 7 to 52 corresponding to about
3
3
970 kg/m to 750 kg/m , but most fall in the 20 to 45 API gravity range.
Although light crude (i.e. 40-45 degrees API) is considered the best, lighter
crude (i.e., 46 degree API and above) is generally no better for a typical
refinery. As the crude gets lighter than 40-45 degrees API, it contains shorter
molecules, which means a lower carbon number. This also means it contains
less of the molecules useful as high octane gasoline and diesel fuel, the
production of which most refiners try to maximize. If a crude is heavier than
35 degree API, it contains longer and bigger molecules that are not useful as
high octane gasoline and diesel fuel without further processing.

For crude that has undergone detailed physical and chemical property
analysis, the API gravity can be used as a rough index of the quality of
crudes of similar composition as they naturally occur (that is, without
adulteration, mixing, blending, etc.). When crudes of a different type and
quality are mixed, or when different petroleum components are mixed, API
gravity cannot be used meaningfully for anything other than a measure of the
density of the fluid.

For instance, consider
a barrel of tar that is
dissolved in 3 barrels
of naphtha (lighter
fluid) to produce 4
barrels of a 40 degree
API mixture. When
this 4-barrel mixture is
fed to a distillation
column at the inlet to
a refinery, one barrel
of tar plus 3 barrels of
naphtha is all that will
come out of the still.
On the other hand, 4

20