Page 23 - Oil and Gas Production Handbook An Introduction to Oil and Gas Production

barrels of a naturally occurring 40 degree API South Louisiana Sweet crude
fed to the distillation column at the refinery, could come out of the still as 1.4
barrels of gasoline and naphtha (typically C 8H 18), 0.6 barrels of kerosene (jet
fuel C 12-15 ), 0.7 barrels of diesel fuel (average C 12H 26), 0.5 barrels of heavy
distillate (C 20-70), 0.3 barrels of lubricating stock, and 0.5 barrels of residue
(bitumen, mainly poly-cyclic aromatics).

The figure above to the right illustrates weight percent distributions of three
different hypothetical petroleum stocks that could be fed to a refinery with
catalytic cracking capacity. The chemical composition is generalized by the
carbon number which is the number of carbon atoms in each molecule -
C nH 2n+2. A medium blend is desired because it has the composition that will
yield the highest output of high octane gasoline and diesel fuel in the
cracking refinery. Though the heavy stock and the light stock could be mixed
to produce a blend with the same API gravity as the medium stock, the
composition of the blend would be very different from the medium stock, as
the figure indicates. Heavy crude can be processed in a refinery by cracking
and reforming that reduces the carbon number to increase the high value
fuel yield.

3.1.2 Natural gas

The natural gas used by consumers is composed almost entirely of
methane. However, natural gas found at the wellhead, although still
composed primarily of methane, is not pure. Raw natural gas comes from
three types of wells: oil wells, gas wells, and condensate wells.

Natural gas that comes from oil wells is typically termed 'associated gas'.
This gas can exist separate from oil in the formation (free gas), or dissolved
in the crude oil (dissolved gas). Natural gas from gas and condensate wells,
in which there is little or no crude oil, is termed 'non-associated gas'.

Gas wells typically produce raw natural gas only. However condensate wells
produce free natural gas along with a semi-liquid hydrocarbon condensate.
Whatever the source of the natural gas, once separated from crude oil (if
present) it commonly exists in mixtures with other hydrocarbons, principally
ethane, propane, butane, and pentanes. In addition, raw natural gas
contains water vapor, hydrogen sulfide (H 2S), carbon dioxide, helium,
nitrogen, and other compounds.

Natural gas processing consists of separating all of the various
hydrocarbons and fluids from the pure natural gas, to produce what is known
as 'pipeline quality' dry natural gas. Major transportation pipelines usually
impose restrictions on the composition of the natural gas that is allowed into

21