Page 18 - Oil and Gas Production Handbook An Introduction to Oil and Gas Production

2.2.4 Gas compression

Gas from a pure natural gas wellhead might have sufficient pressure to feed
directly into a pipeline transport system. Gas from separators has generally
lost so much pressure that it must be recompressed to be transported.
Turbine driven compressors gain their energy by using a small proportion of
the natural gas that they compress. The turbine itself serves to operate a
centrifugal
compressor,
which contains a
type of fan that
compresses and
pumps the natural
gas through the
pipeline. Some
compressor
stations are
operated by using
an electric motor
to turn the same
type of centrifugal
compressor. This
type of
compression does not require the use of any of the natural gas from the
pipe; however it does require a reliable source of electricity nearby. The
compression includes a large section of associated equipment such as
scrubbers (to remove liquid droplets) and heat exchangers, lube oil
treatment etc.

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 (H2S), 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 make-up of the natural gas that is allowed into the
pipeline. That means that before the natural gas can be transported it must
be purified.

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