Page 124 - Compression Machinery for Oil and Gas

Centrifugal Compressors Chapter 3 111

of the compressor to compress gas with significant amounts of entrained liq-
uids), as well the design of compressor motors and drive systems to survive
for extended periods of time on the seafloor [24].
The production pattern for most wells follows a pattern where production
will increase for a short period, then peak and follow a long, slow decline.
The shape of this decline curve, how high the production peaks, and the length
of the decline are all driven by reservoir conditions. The decline curve can be
influenced by cleaning out the wellbore to help oil or gas move more easily to
the surface. This is done by fracturing or treating the reservoir rock with acid
around the bottom of the wellbore to create better pathways for the oil and gas to
move through the subsurface to the producing well, or by drilling additional
wells or by employing EOR techniques. This also means that the operating con-
ditions of the compression equipment will change overtime.
The raw natural gas is treated in a gas plant to created marketable products.
Gas plants are designed to produce dry export gas (i.e., gas with very little water,
a low hydrocarbon dewpoint, limited amounts of CO 2 , and other contaminants)
and liquefied petroleum gas (LPG) products (ethane, propane, and butane). For
the range of gas compositions at the inlet, the plants have specified recovery
targets for the heavier hydrocarbons. The process steps inside the plant include:
primary separation,
front-end compression (boost compression, inlet compression),
carbon dioxide removal,
mercury/chloride removal,
gas dehydration,
gas expansion (turboexpander),
LPG/condensate fractionation,
dry (sales) gas compression,
storage, and
utilities.
In a gas plant, several compression duties have to be covered:
- Boost compression (inlet compression) to bring the gas from delivery pres-
sure (from the gas gathering system) to plant pressure.
- Recompression (sales gas compressor) to bring the natural gas from plant
pressure to pipeline pressure. This duty may also be referred to as pipeline
head station (essentially depending on whether the compressor is operated
by the gas plant or the pipeline operator).
- Turboexpander/compressor for the low-temperature cryogenic cycle.
After treatment in the gas plant, and recompression in a sales gas compressor, or
in a head station, the gas is fed into a transmission pipeline.
Natural gas is almost always transported through pipelines, except in cases
where a pipeline cannot economically be built. In that case, the gas can be liq-
uefied (LNG) and transported on a ship.