Page 399 - Compression Machinery for Oil and Gas

Upstream Compression Applications Chapter 8 377

FIG. 8.3 Oil production with flash gas, gas injection, and vapor recovery compressors [2].

These postseparation gas streams are typically recompressed and combined
for use as sales gas, gas injection, or gas lift. The stabilized crude oil is sent to
storage/sales. Typical discharge pressures for associated gas compressors are
about 7–10MPa. In many cases, water in the gas is removed via glycol dehy-
dration prior to pipeline transport in order to prevent hydrate formation in the
lines [3]. An example flash-gas compression system is shown in Fig. 8.3, where
the gas is sent to a gas injection compressor.
Flash-gas compressor trains often consist of multiple bodies to accommo-
date the different gas streams. In general, each independent stream needs a con-
trol mechanism in order to allow stable compressor operation in response to
process variations in individual streams. For centrifugal compressors, it is usu-
ally recommended to incorporate a combination of speed control for the highest
flow stream and suction throttling or variable inlet guide vanes for lower-flow
streams since speed control is the most energy efficient control method. Recip-
rocating compressors can operate stages at lower flows via speed control, cyl-
inder deactivation, and variable volume pockets or valve unloaders. Recycling
is a potential control method both centrifugal and reciprocating compressors,
but is not recommended because it does not reduce power consumption at
lower flows.

Nonassociated Gas Compression
Natural gas wells do not produce oil but may be classified as dry wells that pro-
duce only raw natural gas or condensate wells that produce both gas and natural
gas condensate (wet gas). Natural gas condensate is typically composed of
hydrocarbons (propane, butane, pentane, hexane) and impurities (may include
hydrogen sulfide, thiols, carbon dioxide, alkanes, cyclohexane, and aromatics,
e.g., benzene). For many condensate wells, the gas-liquid mixture at the well is