Page 296 - Hydrocarbon Exploration and Production Second Edition

Surface Facilities 283

wet gas gas to
feed pipeline
heat
glycol exchanger
injection

Joule
Thomson
valve condensate

Figure 11.16 Low temperature separation (LTS).

H 2 S is extremely toxic even in very small amounts (less than 0.01% volume can be
fatal if inhaled). Because of the equipment required, extraction is performed
onshore whenever possible, and providing gas is dehydrated, most pipeline
corrosion problems can be avoided. However, if third party pipelines are used, it
may be necessary to perform some extraction on site prior to evacuation to meet
pipeline owner specifications. Extraction of CO 2 and H 2 S is normally performed by
absorption in contact towers like those used for dehydration, though other solvents
are used instead of glycol.
Historically, CO 2 removed from hydrocarbon gas was vented, in most cases it
still is, but since CO 2 is increasingly associated with global climate change more
companies are making efforts to capture and store the gas. Amine, an organic
compound derived from ammonia, can be used both as a CO 2 and H 2 S absorber in
contact towers. The CO 2 - and/or H 2 S-rich amine output stream is subsequently
heated and de-pressurised so that the CO 2 and H 2 S gas is boiled off and can be
injected into, for example, a depleted gas reservoir or aquifer.

11.1.3.5. Pressure elevation (gas compression)
After passing through several stages of processing, gas pressure may need to be
increased before it can be evacuated, used for gas lift or re-injected. If gas flowing
from wells at a low wellhead pressure requires processing at higher separator
pressures, inter-stage compression may be required.
The main types of compressor used in the gas industry are reciprocating and
centrifugal compressors. The power requirements of a reciprocating compressor are
shown in Figure 11.17. Notice that at compression ratios above 5 and 25, two-stage
and three-stage compression, respectively, becomes necessary to accommodate
inter-stage cooling. Apart from the need for inter-stage cooling, additional
compression capacity may be installed in phases through the life of a gas field as
reservoir pressure declines.
Gas turbine driven centrifugal compressors are very efficient under the right
operating conditions but require careful selection and demand higher levels of