Page 123 - Compression Machinery for Oil and Gas

110 SECTION II Types of Equipment

In some gas fields, the incoming gas is cascaded to a lower-pressure level to
separate the liquids from the gas. This also leads to gas available at different
pressure levels. In other instances, the gas compression is straight through,
without any sidestreams.
Natural gas is thus either a byproduct of oil production (“associated gas”), or
the main product to be produced. In oil production, well fluid is processed in a
gas-oil separation system where its pressure is reduced in several stages. In each
decompression stage, the associated gas (also called flash gas) is released in a
separator until the pressure is ultimately reduced to slightly above atmospheric
pressure. Flash gas is compressed to about 8–10MPa. The compression is in
multiple stages, with sidestreams added the crude oil is then sent to a stabilizer
column where it is heated and cascaded through a series of bubble trays spaced
throughout the column. Hydrogen sulfide (if present) and remaining light
hydrocarbons boil off in this process and are collected at the top of the column,
while the sweetened heavy crude is drawn off from the bottom. The stabilized
oil is then cooled and stored. The streams collected from the top of the stabilizer
unit are treated in accordance with environmental regulations.
The associated gas produced can be used in three different ways: it can be
exported, that is, sent via a pipeline, which runs subsea in case of offshore pro-
duction to a gas treatment facility—the gas plant. Gas export compressors will
compress the associated gas to pressures of 10–24MPa, and then feed a pipeline
to transport the gas to the gas plant.
Another use is for efforts to enhance the oil recovery. Even today, with
advanced technologies, in some reservoirs, more than two-thirds of the oil pre-
sent may not be recoverable. For enhanced oil recovery (EOR), the gas can
either be compressed, or injected in the well (gas lift), or it can be compressed
into the reservoir (gas reinjection).
Gas lift compressors bring the gas to pressures of 6–12MPa, although some-
times 20MPa may be required. It is then injected into the oil well, where it
increases the downhole pressure, thus enhancing the flow from the reservoir
into the well. Gas reinjection compressors have to deliver gas at pressures high
enough to overcome the pressure in the reservoir, thus requiring discharge pres-
sures ranging from 10 to over 80MPa discharge pressure. Recent material tech-
nology advances allow associated sour gases containing high percentages of
H 2 S and/or CO 2 to be reinjected without the need for sweetening.
The pressure ratio in flash gas compression, gas lift, gas export, and reinjec-
tion applications is usually so high that the gas has to be intercooled during the
compression process.
For offshore applications, the services described above are installed on plat-
forms (fixed leg for shallow waters, floating for deeper waters). Potentially,
these services can be located on the seafloor instead. The development of subsea
compressor packages is a current challenge, and there are no subsea compres-
sors in commercial production at this time. Challenges involve the issue of sep-
aration of gas solids and liquids close to the well (or, conversely, the capability