Page 127 - Compression Machinery for Oil and Gas

114 SECTION II Types of Equipment

range of conditions with a high efficiency (Fig. 3.73). The machines are typi-
cally variable speed machines. In many instances, multiple trains are installed in
compressor station.

High Pressure
Injection
Gas injection is used to improve oil recovery (Fig 3.74). This can, in some cases,
more than double the amount of recoverable oil. The gas used for injection is
typically natural gas, N 2 ,orCO 2 . In the case of gas reinjection, well gas accom-
panies the oil to the surface, gets separated from the oil and is reinjected back
into the reservoir. The composition of the well gas is mostly made up of natural
gas along with other gases and impurities. Under the best circumstances, the
other gases and impurities are trace amounts and reasonably inert. Under the
worst circumstances, the other gases and impurities are a significant fraction
and both highly toxic and corrosive. On top of that, both the reservoir conditions
and the gas composition can change overtime and the compressor operation and
control need to adjust accordingly.
In 1974, the highest pressure gas reinjection application was at the Phillips
Ekofisk oil field [25]. The Ekofisk compressor was designed for 634bar
(9200psia) discharge including a discharge gas density of 318.8kg/m 3
3
(19.9lb m /ft ). The maximum pressure achieved in the field for this application
was 614bar (8900psia). Such opportunities do not come along very often, and
the Ekofisk compressor would remain the highest pressure application for over
two decades. That milestone has been eclipsed a few times, most recently on the
Kashagan sour gas project, which achieved a discharge pressure of 828bar

FIG. 3.74 Reinjection compressor used for oil recovery. (From P.C. Rasmussen, R. Kurz, Cen-
trifugal compressor applications-upstream and midstream, in: Proc. 38th Turbomachinery Sympo-
sium, Houston, TX, 2009, Figure 16.)