Page 306 - Hydrocarbon Exploration and Production Second Edition

Surface Facilities 293

lift pumps filtration degasification injection
pumps

to injection
wells
biocides scale
inhibitor

biocide
flocculating antifoam
agent corrosion inhibitor
oxygen scavenger
Figure 11.24 Injection water treatment scheme.

11.2.1.2. Gas injection
Gas can be injected into reservoirs to supplement recovery by maintaining reservoir
pressure or as a means of disposing of gas which cannot be flared under
environmental legislation, and for which no market exists.
Facilities for the treatment and compression of gas have already been described
in earlier sections. However, there are a number of differences in the specifications
for injected gas that differ from those of export gas. Generally, there is no need to
control hydrocarbon dew point as injected gas will get hotter not cooler, but it may
be attractive to remove heavy hydrocarbons for economic reasons. Basic liquid
separation will normally be performed, and due to the high pressures involved it
will nearly always be necessary to dehydrate the gas to avoid water dropout.
Injection gas pressures are usually much higher than lift-gas or gas pipeline
pressures and special care has to be taken to select compressor lubricants that will
not dissolve in high-pressure gas. Such a situation could lead to inadequate
lubrication and may impair well injectivity.

11.2.1.3. Artificial lift
The most common types of artificial lift are gas lift, beam pumping and downhole
pumping, and the mechanics of these systems are described in Section 10.8,
Chapter 10. Gas-lifting systems require a suitable gas source though at a lower
pressure than injection gas. Gas treatment considerations are similar except that
heavy ends are not normally stripped out of the gas, as a lean gas would only
resaturate with NGLs from the producing crude in the lifting operation. Gas
compression can be avoided if a gas source of suitable pressure exists nearby, for
example an adjacent gas field. Since gas lift is essentially a closed-loop system, little
gas is consumed in gas-lifting operations but gas must be available for starting up
operations after a shutdown (‘kicking off’ production). Alternatively, nitrogen
pumped through coiled tubing could be used for kick off, though this is expensive
and may be subject to availability restrictions.