Page 273 - Hydrocarbon Exploration and Production Second Edition

260 Subsea vs. Platform Trees

fluid and lift gas mixes and flows to surface, where some of the gas is recycled. No
gas is ‘consumed’ by gas lift, although the compressor will require power – usually
from fuel gas.
Such a system, although simple downhole, does require a high gas pressure to get
started when the column of liquid inside the completion is dense. This high pressure
can be supplied by a dedicated kick-off compressor. A more common solution
however is the use of unloading valves situated in gas lift mandrels distributed down
the completion. These valves are initially open and allow shallow gas lift. As the
column of liquid in the completion is lightened and either the tubing or the casing
pressure reduces, the valves automatically close and thus push the lift gas
progressively deeper down the well.
The performance of a gas lift system is monitored by observing injection and
produced fluid flowrates and the casing and tubing pressures. Diagnostic surveys such
as production logging can also be undertaken to determine if any valves are not
working properly; if need be they will be replaced. Optimising the amount of gas
going to each well is also critical. Too much lift gas and the system will
underperform due to increased friction. Too little gas and the reservoir fluids will not
be lightened enough.
An alternative to continuous gas lift is intermittent gas lift. The equipment
needed for intermittent gas lift is similar to that needed for continuous gas lift, but
the operating principle is different. Whereas in a smoothly operating continuous gas
lift system the gas is dispersed in the liquid, intermittent gas lift relies on a finite
volume of gas lifting a liquid column to surface at regular intervals, as piston-like
as possible. The lift gas can be separated from the oil by a plunger; plunger assisted gas
lift. This has proven more efficient in viscous crude oils or in crude oils prone
to emulsions. The performance of the system is again monitored by observing
the casing and tubing pressures.
Figure 10.31 provides an overview of the application envelope and the respective
advantages and disadvantages of the various artificial lift techniques.
As can be seen, only a few methods are suited for high rate environments: gas
lift, ESPs and HSPs. Beam pumps are generally unsuited to offshore applications
because of the low rate and bulk of the required surface equipment. Whereas the
vast majority of the world’s artificially lifted wells are beam pumped, the majority of
these are stripper wells producing less than 10 bpd.

10.9. Subsea vs. Platform Trees

With an offshore development, it is often possible to develop the field using
subsea technology and put the Christmas trees on the seabed – hence the term ‘wet’
trees. Alternatively a platform can be constructed with the trees on the platform –
‘dry’ trees. In any offshore development, this is a big decision. It is sometimes viewed
as simply a capital cost analysis. For a small development in deep water, subsea
development to either a host platform or a floating production vessel will be
cheaper. For larger well numbers, or for shallower water, the lower individual well
cost of a platform will promote the use of fixed platform. However, capital costs are