Page 260 - Hydrocarbon

Well Dynamic Behaviour 247

shaped charges

detonating cord

carrier

each charge points
in a different direction
from its neighbours

Figure 10.19 Perforating gun.

can therefore be selective, but often suffer from poor productivity. In order to
increase productivity, the gravel may be pumped at high pressure and the reservoir
fractured – the so-called frac and pack or frac-pack. This aids in packing the
perforations with gravel and provides some stimulation benefit. This technique can
be complex, but is used to particularly good effect offshore in the GoM.
The upper completion also covers a variety of techniques. Figure 10.20 shows
four common methods. The inflow shown here is for a cased and perforated
completion, but similar techniques exist for the other reservoir completion options.
The simplest method consists of producing straight up the casing with no
tubing. It is cheap, can have a large flow area, but it is difficult to control, may
corrode the casing and in most situations is considered unsafe due to a lack of
barriers in the event of a problem. The second option, although at first glance has
the same problems as the tubingless completion, has some distinct advantages. The
flow may be either up the tubing, up the tubing-casing annulus or up both. It is
therefore very useful in low-pressure gas wells where the flow area can be switched
to overcome liquid loading problems. The other main application is with pumped
wells. When a downhole pump is installed, the pumps perform best if they only
produce liquid; gas production can cause problems. The gas is therefore vented up
the annulus (as shown in Figure 10.21) and out of the wellhead. The liquid enters