Page 89 - Well Control for Completions and Interventions

80 Well Control for Completions and Interventions

There are some disadvantages with the mono-bore design. Frictional
pressure drop, particularly in long small ID liners, can be significant. In
addition, obtaining a good cement bond in high angle wells with long
liners can be problematic (Fig. 2.11).

2.2.11 Single string production tubing completed across
multiple reservoir zones
Many fields have a number of separate reservoir compartments stacked
vertically one above another. Reservoirs of this type can be exploited
using a single string, multiple zone, completion. Production zones are
isolated from one another using production packers, and there will nor-
mally be an additional (top) packer isolating the production annulus from
produced fluids.
Fig. 2.12 illustrates a single string multizone completion. In the exam-
ple illustrated, a well is completed across three separate zones. A configu-
ration like this allows simultaneous production from any combination of
zones, or would allow a single zone to be produced on its own. In the
example shown, production from the lower zone enters the tubing
through the tail-pipe below the lower packer. Flow from this zone could
be isolated by installing a wireline set plug (mechanical barrier) in a nip-
ple profile below the packer. Production from the middle and upper zone
would be controlled by opening or closing a communication port (sliding
sleeve) located between the packers. In most wells the communication
port is manipulated using a slickline deployed mechanical shifting tool. In
wells where access is difficult or expensive, for example subsea wells,
communications ports are hydraulically operated from the surface (“intel-
ligent” or “smart” well technology).
One advantage of this design is the number of different reservoir layers
that can be completed using a single wellbore and a single completion
string—in theory there is no limit. Fields are in production where eight
or nine zones are accessed through a single completion string. There are
some obvious disadvantages to completing in this way. Intervention access
is very limited at the zones that are behind pipe. Pressure contrast
between zones can lead to problems with cross-flow. This can compro-
mise well productivity, create problems during intervention, and make
well kill very problematic. There will be additional well control concerns
when running the completion, since the production casing will have to
be perforated before the completion is run.