Page 255 - Well Control for Completions and Interventions

Well Kill, Kick Detection, and Well Shut-In 249

7.3.6 Plugging the well
There are significant advantages to plugging the well below the circula-
tion ports. A plug prevents fluid loss to the formation, meaning the risk
of losing the hydrostatic overbalance is significantly reduced. A plug also
reduces the possibility of kill fluid damaging the formation. Furthermore,
an inflow tested plug can be used as one of the two mechanical barriers
that are required before the Christmas tree can be safely removed and the
drilling BOP installed.
If left in place during a workover or intervention, the presence of a
mechanical barrier greatly reduces any possibility of a well control event.
Losses or a kick are prevented. Finally, having a plug below the circulation
ports makes the kill more predictable, since losses are unlikely. However, the
risk associated with placing a mechanical barrier in the well must be assessed:
• Debris settling above the plug can make recovery difficult. Two-piece
plug and equalizing prong barriers are more debris tolerant than those
equipped with an integral equalizing device. The only drawback is the
need for two wireline runs to install the plug and prong type (and two
runs to pull).
• Setting a debris catcher above a plug is recommended, but this does
require an additional wireline run.
• Continued injection or production in offset wells often results in a
change in reservoir pressure (increase or decrease) that might compli-
cate plug recovery.
• It is not possible to monitor reservoir pressure in a plugged well.
• Knowing a plug is present can lead to complacency and a lack of vigi-
lance with regard to monitoring fluid level. Plug integrity is not
guaranteed for the duration of a workover, or even of the kill.
A plug can be set before or after the communication port is open:
• If the plug is set before the communication ports are opened or the
tubing punched, debris from the annulus can move into the tubing
and settle above the plug.
• If the plug is set before the communication ports are opened or the
tubing punched, pressure above the plug can be increased, or reduced,
to equalize between tubing and annulus.
• If the plug is set after the ports are open, debris contaminated heavy
fluid from the annulus can be lost to the formation.
It is usually better to set the plug before opening the communication
path. Debris above a plug can usually be removed, whereas formation
damage is sometimes irreversible.