Page 98 - Well Control for Completions and Interventions

90 Well Control for Completions and Interventions

Completion and intervention personnel are unlikely to be directly
responsible for the installation of the wellhead and its associated compo-
nents. However, many completion, workover, and intervention opera-
tions involve working on the wellhead. For example, wellhead
maintenance and integrity assurance work is often carried out by well
intervention crews. Some intervention operations will require the inner
(production) annulus to be accessed and monitored, and there will be
occasions where there is a requirement to pump fluid into, or take returns
from, the inner annulus.
Ideally each annulus will have a pressure monitoring facility and a
sample/bleed point.
• In subsea wells only the “A” annulus can be monitored and bled off. The
outer (B and C) annulus cannot be accessed. Subsea wellheads and trees are
covered in the subsea section of this book.
The “A” annulus is treated a little differently to the B and C annulus.
This is because there is a greater potential for it to become pressurized,
either because of leaks from the production tubing and packer, or because
of annulus fluid expansion.
Although all of the annulus have the potential to undergo pressuriza-
tion when production begins, the effect is normally greater in the inner-
most annulus. Hot fluids, flowing from the reservoir, begin to heat up the
tubing, casing, and surrounding annulus. As fluid in the annulus heats up
it begins to expand. Since the annular volume is finite, fluid expansion
will be limited and where fluid cannot expand, pressure will increase. If
measures are not taken to mitigate against this pressure increase by venting
the annulus, there is a very real risk of tubing collapse or casing burst.
The other major cause of annulus pressure build-up is a leak from the
production tubing or past the packer. If this occurs the annulus will be
exposed to tubing pressure. The worst case would be maximum wellhead
closed-in pressure. For this reason, the production annulus is normally
tested to the same pressure rating as the completion string (production
tubing).
Well integrity requirements for the inner (production) annulus will
normally be:
• Two flanged gate valves, each with the same pressure rating as the
Christmas tree on each outlet.
• Valves should be sized to allow adequate pump rate for well kill/circu-
lation of gas injection. Most valves are 2v nominal diameter.