Page 256 - Well Control for Completions and Interventions

250 Well Control for Completions and Interventions

If the well is plugged, it must be killed to the point of circulation and
not the reservoir depth. This assumes that the wellbore below the plug
remains filled with (lower density) hydrocarbon, but allows for the conse-
quence of a leaking plug.
If there is no plug, the well should be killed to the top of the reser-
voir. Any hydrocarbon between the point of circulation and the reservoir
will migrate upwards (because of density differences) and be swept out
during the kill, leaving the wellbore between the reservoir and circulation
ports filled with kill weight fluid. However, the hydrostatic calculations
for a kill should be based on a worst-case assumption; the interval
between the circulation path and top reservoir remains filled with lower
density hydrocarbon.

7.3.7 Pumping the kill fluid
The procedure described here assumes the well has been shut-in follow-
ing production and is to be killed in preparation for a workover or inter-
vention. In common with other circulating kills, the aim is to maintain a
constant BHP throughout the kill, normally 200 250 psi above reservoir
pressure. If circulating pressure drops below reservoir pressure there will
be an additional influx; if circulating pressure is too high, fluid will be
lost to the formation. Losses or an influx are less of a concern when the
well has been plugged. However, even when a plug is in place it is good
practice to maintain BHP within the parameters that would be used for
an unplugged well. Plugs can leak!
As ECD is normally higher when reverse circulating, a slow circula-
tion rate is normally a requirement. Fluid hydraulics modeling can be
used to determine pump speed where SCRP data is not available
Lines are rigged up to enable kill fluid to be pumped into the annulus. A
pressure relief valve should be fitted to the lines, and non-return valves placed
close to the annulus valve. Returns from the tubing side will be routed
through a choke, and from there, to disposal. If the flowline is still in place,
returns can be handled by the process facilities. This is normally the preferred
option, since there is no need for additional handling and disposal equipment.
Normal procedure for reverse circulation:
1. With the pumps lined up to the annulus and the wellhead valves
open, bring pressure in the casing up to the calculated value whilst
holding back pressure on the tubing using the choke.
2. Continue circulating controlling the casing pressure using the choke.