Page 244 - Well Control for Completions and Interventions

238 Well Control for Completions and Interventions

During a bullhead (or deadhead) kill fluid is pumped down the pro-
duction tubing, pushing any hydrocarbons back into the reservoir and
leaving the wellbore filled with kill weight fluid. Its simplicity makes it
the most popular method of killing a well prior to an intervention or
workover. To succeed, the rate of injection into the well must be higher
than the migration rate of any hydrocarbons in the tubing, meaning a
relatively high pump rate is needed. Bullheading is only possible where
the formation is permeable enough to permit the high rates of injection
required without reaching formation fracture pressure. It is not
suitable for easily damaged reservoirs, since any debris in the tubing (and
some of the kill fluid) is swept into the formation during the kill.
If the completion design will not allow circulation, and if low perme-
ability prevents a bullhead kill, coiled tubing can be used to circulate kill
fluid into the well. Coiled tubing is often overlooked when planning a
well kill, yet it has distinct advantages. The well kill can be performed at
relatively low pressure, and equivalent circulating density (ECD) is low.
Coil tubing circulation should be considered for any well that has an
easily damaged formation, even if bullheading is possible.
Lubricate-and-bleed is used to kill gas wells where circulation cannot
be established, or where it is not possible to obtain a high enough injec-
tion rate (fluid velocity) to overcome the migration velocity of gas during
a bullhead kill. Lubricate-and-bleed involves pumping a measured quan-
tity of fluid into the tubing, then bleeding gas pressure equivalent to the
hydrostatic head of the pumped fluid. The process is repeated until the
well is dead.
Killing the well prior to a workover or intervention is a planned event.
During the running of a completion, or the execution of a workover or
intervention, it may be necessary to respond to an unplanned kick.

7.2.1 Essential information
A great deal of information about the reservoir, completion, and surface
equipment must be obtained to determine which kill method is best
suited for the well. Essential data will include, but need not be limited to:
• Reservoir datum depth (usually top reservoir).
• Formation fracture pressure (P fr ) to set surface and downhole pressure
limits.
• Reservoir pressure (P r ). This will be used to calculate the kill weight
fluid density and static pressure during the kill.