Page 288 - Well Control for Completions and Interventions

Well Kill, Kick Detection, and Well Shut-In 281

than API burst. Conversely, if the completion is equipped with a
seal assembly and PBR (Polished Bore Receptacle) positioned
above the packer, injection pressure will push the base of the tub-
ing into compression. Tubing under compression can have a burst
limit significantly below the API limit. Detailed stress analysis using
proprietary software such as “WellCAT” can be used to establish
tri-axial burst limits during a well kill.
• Where tubing integrity has failed and there is communication
with the annulus, it may be necessary to bullhead down both
tubing and annulus simultaneously to sweep hydrocarbons out of
the annulus.
• Injection pressure may have to be limited in wells where past inter-
ventions have been carried out to isolate unwanted water or gas.
High pressure can result in the failure of isolations such as bridge
plug, straddles, cement squeeze, resins, and polymers.

7.4.2 Bullhead calculations: preparing the kill sheet
The following calculations need to be completed when preparing a bull-
head kill schedule.
1 Calculate the kill fluid density
For most workovers and interventions an overbalance of approxi-
mately 200 300 psi is required. The kill fluid density is calculated as
follows:

ð
reservoir pressure psiÞ 1 over balance psiÞ
ð
Kill fluid ppgÞ 5
ð
ft TVD 3 0:052
2 Calculate the volume of fluid required to kill the well (fluid pump
to reservoir)
This volume must include surface lines from the pump to the wellhead,
the tubing volume, any annular space below the packer, and the volume
of the casing (or liner) below the end of the tubing.

3 Calculate the maximum surface pump pressure (formation fracture
limit) at the start of the kill
At the beginning of a bullhead kill, the pressure acting on the formation
is the HP of the reservoir fluid in the tubing (from the surface to the top
reservoir) plus any applied surface pressure (pump pressure). The maxi-
mum (pump discharge) pressure that can be applied at the surface is