Page 248 - Well Control for Completions and Interventions

242 Well Control for Completions and Interventions

maybemuchdenser thanisneededtokillthe well. When thetub-
ing/annulus communication path is opened, heavy annulus fluid will
drop down the well. There is a risk of this annulus fluid invading
and damaging the formation, particularly if there is debris in the
annulus. It will also be more difficult to displace the heavy annulus
fluid out of the well with the lighter kill fluid unless a viscous pill is
used. The use of a viscous pill will increase frictional pressure drop
in the tubing.
• If the original completion was run before the casing (or liner) was
perforated, a sub-hydrostatic fluid may have been left in the annulus.
If this is the case, there will be a tendency for the lighter annulus
fluid to migrate upwards during the well kill. To sweep the lighter
fluid out of the annulus, the pump rate will need to be increased or a
viscous pill used. This will increase frictional pressure drop in the
tubing.
• If there is no packer in the well, or tubing leaks have allowed oil and
gas to enter the annulus, the circulation rate must be higher than the
hydrocarbon migration rate.

7.3.1 Static and circulating pressure
During a well kill, fluid is dynamic when circulation takes place. It is
therefore necessary to calculate BHP whilst circulating, a more complex
condition than when static. Friction needs to be considered and added to
the hydrostatic pressure (HP). Frictional pressure drop is affected by:
• Fluid density
• Fluid viscosity
• Gel strength in viscos fluids
• Yield point
• Well depth (measured and vertical)
• Annular clearance
• Tubing ID
• Tubing and casing roughness
• Rate of circulation
When forward circulating, fluid is pumped down the tubing and
returns taken from the annulus. Pump pressure when circulating is a result
of friction through the circulation path and is a function of:
Surface line friction1tubing string friction1friction through the BHA
1annulus friction1annulus hydrostatic tubing hydrostatic: