WELL KILLING PROCEDURES
The procedure used to kill the well depends primarily on whether the kick occurs whilst drilling (there is a drillstring in the well) or whilst tripping (there is no drillstring in the well).
1.5 Drillstring out of the Well
One method of killing a well when there is no drillstring in the hole is the Volumetric Method. The volumetric method uses the expansion of the gas to maintain bottom hole pressure greater than formation pressure. Pressures are adjusted by bleeding off at the choke in small amounts. This is a slow process which maintains constant bottom hole pressure while allowing the gas bubble to migrate to surface under the effects of buoyancy. When the gas reaches surface it is gradually bled off whilst mud is pumped slowly into the well through the kill line. Once the gas is out of the well, heavier mud must be circulated. This can be done with a snubbing unit. This equipment allows a small diameter pipe to be into the hole through the closed BOPs.
When the kick occurs during drilling, the well can be killed directly since:
• The formation fluids can be circulated out
• Theexistingmudcanbereplacedwithamudwithsufficientdensitytooverbalance the formation pressure
If a kick is detected during a trip the drillstring must be stripped to bottom, otherwise the influx cannot be circulated out. Stripping is the process by which pipe is allowed to move through the closed BOPs under its own weight. Snubbing is where the pipe is forced through the BOP mechanically.
There are basically two methods of killing the well when the drillstring is at the bottom of the borehole. These are:
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