BAKER HUGHES - Drilling Fluids Reference Manual

PRESSURE PREDICTION AND CONTROL
Figure 12 - 7 Reduction in BHP Corresponding to a Surface-Observed Reduction
in Fluid Weight Due to Gas Cut Fluid
If no flow occurs when the pump is shut down, hydrostatic pressure and fluid weight are
sufficient. There could be a slug of gas in the annulus that could later cause trouble and caution
is indicated, but gas cutting by itself is not serious. A very small amount of gas at bottom hole
conditions can gas cut a large amount of fluid at surface conditions. Drilling a porous sand, a
bridged hole with trapped gas or circulation of bottoms-up after a trip will often produce these
results.
Kicks While Tripping Out of the Hole
It has been estimated that 50% or more of all kicks occur while tripping. The main reasons for
these kicks are generally poor practices and neglect on the part of rig crews. Two major
incidents cause kicks while tripping out of the hole.
1. Failure to fill the hole often enough or failure to add fluid to the hole equivalent to
the displacement of pipe removed. Depending on the displacement of the pipe
withdrawn, there will be a corresponding drop in fluid level (assuming no swabbing
has occurred) which may lower the hydrostatic pressure enough to allow the well to
kick.
For example, five stands of 4½ in. drillpipe (16.6 lb m /ft 93 ft stands) withdrawn from an
8½ in. hole would result in a fluid level drop of 45½ ft. Whereas, five stands of 6¾ in. ×
2¾ in. drill collars (93 ft stands) would result in a fluid level drop of 263½ ft.
Frequency for filling the hole is a function of fluid gradient, hole diameter, pipe
displacement, and formation pressure.
BAKER HUGHES DRILLING FLUIDS
REFERENCE MANUAL
REVISION 2006 12-19