1.4.1 Volumetric Method
Where a gas kick is taken into a well, and the pipe has been fully tripped out of the hole, a problem arises. Gas shut in will slowly migrate through the mud, and if not allowed to expand, will retain its pressure and bring this up the hole. Casing pressure, and bottom hole pressure, will steadily rise until damage occurs.
In many cases, drillpipe is stripped in to allow circulation to be effected. In general this can be done more rapidly than the gas can migrate. It is usually preferred and is described in detail in the Stripping and Snubbing section.
If this cannot be done, mud must be bled from the annulus at intervals to allow the gas to expand, aiming for a steady bottom hole pressure.
The mud weight and the average volume of the hole are known, it is thus possible to work out the volume of mud required to provide a given hydrostatic pressure.
The shut in casing pressure is noted. As gas migrates, the casing pressure rises. Once the pressure has risen to some pre-set amount; say 100 psi, the choke is opened and the volume of mud required to produce that same (100 psi) hydrostatic pressure bled off from the well. The choke is then closed again, and the new casing pressure noted. This figure is somewhat higher than the original shut in casing pressure, since the gas has now been allowed to expand. The bottom hole pressure has, however, been dropped to approximately the original shut in value.
As gas continues to migrate, so the casing pressure slowly rises again. Once it has risen the pre-set amount over the new shut-in pressure, the choke is opened again and the appropriate volume of mud bled off. After the choke is closed, a new shut in pressure is recorded.
In this way gas is allowed to migrate up the well without excessively high bottom hole pressure.
This is not strictly a 'kill' method — since the well has not been killed, however it allows time for other true kill procedures to be implemented, without necessarily losing the well.
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