Page 283 - Drilling Technology in Nontechnical Language

274 Drilling Technology in Nontechnical Language Second Edition

When the surface active system mud is weighted up to ρ , the kill
2
can begin. One person controls the choke valve and can see both drillpipe

and annulus gauges. As the pumps start and flow begins, the pressure
on the annulus will start to rise. The choke is now opened to keep the
choke pressure equal to the shut-in choke pressure of 600 psi, and this
is maintained while the pump speed is slowly increased to 30 strokes a
minute. Once the pump is at the correct speed, the choke operator needs
to watch both choke and drillpipe gauges. Using the table or graph above,
the choke setting is adjusted to keep the pump pressure correct. At this
stage, pump pressure will naturally fall as heavy mud moves down the
drillstring, and only slight choke adjustments are needed.
At the end of phase 1, the pump pressure is then kept constant for the
rest of the killing operation (in this case at 563 psi). The choke setting will
change as the gas moves up the wellbore. As the gas moves up and expands
in accordance with Boyle’s law, it will push out more mud than enters the
well. This reduces the hydrostatic pressure in the annulus, and so choke
pressure has to be increased to compensate. Choke pressure will reach a
maximum once the top of the gas reaches the choke and then will decrease
rapidly as mud replaces gas leaving the annulus.
Once the kill operation is complete, the pumps are stopped. If no
pressure remains on drillpipe or annulus, the BOP is opened. A visual
check is made to ensure that the well is not flowing with the pumps off.

If all is well, drilling can now be resumed, although in practice, in order
to get the mud into proper shape for drilling, another full circulation is
generally required.
There are other techniques for killing a well, depending on the
circumstances. The technique outlined in this example is called wait and
weight or the balanced method. Other methods not described here are
drillers, volumetric, combined volumetric and stripping, and bullheading.

Shallow Gas

Shallow gas was discussed in chapter 3. The emphasis on shallow gas

is prevention. If shallow gas is encountered and the well flows, the chances
of stopping it are remote.

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