Page 281 - Drilling Technology in Nontechnical Language

272 Drilling Technology in Nontechnical Language Second Edition

During a well kill operation, the intention is that bottomhole pressure
should be kept as close to the formation pressure as possible. If the
bottomhole pressure is allowed to drop below pore pressure, it is possible

that more influx will enter the well, which will increase the complexity of
the operation and increase the risk of problems. If the pressure rises much,
a weak formation somewhere in the well might break and allow losses. If

that happens, more influx will enter the well and will move into the loss
zone. This is called an internal blowout and is much more complicated and
dangerous than a simple killing operation, so it must be avoided.
The bottomhole pressure during the well kill is controlled by the choke
on the annulus. Applying pressure here causes pressure to be applied
everywhere in the well, including on the kicking formation.
The bottomhole pressure is monitored during the well kill by watching
the pressure on the drillpipe (the pump pressure). The hydrostatic head of
the mud in the annulus at any particular stage of the operation is known, and
so bottomhole pressure equals pump pressure plus hydrostatic pressure.

During drilling, the driller regularly makes a test of each pump by
circulating at a set slow rate and measuring how much pressure is required
to circulate around the well at that rate. All mud pumps are tested so that
any can be used for killing the well. For purposes of this example, assume
that a slow circulating rate (SCR) of 30 strokes per minute was used. The
pump outputs 5 gal for every stroke, so at a SCR of 30, the pump output is
150 gal/min. There are 42 US gal in a barrel, so the flow rate is 3.57 bbl/

min. At this flow rate, the driller measured a pressure of 500 psi. This is
known as the slow circulating pressure, or PC .
1
When heavier mud is pumped into the system, more pressure is
required to push this heavier mud around. If the mud is changed to heavier
mud, the new pressure (denoted PC ) can easily be calculated, as shown in
equation (11.3): 2

ρ 0.563
PC = PC × —— = 500 × —— = 563 psi (11.3)
2
2 1 ρ 0.5
1
This then is the pressure that is required to force the heavier mud to

flow around the well. In fact, the pressure required to give flow in the

annulus is very small compared to the drillstring, so annular pressure loss
is ignored. The assumption is made that PC and PC are the pressures
2
1

required to give flow through the drillstring and drill bit only.

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