Page 278 - Drilling Technology in Nontechnical Language

Chapter 11 – WELL CONTROL 269

If the permeability of the flowing formation is high, the kick can

develop very quickly. A larger drilled hole will also allow influx to flow in
faster, but this is compensated for to an extent because the capacity of the
hole is greater in a larger hole. It is also possible that if permeability is very
low, little or no influx enters the wellbore even though mud hydrostatic is

less than formation pore pressure.

Killing the Well

The operations involved in restoring primary control are known as
killing the well. In principle it is simple, but in practice, there are many
considerations that are required to execute it safely. Problems may occur
that have to be recognized and addressed quickly. An example of a well
kill will be worked through to describe the main points.

A vertical well is being drilled at 8,000 ft. The bit size is 12¼", and
there are 300 ft of drill collars that have an OD of 8" and an ID of 3".
Above the drill collars, 5" diameter drillpipe is in use with an ID of 4.276".
Casing is set at 5,000 ft; this is 13⅜" OD and 12.615" ID. The mud in use
has a density gradient of 0.5 psi/ft.
A kick is taken, and after closing the BOP, the driller has the following
information about the kick:
1. The pressure inside the drillpipe at surface is 500 psi.

2. The pressure on the annulus (inside the BOP) is 600 psi.
3. The active mud system volume has increased by 21 bbl.
In any problem situation, it is best to draw a simple diagram showing
the main points. This helps to avoid mistakes. Here is the diagram for this

well (see fig. 11–10).

The first thing to notice is that there is a difference between the pressure
on the drillpipe and on the annulus. Although the pressure at the bottom of

the well is the same, the fluid in the annulus now has a different hydrostatic
pressure than the fluid in the drillpipe because pore fluid (21 bbl of it)

has entered the wellbore. This is lighter than the mud, so the hydrostatic
pressure in the annulus is less (by 100 psi, the difference between drillpipe
and annulus pressures).

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