Page 203 - Drilling Technology in Nontechnical Language

194 Drilling Technology in Nontechnical Language Second Edition

on the Wytch Farm oil field in the South of England. For these very long
horizontal wells, it is better to keep the wellbore close to the top of the
reservoir without penetrating the roof of the reservoir.

Perhaps a well is required where the horizontal wellbore in the reservoir
can be steered to stay very close to the top of the reservoir. The reservoir
is sandstone, and the seal rock above it is shale. Shales are naturally
(slightly) radioactive, much more so than sandstone. Getting closer to the
shale is indicated by increasing radioactivity. The LWD tool measures
the radioactivity level, interprets that to give the distance from the shale,
and sends steering signals to the rotary steerable tool to increase, hold,
or decrease inclination. Rotary steerable tools such as the Schlumberger
PowerDrive can be commanded by the LWD tools so that the BHA steers
the bit in the reservoir without having to communicate with the surface.

Directional Drilling Problems

Hole cleaning

With a vertical hole, mud flows vertically upwards in the annulus,

lifting the cuttings upwards. As the hole angle increases, the cuttings move
up partially by lifting and partially by rolling along the low side of the
hole. When the hole angle approaches about 45°, cuttings can start to settle
on the low side of the hole. As they pile up, it can get hard to remove them.
Eventually the pile can become unstable and avalanche down the hole. As
long as the pipe is kept rotating while drilling, this stirs up any cuttings
beds and moves them back into the moving fluid stream, so sliding the

pipe while drilling (especially in high angle or horizontal wells) can lead
to serious trouble, as detailed in the following text.

Stuck pipe

If hole cleaning is not adequate, solids will build up in the well. One
possible result of this is getting the pipe stuck. If the pipe is stuck on a pile
of solids, it will be hard to get free, as the string will likely not be able
to move up or down or to rotate. It will be impossible to pump to clear
up the hole. The longer pipe does not move, the more the sticking forces
will increase.

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