Page 109 - Drilling Technology in Nontechnical Language

100 Drilling Technology in Nontechnical Language Second Edition

azimuth at the depth of the tool, and the TFA. This information allows the
driller to guide and correct the direction that the well drills.
When drilling with a downhole motor, the motor exerts a torque on
the drill bit so that the bit turns and drills. There is also a reactionary
torque exerted above the motor. A long string of drillpipe is pretty flexible

and will act like a long spring, which will get a little wound up with the
reactionary torque from the motor. For this reason, while drilling, the tool
face azimuth will move around even though the drillpipe on the surface
does not rotate. Without a constant surface readout of the TFA, it would

be much more difficult to align the drilling assembly in the right direction.

The directional BHA is assembled on the drill floor, and the motor
is tested by pumping mud through it. If that is okay, the MWD will be
added, and that too will be tested. When all is ready, the assembly is
run to just above the bottom of the hole. Once mud is pumped through
the drillstring, the motor will start turning, and the MWD will start
working and transmitting data to surface. As data is received, the tool face
azimuth is known, and the drillstring is turned to align the TFA in the
correct direction.
With the tool face azimuth aligned, the drillstring is lowered until it
touches bottom. The motion compensator is about half open. Then the
pressure on the compensator is reduced (so that it supports less of the
drillstring weight); the bit takes weight and starts to drill.
While drilling, the main determinant of the buildup rate is the angle
of the bent sub and the distance from the bent sub to the bit. However,
if the weight on bit increases, the buildup rate will tend to increase. The
adjustment is limited, because if too much weight is put on the bit, the
torque required to drill exceeds the motor output torque, and the motor
stops rotating (it stalls).

Drilling the tangent section

After first build is complete, the well must be drilled in a straight path
to the second build position. This is done with a rotary drilling assembly
that is designed to drill straight ahead.

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