Page 197 - Drilling Technology in Nontechnical Language

188 Drilling Technology in Nontechnical Language Second Edition

and hole. For long horizontal or extended reach wells, rotary steerables
have allowed holes to be drilled that are not possible with sliding
motor assemblies.

Historically, a major cause of lost horizontal holes (which could not be
completed to reach their objective) was inadequate hole cleaning. Cuttings
build up in the hole next to the sliding drill pipe. Rotating the pipe stirs up

the cuttings and gets them into the moving fluid stream.
As well as the possibility of commanding the tool from the surface,
the tool can also link to a logging while drilling tool, which measures
electrical or radioactive properties of the rock being drilled through. This
can then be used to steer the bit in relation to rock properties.
Rotary steerable tools can cost over $20,000 a day. Combined with the
cost of LWD tools, the system cost can exceed $30,000. Clearly these tools
will only be used when the advantages are worth the cost compared with
alternative tools. High-cost rigs, complex well paths, long horizontal holes,
or a requirement for geosteering all demand this technology.

Navigating to the Target

Two factors are necessary to navigate while drilling. Measurements
must be taken and calculations made to work out the position of the
wellbore.

Wellbore position surveying tools

All wellbore surveying tools measure the same parameters, but using
different equipment. This is called surveying. As previously explained,
the two pieces of information measured by the tool are the inclination (the
angle between the centerline of the wellbore and vertical) and the azimuth
(the direction). The azimuth may be measured with a magnetic compass,
in which case the direction relative to magnetic north is converted to grid
north. Gyro tools will measure relative to geographic north (the axis of
rotation of the earth). The depth at which these measurements are taken
is known from the depth of the tool at the time of the survey, so a survey
point will show the measured depth, the inclination, and the azimuth. A

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