Page 192 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 168

[ ] Well Programming
2.3.2

closer to the cap rock because GR would increase and resistivity would
decrease. Orientating your steerable assembly downwards for a dis-
tance would then bring the wellbore back to the desired position.
One major consideration with geosteering high-angle wells is that
the LWD sensors can be some distance behind the bit, especially if a
motor is used. By the time the sensor tells you a correction is needed,
you could have drilled 60 ft or more. The closer the sensor is to the bit,
the closer you can control the wellpath.
The Anadrill Resistivity at the Bit (RAB™) is a resistivity LWD tool
that actually measures formation resistivity at the bit as well as resis-
tivities at different depths of investigation a few feet behind the bit.
This allows real-time decisions while drilling so that corrections can be
made immediately and the results can be monitored.
LWD tools and applications are developing very quickly. If your
well might benefit from this kind of measurement, check to see what
tools are currently available. Also, check on prices, especially
because, if they are lost in a hole, some of these tools can cost up to
a half a million dollars.
MWD tools in horizontal wells. Most horizontal wells are drilled
to intersect large vertical fractures (such as in the Austin Chalk, Texas).
This can lead to losses. The larger the losses, the weaker the signal
from the MWD tool, which you will be relying on to monitor and con-
trol progress. Baker-Hughes has an MWD tool for horizontal drilling
that uses special techniques to get survey data to surface, even with
total losses. It normally sends a conventional mud pulse signal along
the mud column in the string, but once an hour it will send the special
signal to surface. Unfortunately, if you have total losses, a signal once
an hour (while better than no signal) is of limited use if you need to
align the string to drill ahead.
There is a cable-run survey tool that can be used in horizontal
holes. This is run after tripping part of the way in and latching into a
special UBHO sub. The wire is cut and a wet connector is made up and
suspended in the tool joint in a basket. Tripping in continues and to
take a survey, the cable is run with the mating connector and latched
into the bottom cable. Thus, the cable only has to be run in the top part
of the hole. The connector can be a problem in conductive muds

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