Page 113 - Drilling Technology in Nontechnical Language

104 Drilling Technology in Nontechnical Language Second Edition

modified by traveling through the rock, and this change can indicate,

among other things, compressive strength.

Fig. 4–14. A four-arm caliper, which measures the hole size in two axes

Logging in a directional hole is more difficult than logging a vertical

hole. Any roughness of the borehole wall will impede progress of the
logging tool. The tool may stand up on a ledge and not run any further. It
is more likely to get stuck, as its own weight presses it into the side of the
hole. Generally, if the wellbore is smooth, logging tools can be successfully
run on wireline up to around 60° inclination.

In bad wellbore conditions or if the hole inclination is too high for
wireline logging, the logging tools can be attached to drillpipe and run in
the hole, with the electrical cable inside the drillpipe. Logging on drillpipe
takes a long time and thus is very expensive. In this case, only the most
important logs needed for decision making on the well will be run.

An alternative is to run a logging while drilling (LWD) tool as part of

the drilling assembly. LWD tools were fairly crude when they were first
developed, but modern LWD tools can take logs of sufficient quality that

they can replace wireline logs in some cases.

Bringing the well to horizontal

The well is now drilled at an inclination, towards the second kickoff
point. The problem with the second kickoff is that it must be exactly right

(fig. 4–15). If the well becomes horizontal just a few feet too low or too
high, it will take a lot of drilling to get the well back to the desired TVD. If

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