Page 252 - Drilling Technology in Nontechnical Language

Chapter 10 – EVALUATION 243

Radioactivity tools

Radioactivity tools measure the natural radioactivity of the rocks
(gamma rays). Some tools can even tell what kind of mineral is responsible
for the radioactivity, thus identifying different shale types, for instance.
Other radioactive tools bombard the formations with neutrons and
measure the response from the formation, which can identify the amount

of hydrogen in the formation fluids (which would, for instance, identify the
presence of hydrocarbons).
Using tools that contain radioactive sources requires stringent safety
precautions to ensure that personnel are not subjected to damaging
levels of radiation. Most authorities around the world also mandate that
any radioactive sources lost down the hole cannot be abandoned but
must be recovered, even if the expense of recovery is very high. Special
permission is then needed to abandon it if it cannot be recovered even with
great effort.

Mechanical tools

Mechanical tools deploy arms that press against the wellbore and
measure the diameter as the tool ascends. These caliper tools may deploy
one arm, two arms, or four arms for measuring the open hole. A four-
arm caliper is especially useful because it gives a good indication of the
wellbore profile. A wellbore may be the same size as the drill bit (in gauge),

uniformly enlarged (overgauge), or enlarged more in one direction than
another. The type and extent of hole enlargement is an indication of how
stable the wellbore is and also of the size and direction of the highest stress
in the rock.
A caliper tool also allows the annular volume to be accurately calculated,
so that the correct volume of cement can be pumped around the casing.
Special calipers are also used inside casing or tubing strings to
evaluate the internal condition (wear, erosion, or other physical damage).
These tools contain many caliper arms (15 or more) so as to give a good
coverage of the casing ID. One of these tools, the Kinley caliper, is shown

in figure 10–9.

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