Page 250 - Drilling Technology in Nontechnical Language

Chapter 10 – EVALUATION 241

Measuring resistivity helps to locate porous formations, show
boundaries between formations, and identify hydrocarbon-bearing zones.

Sonic tools

A pulse of sound is transmitted at the lower end of the tool, and the
time it takes the sound to travel a known distance through the rock is
measured. Sonic tools can measure formation densities and compressive
strengths and can identify formation fractures. Information on permeability
can be obtained. When run inside casing, sonic tools can determine the
quality of the set cement outside of the casing and can also measure the
inside diameter and thickness of the casing, thus identifying areas of
casing damage.

Sonic tools give much information of direct interest to the drillers.

One such log is shown in figure 10–7. This is a tool used to evaluate the
quality of cement after it has set outside casing. The log is depth based on
the vertical axis.

In the left track on the log is shown some raw measurements and depth
correlation information, such as a gamma ray (GR) log. (GR can be read
through steel casing, and this can be used to check that the depths are
correct by comparing this log with earlier logs, including a GR track).
In the next track (second from the left) are the sonic velocities and
other sonic data.
In the third track is shown a color-coded log indicating directly how
good the cement is (different gray-shaded blocks on this monochrome
photo). This makes interpretation by nonexperts relatively simple.

Downhole seismic tools can also be included in this group.
Hydrophones (very sensitive microphones) listen on the surface while a
sound transmitter is lowered into the well. The transmitter is slowly pulled
out while it transmits, and the measured sound returns at the surface give
a good indication of the sonic properties of the formations between the
tool and the surface. This can then be used to improve the interpretation of
surface seismic surveys because the transmission time of the sound signal
is one-way only. (In a surface seismic survey, sound is transmitted at the
surface and the echoes are listened for, so the sound has to travel down and
up again. The speed of sound through each formation has to be known or
estimated to interpret the surface seismic.)

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