Page 248 - Drilling Technology in Nontechnical Language

Chapter 10 – EVALUATION 239

left in place for a period of time, which can be up to 30 minutes. The early
version of this Schlumberger tool was called the Repeat Formation Tester
or RFT. In drilling circles, it was known as the Repeat Fishing Tool as it
had to be fished out of the hole so often. The current version is called the

Modular Dynamics Tester, or MDT; figure 10–6 shows one being prepared
to run in the hole.

Fig. 10–6. MDT tool being prepared

Electrical Logging

The first electrical log was run in France by Conrad and Marcel
Schlumberger in 1927 after they had experimented with measuring
electrical properties of the earth’s crust. Their electric log measured the
electrical potential between the surface of the earth and a probe lowered

into a well. This “spontaneous potential” log allowed the identification
of different downhole formations by the depths at which the electrical
potential was seen to change.
Since that first log, Schlumberger and its competitors have developed

many different techniques for measuring downhole physical, chemical,
radioactive, and electrical properties. Taken together, these measurements

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