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evaluation include examining cuttings and drilling mud, well logging, drill stem
testing, and coring.

Examining cuttings and drilling mud

Several techniques are available to help the operator decide whether to
complete the well. One of the simplest is looking at the cuttings the drilling mud
carries from the bottom of the hole. A geologist can test the cuttings to determine
whether they contain hydrocarbons. The mud logger, using various kinds of detection
equipment, can also spot hydrocarbons in the drilling mud. An operator probably
would not decide to complete or abandon a well using only information from cuttings
and mud returns. Careful examination of them, however, can indicate whether the
well is likely to produce.

Well logging
Well logging is a widely used evaluation technique. Many kinds of logging
tool are available. Some measure and record natural and induced nuclear, or
radioactive attributes of a rock. Others measure and record the way in which
formations respond to electric current. Another log measures and records the speed
with which sound travels through a formation. These are only a few of many logs
available to the operator. By interpreting the recordings, or logs, the operator can
usually tell if the well will be a producer.
The operator calls the logging company to the well while the drilling crew trips
out the drill string. From a portable laboratory, truck-mounted for land rigs or in a
small cabin on offshore rigs, the well loggers lower logging tools into the well on

wireline. They lower the tools to bottom and then slowly reel them back up. When
activated, the tools measure formation properties.
The tools transmit the data they gather to the truck or logging shack. There,
special recorders and computers store the information. For on-site evaluation,
computers in the portable laboratory print the data. These logs give the operator a
first look at what a formation may yield. For thorough evaluation, the portable lab
can transmit the log's data to powerful computers located at central processing
facilities. By carefully examining well logs, the operator can determine whether to
complete the well. Well logs not only indicate the presence of oil and gas, they also
indicate how much may be there.
During drilling, the operator can run logging while drilling (LWD) tools in the
drill stem. These instruments incorporate sophisticated electronic devices that sense,
transmit, and record formation characteristics as the bit drills ahead. The LWD tool
transmits formation information on a pulse the tool creates in the drilling mud. Much
as radio waves transmit sound information through air, mud pulses transmit
formation information to computers on the surface. The computers analyze and
display the information in readouts that experts on the site can interpret and evaluate.
Drill stem testing

To further determine the potential of a producing formation, the operator may
order a drill stem test, or DST (say "dee-esstee"). The DST crew makes up the test

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