Page 81 - Drilling Technology in Nontechnical Language

72 Drilling Technology in Nontechnical Language Second Edition

Running this design of BHA also has some drilling advantages. In the
absence of stabilizers in the BHA, if a lot of weight is placed on the drill bit
(to make it drill as fast as possible), the lower part of the BHA will start to
buckle. This will give unpredictable results, as the bit is no longer aligned
with the center of the hole and is likely to drill a spiral hole or even veer

off course. If the BHA buckles while the drillstring is rotating, significant
fatigue damage will occur to the drill collars, which may eventually cause
something to break. Using a locked assembly allows the well to be drilled
faster and straighter. Some people consider that running more stabilizers
will give more frictional force against the BHA when rotating and tripping
in and out. In fact, drilling a straight hole reduces friction far more than
might be increased by having a few bits of steel touching the side of a
straight hole.
Now the well can be drilled ahead towards the depth to run the next
casing, which is called intermediate casing because it comes after surface
casing but before production casing. A well might have more than one
intermediate casing, or it may contain none if it is a shallow well and the
reservoir can be reached with only one casing string after surface casing.

Once the casing point is reached, after one or several bit runs, the bit is
pulled out of the hole. Electric logs will most likely be run at this time.

Logging

Logs are discussed in detail in chapter 10, “Evaluation.”
Tools are run into the hole on special steel wireline, usually 9/16" (14
mm) in diameter. Inside the wireline are electrical wires that connect the
tool to a computer unit on the rig. Logs measure physical and chemical
characteristics of the formations. In an exploration well, the well is drilled
to gain information, and most of this information will be obtained by using
logs (fig. 3–20).

This hole section is an intermediate hole section; the reservoir is not
yet penetrated. Even though the reservoir is not exposed by the wellbore,
there is a lot of information that can be used to help improve the well design
and drilling program for the next well. Many people have an interest in
the log results, including drilling engineers, geologists, geophysicists, and
other specialists.

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