Page 247 - Drilling Technology in Nontechnical Language

238 Drilling Technology in Nontechnical Language Second Edition

With explosive sidewall coring, the sample will probably not give
useful information on physical structures, such as fractures or bedding
planes. The force of the bullet hitting the wall will fragment the formation.
Information usefully gained will include porosity and permeability,

confirmation of hydrocarbon presence, determination of clay content, and
grain density and lithology.
The explosive sidewall coring tool may carry up to 90 sampling bullets.
Different bullets can be installed for different formation hardness.

Rotary sidewall coring

A wireline tool has a small core barrel and diamond bit. The core
barrel moves out of the side of the tool to contact the formation and starts
to turn. It drills into the formation and recovers a standard-sized plug of
formation, 0.94" (2.4 cm) in diameter and up to 1.73" (4.4 cm) in length. Up
to 50 samples can be taken in one trip in the hole, with the samples being
stored inside the tool.

The major advantage over explosive sidewall coring is that a relatively
undisturbed core sample is recovered. Internal structures and fractures
can be seen. One disadvantage of the rotary sidewall coring tool is
that the samples are stored in a tube, and if there are any misruns (no
core recovered), it might be tricky to decide where the cores recovered
came from.

Pore uid sampling and pressure testing

A tool may be run on wireline to a permeable formation of interest.
Once on depth, the tool is “set” by extending a probe until it contacts
the borehole wall. A seal around the probe isolates the tool from the
surrounding drilling fluid.

Once the seal is established, it is possible to measure pore fluid pressure
in the formation. Pore fluid samples can be captured.

The data recorded during sample taking may be used to calculate
permeability at the point where the probe contacts the formation.
These sampling tools have a nasty tendency to get stuck. To take
samples and pressures, the tool is forced against the side of the hole and

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