Page 17 - Well Logging and Formation Evaluation

Basics 7

be used for establishing the stratigraphic or sedimentary dip and/or
presence of fractures/vugs.
• Formation pressure/sampling: Unlike the above tools, which all “log”
an interval of the formation, formation-testing tools are designed to
measure the formation pressure and/or acquire formation samples at a
discrete point in the formation. When in probe mode, such tools press
a probe through the mudcake and into the wall of the formation. By
opening chambers in the tool and analyzing the fluids and pressures
while the chambers are filled, it is possible to determine the true pres-
sure of the formation (as distinct from the mud pressure). If only pres-
sures are required (pretest mode), the chambers are small and the
samples are not retained. For formation sampling, larger chambers are
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used (typically 2 / 4 or 6 gallons), and the chambers are sealed for analy-
sis at the surface. For some tools, a packer arrangement is used to enable
testing of a discrete interval of the formation (as opposed to a probe
measurement), and various additional modules are available to make
measurements of the fluid being sampled downhole.
• Sidewall sampling: This is an explosive-type device that shoots a sam-
pling bullet into the borehole wall, which may be retrieved by a cable
linking the gun with the bullet. Typically this tool, consisting of up to
52 shots per gun, is run to acquire samples for geological analysis.
• Sidewall coring: This is an advanced version of the sidewall sampling
tool. Instead of firing a bullet into the formation, an assembly is used
to drill a sample from the borehole wall, thereby helping to preserve
the rock structure for future geological or petrophysical analyses.
• NMR: These tools measure the T 1 and T 2 relaxation times of the for-
mation. Their principles and applicability are described in Chapter 5.
• Vertical seismic profiling (VSP): This tool fires a seismic source at the
surface and measures the sound arrivals in the borehole at certain depths
using either a hydrophone or anchored three-axis geophone. The data
may be used to build a localized high-resolution seismic picture around
the borehole. If only the first arrivals are measured, the survey is typi-
cally called a well shoot test (WST) or checkshot survey. VSPs or
WSTs may also be performed in cased hole.

1.2.3 Wireline Cased Hole Logging

When a hole has been cased and a completion string run to produce the
well, certain additional types of logging tools may be used for monitor-
ing purposes. These include: