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Borehole Geophysics in the
Petroleum Industry: Wireline Well Logging
Image 1
Evan Calhoun
Geophysics: October 30, 2014
Image 2
Beginning early in the last century, and even more so in
recent years, geophysical methods have become an invaluable tool
for understanding the world around us. Many of these methods
have become industry standards for practical field research,
which often results in far more precise interpretations and
distinctions of subsurface features. You could argue that no
industry has received a more positive impact from the
development of these methods than the petroleum industry. This
multi-billion dollar, worldwide industry has come to rely on
geophysics to efficiently, practically, and economically
complete their mission to extract energy from deep within the
earth. This industry has come to recognize the geophysical
method of borehole logging as a keystone technology and practice
within the petroleum field whose primary function is to locate
hydrocarbon reserves deep within the earth.
Geophysical well logging was
invented by brothers Conrad and Marcel
Schlumberger in Alsace, France. They
were the first to develop the method
and used it in 1927 for the petroleum
industry. The method utilized the measurability of the
electrical resistivity of the earth. The tool they developed
using this resistivity detected variances in porosity of the
sandstones of the oilfields of eastern France. The resistivity
was then recorded on a log per interval of depth from the
surface in order to give the men a very accurate depiction of
the earth’s subsurface. Since 1927, the overall field of
borehole logging has been greatly expanded. Many more methods of
recording subsurface features have also since been developed as
time progresses and technology advances. Improvements have been
brought to ease of use, equipment reliability, interpretation,
as well as improvements to accuracy. Though, the original
porosity method utilizing electrical resistivity developed by
the Schlumberger brothers is still and will presumably always be
an industry standard.
Through the years since 1927, many new types of borehole
logging have been developed. Each of these methods utilize
different characterizations and traits about materials or
geologic structure which allows geoscientists to create a more
accurate depiction of what lies below. Geophysical logs permit
individuals to obtain information about
hydrogen content, well construction,
and underlying geologic lithology,
porosity of materials, water quality,
and fracturing of rock body material.
All of this information is readably
obtainable without conducting risky
operations to retrieve rock cores, which can commonly break or
Wireline truck (white rig to the left) on a typical
shallow well frack pad. (Photo by. Evan Calhoun)
be lost during extraction. Logs are easily conducted from
vehicles with highly specialized equipment and sophisticated
computer systems that record the geophysical data using a
wireline cable that sends a sonde downhole. The sonde is the
device that physically records the information as the cable
traverses it vertically in the borehole. In the petroleum field,
a well is drilled in an area that is likely to bear natural gas
or oil deposits. Before the final completion of the well, a
geophysical log must be recorded and then interpreted. This can
be done during the drilling process. However, it is also very
common to do this after the initial drilling process has been
completed. Prior to fracking operations beginning, a wireline
crew will go in and either re-log the well or do the initial
logging to decided which zones, if any, will be fractured.
Wireline logging is a broad term that encompasses many methods
of geophysical data collection. The most commonly practiced
methods include: Caliper Logging, Sonic Logging, Electrical
methods, Self-Potential, Natural Gamma Logging, Neutron Porosity
Logging, and detecting radiation. Often, many of these methods
are used in the data collection and included on one log.
One of the primary, most useful, and simplest method of
borehole logging in caliper logging. The primary functions of
this method are to record borehole diameter, detect
irregularities and measure the geologic stratification of the
drill hole. Of these uses, one of the most critical functions is
the recording and logging of average borehole diameter of the
borehole. This critical data is used to later
correct other logging methods to improve their
accuracy due to the effects that variations in
borehole diameter can impose on them. Methods that
used caliper logging corrections include neutron
porosity and density logging. Caliper logging
probes usually consist of one to four pads.
Calipers with four or more arms are usually the most beneficial
to creating a true picture for loggers of what lays downhole by
essentially mapping the entire depth of the borehole. The highly
sensitive pads are sending data to be recorded as they travel
upwards along the borehole wall recording any variations in
diameter. Many of these variations result from drilling
techniques due to the lithology of the subsurface. The caliper
logging method can also be used to look for fractures, faulting,
water tables, and openings within the units drilled. Knowing
the location of these fractures and other subsurface anomalies
is critical to understanding subsurface water movements, as well
as their added value in any hydraulic fracturing or further
drilling operations. From the combination of data resulting from
each of the anomalies, a much more accurate interpretation of
the geology can be made.
Common Caliper Logging
Tools (Image 3)
Another highly useful method of logging geologic
stratification is Natural Gamma Logging. It has been actively
used by the industry since 1927. This method is
commonly used to distinguish the stratification
of the subsurface. The downhole tool only
records total gamma value in terms of energy
level emissions from radioactive isotopes. It
is measured by the variability in the API
radioactivity units. The isotopes commonly seen
are the highly radioactive decay or daughter
products of potassium 40, Thorium 232, and
uranium 238. Variances in gamma readout are used to distinguish
shale materials from non-shale material in the petroleum
industry. Shale and hot shale material will show “kicks” or
trends to the right of center on the log due to high gamma ray
values. Sandstone material will however display much lower
values on a log and will trend to the left of the log. Other
clay rich material also can be detected due to their relatively
high level gamma ray counts. Gamma ray recording has a maximum
penetration of one to two meters into any solid rock material.
However, depth of penetration greatly increases as the rock
density decreases. Therefore, coals and highly porous material
can be much more accurately depicted on the log.
Typical Gamma Log
(Photo by Evan Calhoun)
Neutron porosity logging has also become an industry
standard. This method relies on an active source of neutrons,
which will then be emitted from the transmitter into the
borehole wall with extremely high energy. The
application of this method is invaluable to the
petroleum industry due to one specific criteria.
When large deposits of hydrocarbons are found, the
penetration power of the neutrons are significantly
reduced, if not stopped. The neutrons will
instantaneously lose their high energy output from
the rapid collision with the nucleus of the
hydrocarbon. The measurement difference produced
by this loss in energy permits petroleum deposits
to accurately be located and then extracted. When conducting
this method, a highly sensitive probe is used. Many assumptions
have to be made in order to interpret the data collected. First
and foremost, the pore space within the material is assumed to
be filled with water. Therefore, if a decrease in neutron speed
is encountered, we can then draw the conclusion that a deposit
has been encountered. Loggers must also take into consideration
that drilling practices can alter the borehole, which can in
turn alter the measurements of the neutron logging. This method
can be conducted in boreholes of many different types. Depth is
usually not a factor in this. Also, the method can be completed
Common Well log.
Shaded areas are
possible hydrocarbon
deposits (Photo by
Evan Calhoun)
in either open boreholes or ones that have been cased in steel.
One of the few exceptions to this are boreholes that have been
cased using a PVC material. This is due to the very high
hydrogen content in the PVC material. Once the data has been
collected, the greatest use it to plot it along with many other
geophysical logs in order to precisely pinpoint distinct
lithological units deep within the earth.
Electrical methods are also widely used and very common in
geophysical borehole logging. The most common of these methods
are resistivity, conductivity, and self-potential methods.
These methods are commonly used to detect water chemistry,
mineralogy, rock alteration, acid content, and temperature.
Hydrocarbon deposits are easily detected using electrical
methods due to their high resistivity in comparison to the
variably salty water that is usually found deep within the rock
material in the areas surrounding the deposits. It is measured
by using electrodes and electrical currents. The current is
generated and forced to flow through the rock formations from
and electrode and then received again by another electrode.
Induction coils are commonly used in the conductivity plots,
conductivity being the inverse of resistivity.
The first electrical method which is very common in
borehole surveys is electrical resistivity. The predominant unit
for this survey when logging is the SI unit of ohm-meters. When
rock material is subjected to voltages, electrical current will
flow. Certain materials however are highly resistive to
electrical flow. This method measures that resistance to
electrical current when a medium that does not conduct the
current well is encountered. The value of resistivity can be
comparatively used with other methods of geophysical logging to
better determine the lithology of the underlying subsurface
materials. Quartz and muscovite bearing materials do not conduct
the current well, and therefore have very high resistivity
values. Meanwhile, materials such as clays and salt water
bearing material have very low values of resistivity because
current is easily conducted through them. Materials such as
sandstone also have considerably lower values of resistivity due
to the fair amount of pore space present within the rock
structure. The present
pore spaces will
typically be filled
with salt water of
some concentration,
giving it the low
value of resistivity
as before mentioned.
This water is commonly a result of drilling or fracturing
processes and does not need to be naturally occurring.
Common Resistivity Values of Materials. Image 4
Resistivity logs are also greatly affected by lithological bed
thickness as well as the diameter of the borehole. Unlike the
next method of surveying, resistivity logs can only be recorded
in situations where they are surveying water or mud filled open
borehole.
The next method of geophysical borehole electrical
surveying is conductivity. Conductivity is commonly used in
reference to electromagnetic induction and is measured in
Siemens per meter. As
mentioned earlier,
conductivity is the inverse
of resistivity. These
surveys have the versatility
to either be completed in a
dry borehole or a fluid
filled borehole. Factors that can cause great influence on
conductivity include the borehole mineralogy, drilling
alterations to the borehole structure, fracturing, porosity of
the material, water salinity, and pore connectivity. Corrections
for these factors have to be taken into consideration and in
most instances completed in order to collect the most precise
data possible. Many of these factors have to deal with fluid
mechanics. A highly fractured, very porous, well connected
material filled with solution of water will be less resistive to
flow and more conductive. This increases your chances of
petroleum extraction and is commonly sought as a result in these
surveys. However, the accuracy of this survey greatly suffers in
material that exhibits high resistivity. A high resistivity
means that the material will not be very conductive. For this
reason, it is very rare to apply this method is solid rock body
environments.
Self-potential logging is also another electrical method
that is also very commonly used within the industry. This method
of logging is commonly conducted at the same time that the
resistivity data is being collected. First developed by the
Schlumberger Company, it has been widely used since its creation
in 1931. Self-potential utilizes small changes that occur in
voltage between the recording electrode and the wall of the
borehole. Lithology, chemical variances, mineralogy, and many
more factors influence the method. The other factors that cause
the variances in voltage are not fully understood in the field,
therefore interpretation of the data can become very difficult
and debatable. Even with what confusion exists about the method,
the few concrete facts and comparisons that can be drawn have
proven invaluable what put in context with other sources of
geophysical data.
Each and every method of geophysical borehole logging has
experienced advancements in technology. No one method is the key
to understanding the lithological differenced that lay deep in
the earth beneath us. The key to extracting
the full potential of this information is to
use it in unison with multiple other logging
techniques. When multiple log arrays are put
together in one integrated set, on a
definitive scale and correlated, only then can
accurate conclusions be drawn about the vast
subsurface of our world. As the industry continues to grow, it
is very possible that each of these methods will experience
further advancements in accuracy and reliability. It is also
very plausible that with time, many new methods could be
developed. The petroleum industry is ever changing, but many of
the core principals it was founded on remain true. Wireline
logging has become one of the hallmark tools of the trade, and
will continue to be so for the foreseeable future.
Shallow Well Log book
(Photo by Evan Calhoun)
Works Cited
Anderson, Mark. "Discovering the Secrets of the Earth." Defining Logging. Schluberger, 1 Jan. 2011.
Web. 1 Jan. 2014.
<https://www.slb.com/resources/publications/oilfield_review/~/media/Files/resources/oilfield_review/ors
11/spr11/defining_logging.ashx>. (Anderson)
Chopra, Prame, Eva Papp, and David Gibson. "Geophysical Well Logging." (2002): 105-15. Print.
(Chopra)
Keys, W. Scott, and L. M. MacCary. "Application of Borehole Geophysics to Water-Resources
Investigations." Book 2 (1971): 94-97. Print. (Keys)
"Reading the Rocks from Wireline Logs." Www.kgs.ku.edu. Kansas Geological Survey, 1 Nov. 2003.
Web. 1 Jan. 2014. (Reading the Rocks)
Williams, John. "Introduction to Borehole Geophysics." New York Water Science Center. USGS, 1 Jan.
2013. Web. 1 Jan. 2014. <http://ny.water.usgs.gov/projects/bgag/intro.text.html>.(Williams)
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