Borehole Geophysics in the

Petroleum Industry: Wireline Well Logging

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Evan Calhoun

Geophysics: October 30, 2014

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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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