Petrophysical Techniques; Geology/Geophysics 449

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Questions on Petrophysical Log Interpretation will not appear in the following exam. Log questions are published under a separate section.

Go to: Midterm II | Well Log Interpretation

Geology (GLGY 449-UCAL) Midterm I

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Question 1

Calculate the porosity, if the formation density was 2.25 g/cc on the density log and the core grain density was 2.80 g/cc. Assume the fluid density is 1.20 g/cc. (ID-PT1-37)

A	35%
B	29%
C	2.9%
D	34%
E	3.0%

Question 1 Explanation:

porosity = (density_matrix - density_bulk) / (density_matrix - density_fluid)
porosity = (2.80 - 2.25) / (2.80 - 1.20) = 0.34 = 0.34%

Question 2

Which of the following might be the best process to introduce significant amount of effective porosity at higher depths? (ID-PT1-44)

A	Dissolution of grains at the grain to grain contact boundaries.
B	Fractures of cements and minerals.
C	Dissolution of early diagenetic cements.
D	Break up of mineral compounds causing in situ porosity.

Question 3

A typical neutron log response to presence of shale is characterized by... (ID-PT1-19)

A	...a response close to the calibrated zero position.
B	...no response at all (at zero level).
C	...a very low negative response.
D	...a very high positive response.

Question 4

Which of the following most likely the primary cause of porosity in deep lithologies? (ID-PT1-29)

A	Organic activity
B	Dissolution of early diagenetic cements
C	Dissolution of cemented materials due to high temperature and pressure
D	Disintegration of minerals
E	Fracture development

Question 5

Which of the following situations will drastically reduce the effectiveness of neutron tools? (ID-PT1-25)

A	Porous formations filled with oil or gas.
B	High vibrations caused by drilling tools.
C	Porous formations filled with water.
D	Fractures formed due to drilling itself (non-natural fractures).

Question 5 Explanation:

Neutrons are absorbed by the hydrogen nuclei in water.

Question 6

What is the typical range for geothermal gradient for planet Earth? (ID-PT1-06)

A	15^o to 25^o Celsius per kilometer of depth.
B	20^o to 30^o Celsius per kilometer of depth.
C	15^o to 50^o Celsius per kilometer of depth.
D	30^o to 50^o Celsius per meter of depth.
E	30^o to 50^o Celsius per kilometer of depth.
F	20^o to 30^o Celsius per meter of depth.

Question 7

What does the bulk density and neutron porosity cross overs indicate? (ID-PT1-20)

A	sudden increase in density
B	high natural radiation levels
C	gradual increase in density
D	gas effect

Question 8

What do we used for calibration of neutron tools? (ID-PT1-18)

A	clean coal
B	clean clay
C	clean shale
D	clean sandstones
E	clean limestones

Question 9

Which of the following conditions would Geologists prefer? (ID-PT1-02)

A	Fast mudcake buildup with very shallow mud filtrate.
B	Fast mudcake buildup with very deep mud filtrate.
C	Slow mudcake buildup with very shallow mud filtrate.
D	Slow mudcake buildup with very deep mud filtrate.
E	It all depends on the type of mud used for the drilling process.

Question 10

Where are the logging equipment placed (sensor processors, emitters, etc) in a typical logging while drilling (LWD) string? (ID-PT1-45)

A	In the electronics bay.
B	In the bottom hole assembly
C	In the logging equipment casing.
D	Inside the equipment pads.
E	Outside of the borehole; in the data collection truck.

Question 10 Explanation:

Bottom Hole Assembly is also known as BHA and it is right above the drill bit.

Question 11

What type of log is used to measure borehole size and shape? (ID-PT1-07)

A	Gamma ray logs
B	Spectral gamma ray logs
C	Spontaneous potential logs
D	Neutron logs
E	Density logs
F	Caliper logs

Question 12

Given a geothermal gradient of 25^o C/km, what is the approximate temperature at 2200 m below surface if the surface temperature is 10^oC? (ID-PT1-09)

A	~ 65 ^o C
B	~ 45 ^o C
C	~ 43 ^o C
D	~ 55 ^o C
E	~ 49 ^o C

Question 12 Explanation:

(25^o C/km)(2200m)(1km/1000m) + 10^o C= 65 ^o C

Question 13

Fundamentally neutron tools measure... (ID-PT1-31)

A	...the neutron radiation omitted by the formation nuclei.
B	...the radiation omitted by elastic collisions between the neutron and the formation nuclei.
C	...the radiation that was not absorbed by the formation.
D	...the radiation omitted by inelastic collisions between the neutron and the formation nuclei.

Question 14

In a typical mud system used in Alberta, we have the following components. Place them in order starting from dirty mud returns. (ID-PT1-03)

(START) Dirty/used mud return to...
I. Distiller
II. Mud storage and settling tubs
III. Annulus spit out
IV. Desander
V. Shale shaker
VI. Mud pumps

A	I --> V --> VI --> II --> IV --> III
B	V --> IV --> I --> II --> VI --> III
C	III --> V --> I --> II --> VI --> V
D	II --> V --> I --> III --> VI --> V
E	I --> III --> VI --> IV --> V --> II
F	I --> V --> IV --> II --> VI --> III

Question 15

What is the typical purpose of spectral gamma rays (SGR) logs? (ID-PT1-42)

A	Identification of non-radioactive clays and dolostones.
B	Primary depth control.
C	Calculate the volume of shale to sand ratio.
D	Analysis of grain volumes and grain characteristics of formation.
E	Calculate the bulk porosity of formation.

Question 16

What type of logging tool is described bellow? (ID-PT1-35)

Depth of Investigation: 15 -25 cm
Resolution: 50 cm
-Very high readings when shales or coals detected

A	Gamma Ray or Special Gamma Ray long
B	Spontaneous Potential log
C	Neutron log
D	Density log
E	Caliper log

Question 17

Gamma rays interact with the formation primarily through what process? (ID-PT1-27)

A	Gamma ray bouncing effect
B	Radioactive capture
C	Compton scattering
D	Natural gamma rays
E	Pair production

Question 18

What is the primary reason why we are interested in density logs? (ID-PT1-14)

A	A high log value of density indicates a very good reservoirs.
B	The density can be used to calculate porosity of formations.
C	It can help determine the type of drill bit to use to cut into the formations.
D	It provides an overview of the lithological formations (for example, quartz from calcite).
E	The density logs can be used to resolve extremely thin formations and transitional zones.

Question 18 Explanation:

While used with combination of other logs, density logs can be used to derive lithological interpretations, the primary use is the calculation of porosity.

Question 19

Which of the following tool's measurements are effected by salinity of the drilling mud/fluid? (ID-PT1-26)

A	Spontaneous potential tools
B	Neutron tools
C	Caliper tools
D	Gamma ray tools
E	Density tools

Question 20

Which of the following is the best log for determining permeability? (ID-PT1-21)

A	Gamma ray logs
B	Neutron logs
C	Spontaneous potential logs
D	Caliper logs
E	Density logs Hint: Nope, that's an indicator of porosity.

Question 21

What is the minimum vertical bed resolution of gamma ray tools running at normal speeds? (ID-PT1-11)

A	60 - 70 cm
B	10 - 15 cm
C	5 -10 cm
D	20 - 30 cm
E	40 - 50 cm

Question 22

Which of the following statement is correct about mud invasion? (ID-PT1-23)

A	The mid invasion is enhanced by the fast build up of mudcake.
B	The invaded zone is larger in low porosity and permeability formations because slower flow result in slow build up of mudcake.
C	The invaded zone is larger in high porosity and permeability formations because faster flow result in slow build up of mudcake.
D	Larger invaded zones provides the best resolutions of LWD logs because it allow the waves/signals to be penetrated deep into the formation.

Question 23

Which of the following allow gamma rays to penetrate deep into the formation? (ID-PT1-10)

A	High radioactive formations hence with more constructive interference.
B	Very low density where fewer gamma rays are absorbed hence increasing the propagation distance.
C	Formations that naturally produce very high rates of gamma rays hence allowing the artificial gamma rays to be amplified.
D	Low radioactive formations hence less destructive interference.
E	Very high density allowing gamma rays to bounce off from atom to atom deep into the formation.

Question 24

What is the temperature regime of Logging While Drilling (LWD) tools? (ID-PT1-24)

A	It is that of the drilling mud.
B	It is dependent on what type of logging tool is used.
C	It is that of the formation.
D	It is that of the surface.

Question 25

What is the typical vertical minimum bed resolution for density log running at normal speeds? (ID-PT1-16)

A	25 - 35 cm
B	40 - 50 cm
C	15 - 20 cm
D	0 - 10 cm
E	10 - 40 cm

Question 26

Which of the following will increase the depth of investigation (DOI) of gamma ray logs? (ID-PT1-40)

A	Higher formation fluid content allowing gamma rays to penetrate the formation.
B	Lower formation density resulting fewer gamma rays being absorbed.
C	Higher formation density resulting more gamma rays being absorbed.
D	Higher formation porosity allowing more gamma rays to penetrate the formation.
E	Higher radioactivity within the formation allowing gamma rays to interact with minerals within the formation.

Question 27

What is the typical depth of investigation for density log running at normal speeds? (ID-PT1-15)

A	40 - 50 cm
B	25 - 30 cm
C	30 - 35 cm
D	0 - 10 cm
E	15 - 20 cm

Question 27 Explanation:

To be fair, you could argue it is always less than 10 cm and not 0 - 10 cm. However, in multiple choice exams, you must pick the best possible answer.

Question 28

What is the best option to increase the extraction of hydrocarbon fluids from a formation? (ID-PT1-22)

A	Use high pressured drilling mud.
B	Use larger boreholes and wider cutting tools.
C	Use fracking to fracture the formations to increase permeability.
D	Use horizontal drilling.

Question 28 Explanation:

Why not fracking the best option? This is expensive and fracking not always produce an increase in output. Also there are too many unknowns in fracking compared horizontal drilling.

Question 29

Thorium-232 isotope interacts with gamma rays used by logging tools. Where do you find Thorium-232 in nature? (ID-PT1-41)

A	Subsurface formation waters as trace substance.
B	In radioactive natural deposits of uranium as a by product.
C	In stable minerals such as phosphates.
D	Within clay minerals such as illite and smectite.

Question 30

What is the primary purpose of controlling density of the mud during drilling? (ID-PT1-08)

A	To allow cutting to be transported to the surface.
B	To balance the lithostatic pressure of the formation to prevent borehole collapses.
C	To increase the speed of the drilling by applying pressure to the drill bit.
D	To prevent mud filtrate from reaching inner parts of the formation of interest.
E	To balance the hydrostatic pressure of the formation to prevent borehole collapses.

Question 31

Why do we use gamma rays instead of alpha or beta rays for logging? (ID-PT1-01)

A	Gamma rays do not react with any chemical components in the natural world.
B	Gamma rays are more penetrative than the others.
C	Gamma rays are not naturally produced in the subsurface hence removes the uncertainty factor.
D	Gamma rays produce a much higher energy difference that can be easily measurable by logging tools.
E	Gamma rays travel faster than other rays.

Question 32

What is counter acting the fluid invasion during drilling and after the well casing is placed? (ID-PT1-38)

A	Hydrostatic pressure of the formation.
B	Lithostatic and hydrostatic pressure of the formation.
C	Lithostatic pressure of the formation.
D	Thin mud film between the casing and the formation.
E	Mudcake itself surrounding the borehole.

Question 33

A very usual high response in gamma ray logs most likely caused by which of the following lithologyy? (ID-PT1-28)

A	Clean coal
B	Black shale
C	Clean sand
D	Carbonates
E	Mudstone
F	Organic content poor shale

Question 34

How is the count of gamma ray remaining after interaction with the formation is represented on a density log? (ID-PT1-39)

A	As neutron density.
B	As fluid density.
C	As matrix density.
D	As a difference between bulk and fluid density.
E	As bulk density.

Question 35

Why does the top drive drilling system considered an improvement in drilling technologies? (ID-PT1-04)

A	Improved cost efficiency (low cost).
B	Improved drill bits.
C	Improved logistics due to smaller equipment.
D	Improved logging tools.
E	Improved speeds of drilling.

Question 36

You noticed a very high gamma ray response. Given you do not have access to any other form of logs, what is the most suitable expiation for the observed response? (ID-PT1-13)

A	Highly radioactive formation;
B	Highly non-radio active formation;
C	Shale;
D	Coal;
E	Very thin formation;

Question 37

While neutrons used in neutrons tools have significantly shorter half life, what is the half life of free neutrons in nature? (ID-PT1-36)

A	About 2 years
B	About 15 minutes
C	About 4 hours
D	About 10 hours
E	About a day

Question 38

What is the typical depth of investigation for gamma ray log running at normal speeds? (ID-PT1-12)

A	60 - 70 cm
B	5 - 10 cm
C	10 - 15 cm
D	40 - 50 cm
E	20 - 30 cm

Question 39

Which of the following most likely to occur during burial of sandstone, shale and clay bearing formations? (ID-PT1-43)

A	Lower porosity loss in clay sequence compared to sandstone and shale due to clay being very dry in nature.
B	Rapid loss of porosity within the sandstone sequence due to sandstone having higher permeability.
C	No porosity loss within shale sequence due to shale being already compacted by the time of burial.
D	Increased rate of friction between clay particles due to overburden pressure.
E	Lower rate of porosity loss within the shale sequence due to tightly packed ionic bonds in shale.

Question 40

A density log is run in a producing formation giving a bulk density reading of 2.50 g/cc. From core measurements, the formation’s rock matrix density is measured as 3.40 g/cc and the fluid density is 1.41 g/cc. Based on this information what is the porosity? (ID-PT1-34)

A	~ 5%
B	~ 45%
C	~ 15%
D	~ 20%
E	~ 35%

Question 41

Most hydrogen in subsurface exists in what form? (ID-PT1-30)

A	As basic solutions.
B	Within water molecules.
C	As a part of hydrous minerals.
D	In the chemical structures of clay.
E	As saline solutions.

Question 42

You were tasked with choosing the cheapest but environmentally most friendly type of mud for a drilling operation. The area is known to have a large formations of evaporates. Which of the following type of drilling mud would you choose and why? (ID-PT1-05)

A	Oil-based mud because oils do not react with evaporates and the mud can be formulated to not to react with subsurface formations.
B	Water-based mud because it the most effective way to reduce the environmental impact.
C	All-oil based mud because it is the cheapest and can overcome the environmental cleanup costs.
D	Synthetic-based mud because the mud can be engineered to be inactive with the evaporate formations and reduce the environmental impact.

Question 43

The following diagram is a cross section of two identical boreholes. The only difference is that they were placed in two different locations. The brown area indicates the mud invasion. Given all other conditions are being the same, which of the following statements are correct? (ID-PT1-33)

A	The Well-I most likely surrounded by a lithology that is very porous and permeable than Well-II; hence faster mudcake build up have lead to smaller invaded zone.
B	The Well-I most likley surrounded by a lithology that poor in porosity and permeability than Well-II; hence faster mudcake build up have lead to smaller invaded zone.
C	The Well-II most likley surrounded by a lithology that poor in porosity and permeability than Well-II; hence the faster mud flow resulted in a large invaded zone.
D	The Well-II most likely surrounded by a lithology that is very porous and permeable than Well-I; hence the faster mud flow resulted in a large invaded zone.

Question 44

Spontaneous potential (self potential/SP) tools usually use NaCl. Which of the ions from this solution diffuse causing a potential difference and why? (ID-PT1-32)

A	The Cl^- ion diffuse because it is much smaller in size than Na⁺.
B	The Na⁺ ion diffuse because Cl^- attracts more water molecules.
C	The Cl^- ion diffuse because it has a stronger charge (attraction) than Na⁺.
D	The Na⁺ ion diffuse because it is much smaller in size than Cl^-.
E	The Cl^- ion diffuse because Na⁺ attracts more water molecules.
F	The Na⁺ ion diffuse because it has a stronger charge (attraction) than Cl^-.

Question 45

For any type of logging tool, the minimal vertical bed resolution may be higher than what is typical when... (ID-PT1-17)

A	...the depth of investigation is much higher than expected hence increasing the vertical minimum bed resolution.
B	...the contrast between the nearby formations and the resolved formation is higher.
C	...the radioactivity of the formations are much higher.
D	...the distance between formations are lower.
E	...the porosity and permeability of formations are higher.

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Credits: Based on the excellent class notes provided by, Dr. Rudi Meyer during Fall 2014.
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If you are a company or an organization willing to donate copies of well logs, please contact me. I really appreciate your contributions.

Type of tool	DOI	Resolution	Used for…	Processes	Notes
Caliper	NA	NA	-Borehole size and shape	Spring loaded arms	2 arm = diameter 4 arm = shape, orientation
Gamma Ray	10 – 15 cm	~40 cm	-Lithology(vol. Shale/Sand often due to high K), geosteering, facies, depth control	-Photoelectric effect (dom) -Compton scattering -Pair production -Scintillation counter + photomultiplier	High (right kick): -Shale line -Organic content -K-Feldspars Low (left kick): -Clean sand -Carbonates -Clean coals
Spectral Gamma Ray			-Radioactive anomalies -Clay types -Volume of shale(clay)	-Same as GR, but much better/larger scintillator
Density & Photoelectric Factor	< 10 cm	11 – 40 cm	-Porosity, general lithology, min ids
Neutron	15 – 25 cm	~50 cm	-Fluid, porosity, density, cross-over gas effect -Meassures porosity; used for calulating density	Hydrogen	High (left kick): -Shales and coals Low (right kick): -Gas breaing fm./rocks Use Dphi & Nphi chart *Decreased with increased permeability
Spontaneous Potential	Depends	Depends	-Permeability(even small), lithology, facies, shale volume(not so much)	Natural potential differences between fms. caused by: -Diffusion -Mambrane or shale potential	Important: -Salinity of drilling fluid to fm. -Conductive drilling fluid -Constrasting permeability in adj. beds -Surface electrode(earth) No absolute scale(relatove +/-) Positive kick (relative): -Clean sand or sand stone, shaly sand Negative kick (relative): -Shale *Decreased with increased permeability

Sanuja Senanayake

Geology 449-University of Calgary

Go to: Midterm II | Well Log Interpretation

Geology (GLGY 449-UCAL) Midterm I

Standing on the shoulders of giants

Go to: Midterm II | Well Log Interpretation

Geology (GLGY 449-UCAL) Midterm I

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Standing on the shoulders of giants