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Introduction

Porosity Logging Core

Learning Objectives

By the end of this lesson, you will be able to:

Identify the different logs used to determine porosity

Explain the definition of porosity and how it is determined

Porosity Logging Core ═══════════════════════════════════════════════════════════════════════════════════

©PetroSkills, LLC. All Rights Reserved. _________________________________________________________________________________________________________

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Primary Porosity Logs

Density Log

Neutron Log

Sonic Log

Nuclear Magnetic Resonance Log

A “Porosity Reminder” (1)

Porosity – Definition and Computation

Scanning Electron Microscopy (SEM) photograph of quartz sand. The total

Bulk volume (v) comprised of grains and fluid-filled pores (v = A x h)

Porosity = pore volume per unit volume of the formation. Porosity indicates how much fluid can be held.



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General and Special Porosity Cases

Dependent upon lithology identification• We can measure formation density in grams per cubic centimeter,

(gm/cc).• We can measure neutron count rates and convert to porosity.• We can measure the sonic travel time, slowness, in microseconds

per foot or per meter.

Single sensors• Density log, fD (radioactive source of gamma rays)• Neutron log, fN (radioactive neutrons)• Sonic fS (acoustic device)

Combination Sensors• fN-D (fXP from density and neutron)• fN-S (fXP from sonic and neutron)

Gas bearing formations

Shaly formations

Learning Objectives

Identify the different logs used to determine porosity

Explain the definition of porosity and how it is determined



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Density Log


Learning Objectives


Describe the logging tool configuration in the borehole and the basic physics of the Density Log and Litho-Density Logs

Discuss the units and the scale used for Density log data

Explain how Density log data are used to calculate porosity and what are the required “inputs”

Describe the effect of free gas saturation on the bulk density (b) log and the impact on calculated porosity if 100% liquid is assumed

Discuss how the correction curve and caliper curve are used together to quality check bulk density log data



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Porosity from Density as Single Sensor

Several applications

• Porosity estimation

• Lithology estimation

• Prediction of hydrocarbon density

• Input to synthetic seismograms (density x velocity)

• Formation mechanical properties and rock physics

Formation Density Compensated (FDC)

Density Tool responds to electron density of the formation in front of the tool.

Density Log response; combination of matrix density and density of fluids in the invaded zone in their relative proportions. For simplicity, we have considered ρw = ρmf.

The Density Log measures bulk density of formations in open-hole (g/cc).

In a known lithology, the bulk density is used to determine formation porosity.

Also an important reference for detecting light hydrocarbons.

The Density Log has earned its reputation as the most reliable and useful porosity log.

Typical Density Tool Configuration



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Gamma Ray Interactions

Logging Tools Available

Wireline Density Measurements• LDT = Litho-Density Tool, Schlumberger• ZDEN = Litho-Density Tool, Baker Atlas• SDLT = Litho-Density Tool, Halliburton• MPD = Litho-Density Tool, Weatherford

LWD Density Measurements• ALD = Litho-Density, Sperry (Halliburton)• ADN & CDN = Litho-Density, Anadrill (Schlumberger)• MDL = Litho-Density, Baker-Huges Inteq• AZD = Litho-Density, Weatherford



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Density Correction by Spine and Ribs

Log of Corrected Bulk Density

(0.1524 m) (0.4064 m)



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Good qualitative indication of log quality

Should be ~ 0 in smooth borehole

• Barite mud exception

Can be good fracture indicator as well

A. This hole was drilled with an 8-½ inch (18 mm)

drill bit.

(0.1524 m) (0.4064 m)

(454 kg)

vs. Caliper Curves

b = Φ [Sxow + (1-Sxo)h] + (1 - Φ)ma

b : Formation bulk densityma : Matrix densityf : Fluid densityw : Water densityh : Hydrocarbon densityΦ : Formation porositySxo : Invaded zone water saturation

Rearrange to calculate porosity, Phi

Quartz 2.65 g/cc

Calcite 2.71 g/cc

Dolomite 2.87 g/cc

Density Tool Response Equation

ma b

ma f

- =

-



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Density Values in Formation Evaluation (g/cc)

Reservoir matrix

Quartz (sandstone) 2.65

Calcite (limestone) 2.71

Dolomite 2.87

Pore fluids

Fresh water 1.00

Salt water (200 g/l (200 kg/m3)) 1.13

Fresh water with 30% residual oil 0.90 – 0.94

Fresh water with 30% residual gas 0.73 – 0.74

Other minerals

Halite (rock salt) 2.03

Anhydrite 2.98

Core Porosity Calibration

Co

re p

oro

sity

(%

bv)

Log density (g/cc)

Apparent fluid

Density

fl

Core matrix density

ma



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Density Log Response Chart

Porosity = 25%

Remember:Sand (quartz rich)Limestone (calcite rich)

PEF (or Pe) Curve

“Essentially” porosity independent

Typical values• Sand = 1.8• Limestone = 5.1• Dolomite = 3.1• Salt = 4.7• Anhydrite = 5.1

Original density log was developed in the 1960s

• Measured bulk density • Called Formation

Density Compensated Log (FDC)

Photoelectric Factor Curve (PEF) was later developed

Density log was renamed Litho-Density Tool (LDT)

• Single curve lithology indicator



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Litho-Density Tools: PEF (or Pe) Curve

Photoelectric Factor (PEF)• Incident low energy gamma ray absorbed by electron and electron

ejected from the atom• PEF = (Z/A)exp 3.6

– Where: • Z = atomic no.

• A = atomic weight

Measures different part of decay spectrum

Can be highly affected by barite muds

Single curve lithology indicator

Pe vs. Porosity: Litho-Density Tools

Chart Use:

Read PEF

Read Bulk Density

Determine Porosity %

Porosity = 20%



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Summary: Density as Single Sensor

Source for porosity and lithology information

Litho-density (PEF) provides additional lithology information

Pad contact tool • Rugose hole implications• Barite mud implications• Fracture identification implications

Can be combined with other porosity devices to define lithology, porosity, shale content

Depth of investigation shallow (< 50 cms)

Vertical resolution ~ 20 cms

Learning Objectives

Describe the logging tool configuration in the borehole and the basic physics of the Density Log and Litho-Density Logs

Discuss the units and the scale used for Density log data

Explain how Density log data are used to calculate porosity and what are the required “inputs”

Describe the effect of free gas saturation on the bulk density (b) log and the impact on the calculated porosity if 100% liquid is assumed

Discuss how the correction curve and caliper curve are used together to quality check bulk density log data



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Neutron Log


Learning Objectives


Describe the Neutron tool configuration in the borehole and the basic physics of the CNL log

Identify the units and the scale used for Neutron log data

Identify what element in the formation has the most effect on the Neutron porosity reading

Recognize the typical log response of the CNL log in the shale intervals and high porosity gas bearing intervals



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Porosity from Neutron as Single Sensor

Neutron responds to:• What’s in pore spaces

– Pore fluid chemistry

• Matrix material– Shale (clay minerals +

other minerals)

Factors affecting neutron:• Anything with hydrogen

– Water

– Oil– Gas

– Clay Minerals

– Other hydrated minerals

Compensated Neutron Log (CNL)

Developed to reduce inaccuracies caused by borehole and environmental effects on single detector tools.

CNL porosity if affected by borehole environment

• Temperature• Salinity

CNL log is recorded in linear porosity units

When combined with another porosity survey, both logs are recorded on the same porosity scale

• Scales are made compatible



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Neutron Log Animation

Compensated Neutron Measurement

Bombard formation by 5 Mev neutrons• Elastic collision with nuclei• Energy loss in collision function of nuclei relative mass• Hydrogen is most effective

Measure resultant products at a near and far detector• Size of the neutron cloud is inversely proportional to hydrogen

content• Population of neutron cloud is a function of absorption qualities

The CNL is calibrated to read accurate porosities in water saturated limestone.



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Elastic Neutron Scattering

Neutron Interactions (1)

The rate the rate at which neutrons lose energy by collision is related to the amount of hydrogen in the formation

Since both water and hydrocarbons contain hydrogen, the quantity of hydrogen present is related to porosity

LOW Energy Loss

Non-HydrogenNucleus

HIGH Energy Loss

Hydrogen Nucleus

Non-HydrogenNucleus

Neutron Interactions (2)

Relative slowing down effectiveness

• H = 100.0• C = 15.8• O = 12.0• Si = 7.0• Ca = 4.9• Cl = 5.6

Neutron Slowing Down Length

SFast

Neutron

Epithermal Neutron

Reaches Thermal

Energy

Capture

Capture

Gamma Ray



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The Neutron Porosity Response Equation

Neutron Logs register anomalously low apparent porosity in gas-bearing reservoir zones because of the low hydrogen index of gas.

• This was called “Excavation Effect.”

The Compensated Neutron Log was calibrated to read the correct porosity in water saturated limestone.

• For example, a 30% porosity reservoir rock that is saturated with gas may have an “apparent” Neutron porosity of as low as 10%.

A high porosity gas-bearing zone will appear as a low porosity zone on the Neutron Log.

This difference is called the “Gas Effect.”

The combination with a different type of porosity tool, such as the density, makes a powerful gas-liquid discriminator.

The Neutron Porosity Response Equation

The Neutron tool responds to the hydrogen index of the formation in front of the tool.Neutron Log response is a combination of the matrixhydrogen index and the hydrogen index of the fluidsin the invaded zone in their relative proportions.

n = [SxoHw + (1-Sxo) Hh] + (1 - )Hma

n : Neutron porosity

Hma : Matrix hydrogen index

Hw : Water hydrogen index

Hh : Hydrocarbon hydrogen index

: Formation porosity

Sxo : Invaded zone water saturation

The Neutron Tool (CNL)



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Neutron Logging Tools Available

Wireline Neutron Measurements• CNL = Compensate Neutron Tool, Schlumberger• APS = Accelerator Porosity Sonde (epithermal), Slb• CNT = Compensated Neutron Tool, Baker Atlas• DSNT = Compensated Neutron Tool, Halliburton• CNS = Compensated Neutron Sonde, Weatherford

LWD Neutron Porosity Measurements• CTN & CNO = Neutron Porosity, Sperry (Halliburton)• ADN & CDN = Neutron Porosity, Anadrill (Schlumberger)• MNP = Neutron Porosity, Baker-Hughes Inteq• TNP = Neutron Porosity, Weatherford

Neutron Log Presentation

(m)

(0.15) (0.2) (0.25) (0.3) (0.36)



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Neutron Log Response Chart – Single Sensor

The Neutron porosity log is calibrated to read correct porosity directly in a limestone matrix

Sand = + 4% p.u.

Dolomite = -7% p.u.

Summary: Neutron as Single Sensor

Great variety of vintage neutron logging tools

Compensated dual thermal detector type (CNL) most common

Highly sensitive to shale (clay) and gas

Combination with density log is “standard”

Special attention to “compatible” scaling

Old neutron logs may not be scaled in porosity and require conversion

Depth of Investigation ~70 cms

Vertical resolution ~40 cms



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Learning Objectives

Describe the Neutron tool configuration in the borehole and the basic physics of the CNL log

Identify the units and the scale used for Neutron log data

Identify what element in the formation has the most effect on the Neutron porosity reading

Recognize the typical log response of the CNL log in shale intervals and high porosity gas bearing intervals



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Density and Neutron Combination Log


Learning Objectives


Describe Density/Neutron log compatibility scaling and why they are displayed on the same log track

Recognize the relative positions of the Density log and Neutron porosity curves in typical shales and liquid bearing clean reservoir rocks

Identify “gas effect” on the Density/Neutron log

Identify how the Density/Neutron log may be used to indicate lithology in clean (non-shaly) rock



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The Density Tool Density Tool responds to electron density

of the formation in front of the tool Density Log response; combination of

matrix density and density of fluids in the invaded zone in their relative proportions. For simplicity, we have considered ρw = ρmf

Porosity Determination – Nuclear Devices

The Neutron Tool Neutron Tool responds to hydrogen

index of the formation in front of the tool. Neutron Log response; combination of

the matrix hydrogen index and the hydrogen index of fluids in the invaded zone in their relative proportions. For simplicity, we have considered Hw = Hmf

b = Φ [Sxow + (1-Sxo)h] + (1 - Φ)ma

b : Formation bulk densityma : Matrix densityw : Water densityh : Hydrocarbon densityΦ : Formation porositySxo : Invaded zone water saturation

n = [SxoHw + (1-Sxo) Hh] + (1 - )Hma

n : Neutron porosityHma : Matrix hydrogen indexHw : Water hydrogen indexHh : Hydrocarbon hydrogen index : Formation porositySxo : Invaded zone water saturation

Density/Neutron Combination

Density tool measures bulk density; neutron tool measures hydrogen density

Gas separation of curves; neutron on right and density on left in gas bearing reservoirs

Shale separation of curves; neutron on left and density on right because of high hydrogen density (clay bound water) in shales

Density/Neutron cross plot helps determine mixed lithologies of sandstone, limestone, and dolomite.

Gas-bearing intervals plot away from lithology lines, correction follows “approx. gas correction” line.

Quick Look D/N alternatives; Gaymard’s Method (gas-filled zones), 1/3:2/3 line (see next figures)



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“Quick Look” Porefill Determination with Density/Neutron Logs

The hydrocarbons have density and hydrogen index less than that of water.

Thus the presence of hydrocarbons in the formation results in a decrease of density and neutron log responses, which results in the log separation.

Gas has very low density and hydrogen index compared to water or oil, resulting in a larger separation.

Density/Neutron Limestone Scales

Compatible lithology scales are important

• Density scaling in B1.95 g/cc to 2.95 g/cc

• Neutron presented in limestone units

• Scale compatible in porosity and lithology

– 45% – -15%

– Stack in clean wet limestone



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Density/Neutron Mineralogy Model

Limestone or Sandstone

Density/Neutron Responses with Fluids



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Density/Neutron Cross-Plot

Recognition of Gas in a Sand-Shale Sequence



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Density/Neutron Porosity – Identify the Gas

A Few More Words on Net Sand Selection

Example of Density

Neutron Xplot: Clastic Reservoir

Clean sandsClean sands

Shaly sandsShaly sands

Very shaly sandsVery shaly sands

Pure shalePure shale



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A Few More Words on Porosity

Learning Objectives

Describe Density/Neutron log compatibility scaling and why they are displayed on the same log track

Recognize the relative positions of the Density log and Neutron porosity curves in typical shales and liquid bearing clean reservoir rocks

Identify “gas effect” on the Density/Neutron log

Identify how the Density/Neutron log may be used to indicate lithology in clean (non-shaly) rock



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Sonic Log


Learning Objectives


Describe the basic tool physics of the Sonic tool

Identify the units and scale used for Sonic logs

Explain how Sonic log data are used to calculate porosity and what are the required “inputs”

Recognize what two logs are used for “seismic calibration”

Identify the two acoustic measurements needed for calculating mechanical rock properties



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Sonic Log (Acoustic Log)

The Sonic log (or Acoustic) log, is the third of the so-called porosity logs that are introduced in this module.

From the Schlumberger Oilfield Glossary: • The Sonic log is a type of acoustic log that displays travel time of

P-waves versus depth. • Sonic logs are typically recorded by pulling a tool on a wireline up

the wellbore. • The tool emits a sound wave that travels from the source to the

formation and back to a receiver.

Sonic Log Applications

Petrophysical• Porosity, lithology, and fluid saturation• Mechanical properties and rock physics

Geophysical• Calibrate seismic surveys (Time to Depth)• Synthetic seismograms• Vertical Seismic Profile (VSP) survey • Seismic Inversion

Other• Cement bond logging in cased hole• Overpressure prediction in shales



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Sonic Tool Measurement Principles

Send out a sonic pulse from a magnetostrictivetransmitter(s)

Listen for the received signal at receivers two or more distances from the transmitter

Record the acoustic signal waveform and differences in arrival time for different receivers and wave modes

Multiple wave modes can be excited or converted

• Compressional or P wave – fastest• Shear or S wave• Stoneley (tube)

Sonic Tool Measurement Principle

UncompensatedTwo Receiver Tool

Borehole Compensated (BHC)

Tool

BoreholeCompensation

3 f

ee

t(.

91 m

)

2 ft(.61 m)



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Sonic Tool Principle – Wave Modes

Stoneley

Shear

Compressional

First Motion

Sonic Tool Principle – Wave Modes



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Sonic/Acoustic Logging Tools

BHC and LSS Dipole Array

Tool Combinations Auxiliary Measurement

Sonde Compensated Neutron Log Gamma Ray Litho-Density* Formation MicroScanner* Natural Gamma Ray

Spectroscopy Phasor* Induction

(3.96 m)

(5.49 m)

(1.07 m)

(0.15 m)

(2.59 m)

(5.03 m)

(2.74 m)(3.35 m)

(3.51 m)

Sonic Measurement Principle – Array Processing

Waveforms Semblance/Coherency Processing

Depth-Derived Borehole Compensation



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(μ sec/m)

(148)

(164)

(180)

(197)(213)

(230)

(459)

(246)(262)(279)

(295)(312)(328)

(344)(361)(377)(394)

(410)(427)

(443)

Example of Sonic Log

Delta-t (t) of Common Minerals

Mineralt ma

(sec / ft)Velma

(ft / sec)

Quartz 55.6 18,000

Calcite 47.5 21,000

Dolomite 43.5 23,000

Anhydrite 50.0 20,000

Gypsum 52.5 19,000

Halite 67.0 15,000

Casing (Fe) 57 17,500



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Delta-t (t) of Common Minerals (Metric)

Mineralt ma

(sec / m)Velma

(m / sec)

Quartz 182 5,488

Calcite 156 6,402

Dolomite 143 7,012

Anhydrite 164 6,098

Gypsum 172 5,793

Halite 219 4,573

Casing (Fe) 187 5,334

Sonic Log Porosity – Single Sensor

Time Average Equation

Wyllie Time Average Equation

(164)

(∆t, microsec/ft)

(328) (492) (656)



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Porosity Evaluation from Sonic: Raymer-Hunt-Gardner

1.1

1.2

1.3

1.4

1.5

1.6

Bcp

Vma(ft/s)

- Gardner

(m/s)

(μs / m)

(98.4) (131.2) (164) (196.9) (229.7) (262.5) (295.3) (328) (360.9) (393.7)

Compaction coefficients

Issues with Sonic as a Single Sensor Porosity

A large effect on sonic velocities and therefore travel times with very small increases in gas saturation

A valuable tool for pay identification in complex formations

Fluid substitution to input the proper Dtfluid into the porosity equation is critical in such situations

Sonic waves preferentially propagate via grain to grain contact which means:

• Sonic porosity may under-estimate in presence of secondary porosity:

– Effect based on aspect ratio of pores

– “Round” Vugs may be completely overlooked– Fractures effect based on aspect ratio and orientation



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Sonic as Geophysical Interface

1 1 2 2

1 1 2 2

- V =

+ V

VR

V

Seismic Reflection Coefficient Equation

Sonic Data for Mechanical and Rock Properties

Both Density log data and Sonic data are required to calculate the various elastic moduli required to process seismic data and to design hydraulic fracture treatments.

Must have compressional and shear information

Methods for estimation of shear from compressional

Key inputs into stimulations and AVO processing

Rock physics properties can be valuable in determining pay, especially in complex tight reservoirs



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Dynamic Elastic Constants

µ Poisson’s Ratio

G Shear Modulus

E Young’s Modulus

KB Bulk Modulus

CB Bulk Compressibility

a X t3

4 -

∆t

1 ρ

strain volumetric

stress applied2s

2c

b

1 - /∆

1 - /∆ 1/2

strain allongitudin

strain lateral2

2

cg

cg

tt

tt

a X ∆t

ρ

strain shear

stress applied2s

b

a X μ 12G strainnormal

stress applied

BK

1

stress applied

ndeformatio volumetric

When Are Elastic Constants Important?

Drilling• Avoid exceeding fracture gradients

Engineering unconventional reservoirs• Understanding in situ stress of naturally fractured reservoirs

Completions• Hydraulic fracture designs• Gravel pack decisions vs. sanding/fines migration issues

Seismic modeling• Evaluation of synthetics, seismic attributes• Improving seismic processing



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Sonic as Single Sensor Summary

There is a variety of sonic energy and propagation modes.

These measured properties of the formation can be valuable links to:

• Petrophysical properties– Lithology, porosity, and permeability

• Geophysical properties– Synthetics and Time to Depth conversion

• Rock Mechanical properties– Stimulation and rock physics

Porosity Tools Checklist

Density: First choiceUsually more accurate than neutron or sonicRequired: matrix and fluid densities

Neutron: In combination with densityLimestone scalingSeveral corrections requiredUsed for gas/oil and shale discriminationCased hole porosity log

Sonic: Used for seismic calibrationPorosity tool if density not availableRequired: matrix and fluid travel times



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Learning Objectives

Describe the basic tool physics of the Sonic tool

Identify the units and scale used for Sonic logs

Explain how Sonic log data are used to calculate porosity andwhat are the required “inputs”

Recognize what two logs are used for “seismic calibration”

Identify the two acoustic measurements needed for calculatingmechanical rock properties

PetroAcademyTM Foundations of Petrophysics

Petrophysical Data and Open Hole Logging Operations Core

Mud Logging, Coring and Cased Hole Logging Operations Core

Gamma Ray and SP Logging Core

Porosity Logging (Density, Neutron and Sonic) Core

Formation Testing Core

Resistivity Logging Tools and Interpretation Core

Petrophysical Evaluation Core

Core Analysis Core Knowledge

Special Petrophysical Tools: NMR and Image Logs Core



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