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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
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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
Porosity Logging Core
Learning Objectives
By the end of this lesson, you will be able to:
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
Porosity Logging Core
Learning Objectives
By the end of this lesson, you will be able to:
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
Porosity Logging Core
Learning Objectives
By the end of this lesson, you will be able to:
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
Porosity Logging Core
Learning Objectives
By the end of this lesson, you will be able to:
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
Porosity Logging Core ═══════════════════════════════════════════════════════════════════════════════════
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