1. INSTRUCTOR © 2017, John R. Fanchi
All rights reserved. No part of this manual may be reproduced in any form without the express written permission of the author.
© 2004 John R. Fanchi All rights reserved. Do not copy or distribute.

2. To the Instructor
The set of files here are designed to help you prepare lectures for your own course using the text Introduction to Petroleum Engineering, J.R. Fanchi and R.L. Christiansen (Wiley, 2017)
File format is kept simple so that you can customize the files with relative ease using your own style. You will need to supplement the files to complete the presentation topics.

3. WELL LOGGING © 2017, John R. Fanchi
All rights reserved. No part of this manual may be reproduced in any form without the express written permission of the author.
© 2004 John R. Fanchi All rights reserved. Do not copy or distribute.

4. Outline Well Logging Subsurface Ionic Environment Lithology Logs
Porosity Logs
Resistivity Logs
Induction Logs
Log Calibration with Formation Samples
Modern Log Applications
Homework: IPE Ch. 9

5. WELL LOGGING

6. Well Logging Objectives
Estimate near-wellbore formation properties
Depth
Thickness (net and gross)
Porosity
Formation density
Acoustic velocity
Temperature and pressure
Lithology (rock type)
Fluid saturations
Indication of hydrocarbons
Indicate permeability (e.g. high, low, tight)
Structural trends (e.g. formation dip)
Fracture properties
© 2004 John R. Fanchi All rights reserved. Do not copy or distribute.

7. Typical Applications of Well Logs
Quality of wellbore
Size of wellbore (caliper log)
Integrity of cement bond (cement bond log)
Provide information for
Geologic mapping
Prospect evaluation (where to drill)
Reserve estimates
Indicate presence of hydrocarbons
Measure Sw; infer hydrocarbons
Fluid contacts, e.g. GOC, WOC
Indicate which zones to complete (perforate)

8. Well Log Header and Tracks
Scale
Scale
Scale
Scale
Scale
Track 1
Track 2
Track 1
Track 2
Track 3
Depth Track

9. Well Log Schematic Well Data Logging Data SP Res Track 1 Track 2
Depth Track

10. Types of Well Logs Lithology logs Porosity logs Resistivity logs
Spontaneous Potential (SP)
Now tends to be replace by Gamma Ray
Gamma Ray (GR)
Photoelectric Effect (PEF)
Porosity logs
Density
Neutron
Acoustic (Sonic)
Resistivity logs
Induction
Laterolog
Micro Resistivity
Specialty Logs
e.g. FMI (Formation Micro Image)
© 2004 John R. Fanchi All rights reserved. Do not copy or distribute.

11. Computer Generated Logs
Computer performs corrections and calculations
Graphical view
Easy to see analysis
Estimate lithology, saturations, porosity, etc.

12. SUBSURFACE IONIC ENVIRONMENT

13. Ohm’s Law Ohm’s Law: V = IR where V = voltage (volts)
I = current (amps)
R = resistance (ohms)
Conductance = 1/R (Siemens)
1 Siemens = 1 mho = 1/ohm
Electrical current is charge in motion, e.g. Na+ cation and Cl- anion.

14. Alternative form of Ohm’s Law
E = ρJ
where
E = electric field (volt/m)
J = current density (amp/m2)
ρ = resistivity (ohm-m) L A + −
Carrier of positive charge moves in direction of E, I, J
Carrier of negative charge (e-) moves in opposite direction

15. Resistivity and Resistance
Resistivity is related to resistance.
For uniform conductor with length L and area A:
R = ρ L / A
Resistivity ρ is the inverse of conductivity σ :
ρ = 1 / σ
Alternative form of Ohm’s Law:
E = ρJ
where
E = electric field (volt/m)
J = current density (amp/m2)
ρ = resistivity (ohm-m) L A + −

16. Fluids that Affect Logging Measurements
Drilling mud (resistivity Rm)
Mud filtrate(resistivity Rmf)
Formation water(resistivity Rw)
Hydrocarbons (assumed infinite resistivity)
Resistivity depends on formation temperature

17. Invasion Zones for Drilling Fluids
Adjacent Bed
Borehole
Uninvaded
Zone
Zone of Transition
Flushed
Mud
Cake

18. LITHOLOGY LOGS
© 2004 John R. Fanchi All rights reserved. Do not copy or distribute.

19. Common Reservoir Rock Types and an Illustrative Stratigraphic Column

20. Gamma Ray Log or Natural Gamma Ray Log
Gamma rays (GR) from NORM
Measure in API units
Relative unit
NORM
Potassium
GR energy 1.46 MeV
Thorium series
GR energy 2.62 MeV
Uranium-Radium series
GR energy 1.76 MeV

21. Light-Matter Interaction
Low-energy phenomenon
Photoelectric effect
Mid-energy phenomena
Thomson scattering (elastic)
Compton scattering (inelastic)
High-energy phenomenon
Pair production
Photoelectric Effect
Space
Time
Compton Scattering
Photon wavelength changes

22. Gamma Ray Log Response

23. NORM in West Texas Barnett Shale
Mississippian Barnett Shale above MD = 9606 ft
Mississippian Limestone below MD = 9606 ft
SGR – Spectral Gamma Ray
CGR – Total GR minus URAN
POTA – Potassium 40, wt %
URAN – Uranium, ppm
THOR – Thorium, ppm
Barnett Shale
Limestone
Source: Asquith and Krygowski, Fig. 3.3, Basic Well Log Analysis, 2nd Ed (2004)

24. Lithology Log: Gamma Ray
VARIABLE
RESPONSE
Gamma Ray
Rock Type
Detects shale from in situ radioactivity.
 High GR  shales
 Low GR  clean sands and carbonates
In most cases, shale formations are most radioactive
Most reservoir rocks exhibit low radioactivity
GR log is shale indicator

25. SP (Spontaneous Potential)
SP = Potential difference (voltage) between 2 fluids with different salinities
SP electrode
Grounded on surface
Connected to logging tool
SP affected by shale content
Can calculate formation water resistivity RW from SP
Need RW to calculate saturation

26. SP (aka Self Potential) Log
Measures potential difference between drilling fluids and formation waters
Distinguish permeable beds from shale
Small SP response  impermeable shale
Large SP response  permeable beds
SP log hard to interpret when formation waters are fresh (not salty)

27. Lithology Log: Spontaneous Potential
VARIABLE
RESPONSE
Spontaneous Potential
Permeable Beds
Measures electrical potential (voltage) associated with movement of ions.
 Low response  impermeable shale
 Large response  permeable beds

28. Lithology Log: Photoelectric Effect
VARIABLE
RESPONSE
Photoelectric Effect
Rock Type
Measure absorption of low energy gamma rays by atoms in formation.
 High GR  shales
 Low GR  clean sands and carbonates (absorb GR)
Photoelectric effect log is shale indicator
Photoelectric Effect

29. POROSITY LOGS
Formation Density
Neutron Porosity
Sonic

30. Density Log Gives rock density reading in gm/cc
Water = 1 gm/cc (62.4 lb/cu ft or 8.33 ppg)
Sandstone ~ 2.65 gm/cc
Limestone ~ 2.71 gm/cc
Salt ~ 1.6 – 2.0 gm/cc
Calculate porosity % from log reading and rock type

31. Estimate Porosity from Density given Lithology

32. Porosity Log: Density LOG VARIABLE RESPONSE Density Porosity*
Measures electron density by detecting Compton scattered gamma rays. Electron density is related to formation density. Good for detecting hydrocarbon gas with low density compared to rock or liquid.
 Low response  low HC gas content
 Large response  high HC gas content
* The combination of density log and neutron log provides the most reliable porosity estimate and can be used to indicate gas.
Shale reduces apparent porosity measured by density log
Gas increases apparent porosity measured by density log

33. Porosity Log: Neutron LOG VARIABLE RESPONSE Neutron Hydrogen Content
Fast neutrons are slowed by collisions to thermal energies. Thermal neutrons are captured by nuclei, which then emit detectable gamma rays. Note: hydrogen has a large capture cross-section for thermal neutrons. Good for detecting gas.
 Large response  high H content
 Small response  low H content
Shale appears as high apparent porosity measured by neutron log
Dry gas appears as low apparent porosity measured by neutron log

34. Neutron – Density Crossplot
plot porosity from neutron log vs porosity from density log
1.0
0.0
density
neutron
Clean sand line
density = neutron
Gas sand
density > neutron
Shaly sand
density < neutron

35. Neutron Log – Density Log Comparisons
Gas indicator
Crossplot can identify gas-bearing sands in sand-shale sequences
Lithology indicator
Apparent limestone porosity will appear high in density log if limestone contains anhydrite

36. Gas Effect Density-Neutron Crossover
How do logs respond when gas is present?
Density log reads porosity correctly
Neutron log treats gas as rock so it reads low porosity
Therefore curves separate when gas is present
Gas probably present when density log and neutron log separate

37. Typical Sonic Log Velocities
Velocity (ft/sec)
t (second/ft)
Shale
7,000 – 17,000
144 – 59
Sandstone
11,500 – 16,000
87 – 62
Limestone
13,000 – 18,500
77 – 54
Dolomite
15,000 – 20,000
67 – 50
Natural Gas
1,500
667
Water
5,000
200

38. Porosity Log: Sonic LOG VARIABLE RESPONSE Acoustic (sonic) Porosity
Measures speed of sound in medium. Speed of sound faster in rock than in fluid.
 Long travel time  slow speed  large pore space
 Short travel time  high speed  small pore space
Porous rock slows down sound waves
Porosities calculated from sonic log measurements are generally high in hydrocarbon-bearing unconsolidated sands

39. RESISTIVITY LOGS

40. Gamma Ray and Resistivity Logs
Short
Long
Gamma Ray Log
Resistivity Log

41. INDUCTION LOGS

42. What is induction logging?
Based on Faraday’s law of electromagnetic induction
Oscillating magnetic field induces electric field
Transmitter coil in tool creates primary magnetic field
Primary magnetic field induces toroidal electric field
Toroid = doughnut shape

43. What is induction logging? (cont.)
Toroidal electric field creates electrical “eddy current”
Eddy current is induced in conductor by changing magnetic field
Strength of eddy currents depends on conductivity
Eddy currents create secondary magnetic field
Measure secondary magnetic field with receiver coil

44. What is induction logging? (cont.)
Transmitter coil
Primary magnetic field
Eddy current in conductor
(e.g. ionic environment)
Secondary magnetic field
Receiver coil

45. SI Unit of Conductivity
Conductivity is inverse of resistivity
Conductivity unit is siemens/meter (S/m) or millisiemens/meter (mS/m)
where 1 Siemen = 1 mho = 1/ohm
Common conductivity unit is micromho/cm
1 micromho/cm = 1 μS/cm.
Convert to logged units using 10 μS/cm = 1 mS/m
Example
suppose resistivity is 10 ohm-m
conductivity = 1/(10 ohm-m) = 0.1 mho/m = 0.1 S/m

46. Electrode Log or Dual Laterolog
VARIABLE
RESPONSE
Electrode or Dual Laterolog
Fluid Type
Measures resistivity of formation water.
 High resistivity  hydrocarbons
 Low resistivity  brine

47. Resistivity Logs and Borehole Fluids
Need Conductive Borehole Fluid?
Comment(s)
Induction No
Work with oil-based mud and air-filled boreholes.
Unreliable in boreholes filled with very conductive mud.
Dual laterolog*
Yes
Will not work with oil-based mud and air-filled boreholes
*Laterolog tools use electrodes to measure formation resistivity (shallow and deep) through saline borehole fluids

48. Distinguish between Water-Bearing Zone and Hydrocarbon-Bearing Zone
Dual Laterolog Curves
Distinguish between Water-Bearing Zone and Hydrocarbon-Bearing Zone
Log
Measures GR
Gamma Ray
CALI
Caliper
LLD
Deep Laterolog
True formation resistivity (Rt)
LLS
Shallow Laterolog
Resistivity of invaded zone (Ri)
RXO
Microresistivity
Resistivity of flushed zone
MSFL
Microspherically focused
Source: Asquith and Krygowski, Figs. 1.7 & 1.9, Basic Well Log Analysis, 2nd Ed (2004)

49. Activity Well Log Responses – 1
Place the correct answer in the left hand column.
Log
Response
Gamma Ray 1
Measures electrical potential (voltage) associated with movement of ions.
Density 2
Detects shale from in situ radioactivity.
Photoelectric Effect 3
Measures speed of sound in medium. Speed of sound faster in rock than in fluid.
Electrode (dual laterolog) 4
Fast neutrons are slowed by collisions to thermal energies. Thermal neutrons are captured by nuclei, which then emit detectable gamma rays.
Acoustic (sonic) 5
Measures resistivity of formation water. SP 6
Measure absorption of low energy gamma rays by atoms in formation.
Neutron 7
Measures electron density by detecting Compton scattered gamma rays. 8
None of the above

50. Activity Well Log Responses – 2
Place the correct answer in the left hand column. There may be some duplication.
Log
Identifies
Neutron 1
Porosity SP 2
Fluid Type
Density 3
Rock Type
Photoelectric Effect 4
Hydrogen content
Acoustic (sonic) 5
Permeable beds
Electrode (dual laterolog) 6
None of the Above
Gamma Ray

51. LOG CALIBRATION WITH FORMATION SAMPLES

52. Mud Log ROP Rate of Penetration Gamma rays (GR) from NORM
Measure in API units
Relative unit
Potassium
GR energy 1.46 MeV
Thorium series
GR energy 2.62 MeV
Uranium-Radium series
GR energy 1.76 MeV

53. NORM in West Texas Barnett Shale
Mississippian Barnett Shale above MD = 9606 ft
Mississippian Limestone below MD = 9606 ft
SGR – Spectral Gamma Ray
CGR – Total GR minus URAN
POTA – Potassium 40, wt %
URAN – Uranium, ppm
THOR – Thorium, ppm
Barnett Shale
Limestone
Source: Asquith and Krygowski, Fig. 3.3, Basic Well Log Analysis, 2nd Ed (2004)

54. Evaluate Well Cuttings
Well Site Geologist
Examines Well Cuttings
Makes note of cores
Full-Diameter Cores
Side Cores

55. MODERN LOG APPLICATIONS

56. Principal Applications of Common Well Logs (after Selley and Sonnenberg [2015, page 86])
Log Type
Lithology
Hydrocarbons
Porosity
Pressure
Dip
ELECTRIC SP X
Resistivity
RADIO­ACTIVE
Gamma Ray
Neutron
Density
SONIC
DIPMETER

57. Illustration of a Fence Diagram
(A) A clean sand interval indicated by gamma ray (GR) logs.
(B) Fence diagram displaying clean sand correlation.

58. Interpret Depositional Environment Using Well Logs
Typical electrical log shapes…
S.P.
Resistivity
Well
Shale
Sandstone
Interbedded
SS & Shales
S.P.
Resistivity
Well
Shale
Sandstone
for beach or barrier island marine SS
for fluvial SS
*Fig. 158, P.K. Link, Basic Petroleum Geology, 3rd Edition (2001), Tulsa: OGCI

59. QUESTIONS?

60. SUPPLEMENT