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Slide1
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.
Slide2To 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.
Slide3WELL 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.
Slide4Outline
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
Slide5WELL LOGGING
Slide6Well Logging Objectives
Estimate near-wellbore formation properties
Depth
Thickness (net and gross)
Porosity
Formation density
Acoustic velocity
Temperature and pressureLithology (rock type)Fluid saturationsIndication of hydrocarbonsIndicate permeability (e.g. high, low, tight)Structural trends (e.g. formation dip)Fracture properties
Slide7Typical 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 estimatesIndicate presence of hydrocarbonsMeasure Sw; infer hydrocarbonsFluid contacts, e.g. GOC, WOCIndicate which zones to complete (perforate)
Slide8Well Log Header and Tracks
Depth Track
Track
1
Track
2
Track
3
Header
Header
Track
1
Track
2
Scale
Scale
Scale
Scale
Scale
Slide9Well Log Schematic
Well Data
Logging Data
Depth Track
Track
1
Track
2
SP
Res
Slide10Types of Well Logs
Lithology logs
Spontaneous Potential (SP)
Now tends to be replace by Gamma Ray
Gamma Ray (GR)
Photoelectric Effect (PEF)
Porosity logs
DensityNeutronAcoustic (Sonic)Resistivity logsInductionLaterologMicro Resistivity
Specialty Logs
e.g. FMI (Formation Micro Image)
Slide11Computer Generated Logs
Computer performs corrections and calculations
Graphical view
Easy to see analysis
Estimate lithology, saturations, porosity, etc.
Slide12SUBSURFACE IONIC ENVIRONMENT
Slide13Ohm’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/ohmElectrical current is charge in motion, e.g. Na+
cation
and
Cl
-
anion.
Slide14Alternative form of Ohm’s Law
Alternative form of Ohm’s Law:
E =
ρ
J
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
Slide15Resistivity and Resistance
Alternative form of Ohm’s Law:
E =
ρ
J
where
E = electric field (volt/m)
J = current density (amp/m2)ρ = resistivity (ohm-m)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 / σLA+−
Slide16Fluids that Affect Logging Measurements
Drilling mud (resistivity R
m
)
Mud filtrate(resistivity
R
mf
)Formation water(resistivity Rw)Hydrocarbons (assumed infinite resistivity)
Resistivity depends on formation temperature
Slide17Invasion Zones for Drilling Fluids
Adjacent Bed
Borehole
Adjacent Bed
Uninvaded
Zone
Zone of Transition
Flushed
Zone
Mud
Cake
Uninvaded
Zone
Slide18LITHOLOGY LOGS
Slide19Common Reservoir Rock Types and an Illustrative Stratigraphic Column
Slide20Gamma 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
Slide21Light-Matter Interaction
Photoelectric Effect
Low-energy phenomenon
Photoelectric effect
Mid-energy phenomena
Thomson scattering (elastic)
Compton scattering (inelastic)
High-energy phenomenon Pair production
Compton Scattering
Photon wavelength changes
Space
Time
Slide22Gamma Ray Log Response
Slide23NORM 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)
Slide24Lithology Log: Gamma Ray
LOG
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
Slide25SP (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 R
W
to calculate saturation
Slide26SP (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 bedsSP log hard to interpret when formation waters are fresh (not salty)
Slide27Lithology Log: Spontaneous Potential
LOG
VARIABLE
RESPONSE
Spontaneous Potential
Permeable Beds
Measures electrical potential (voltage) associated with movement of ions.
Low response
impermeable shale
Large response
permeable beds
Slide28Lithology Log: Photoelectric Effect
LOG
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
Slide29POROSITY LOGS
Formation Density
Neutron Porosity
Sonic
Slide30Density 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/ccCalculate porosity % from log reading and rock type
Slide31Estimate Porosity from Density given Lithology
Slide32Porosity 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
Slide33Porosity 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
Slide34Neutron – Density Crossplot
Neutron – Density
crossplot
plot porosity from neutron log
vs
porosity from density log
Clean sand line
density = neutron
1.0
1.0
0.0
0.0
density
neutronGas sand density > neutronShaly sand
density < neutron
Slide35Neutron 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
Slide36Gas 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
Slide37Typical 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
Slide38Porosity 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
Slide39RESISTIVITY LOGS
Slide40Gamma Ray and Resistivity Logs
Resistivity Log
Gamma Ray Log
Long
Short
Slide41INDUCTION LOGS
Slide42What 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
Slide43What 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
Slide44What is induction logging?
(cont.)
Secondary magnetic field
Eddy current in conductor
(e.g. ionic environment)
Primary magnetic field
Transmitter coil
Receiver coil
Slide45SI 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
Slide46Electrode Log or Dual Laterolog
LOG
VARIABLE
RESPONSE
Electrode or Dual
Laterolog
Fluid Type
Measures resistivity of formation water.
High resistivity
hydrocarbons
Low resistivity
brine
Slide47Resistivity Logs and Borehole Fluids
Log
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
Slide48Dual Laterolog
Curves
Distinguish between
Water-Bearing Zone and Hydrocarbon-Bearing Zone
Source: Asquith and
Krygowski
, Figs. 1.7 & 1.9, Basic Well Log Analysis, 2
nd Ed (2004)
Log
Measures
GR
Gamma Ray
CALI
Caliper
LLD
Deep
Laterolog
True formation resistivity (
R
t)
LLS
Shallow LaterologResistivity of invaded zone (Ri)RXOMicroresistivityResistivity of flushed zoneMSFLMicrospherically focusedResistivity of flushed zone
Slide49Activity
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
Slide50Activity
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
Slide51LOG CALIBRATION WITH FORMATION SAMPLES
Slide52Mud 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
Slide53NORM 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)
Slide54Evaluate Well Cuttings
Well Site Geologist
Examines Well Cuttings
Makes note of cores
Full-Diameter Cores
Side Cores
Slide55MODERN LOG APPLICATIONS
Slide56Principal Applications of Common Well Logs
(after
Selley
and
Sonnenberg
[2015, page 86])
Log Type
Lithology
Hydrocarbons
Porosity
Pressure
Dip
ELECTRIC
SP
X
Resistivity
X
X
X
RADIOACTIVE
Gamma Ray
X
Neutron
X
X
Density
X
X
SONIC
X
X
X
X
DIPMETER
X
Slide57Illustration of a Fence Diagram
(A) A clean sand interval indicated by gamma ray (GR) logs.
(B) Fence diagram displaying clean sand correlation.
Slide58Interpret Depositional Environment Using Well Logs
S.P.
Resistivity
Well
Shale
Sandstone
Interbedded
SS &
Shales
Shale
S.P.
Resistivity
Well
Shale
Sandstone
Shale
for beach or barrier island marine SS
for fluvial SS
*Fig. 158, P.K. Link,
Basic Petroleum Geology
, 3
rd Edition (2001), Tulsa: OGCITypical electrical log shapes…
Slide59QUESTIONS?
Slide60SUPPLEMENT