The document provides an overview of well log analysis including typical approaches for analyzing clean formations and shaly sands. Key aspects covered include determining water saturation using Archie's equation, identifying potential water and hydrocarbon bearing zones, performing environmental corrections, and making decisions on a formation's productive capability based on calculations like moveable hydrocarbon index and bulk volume water. Software tools for formation evaluation and probabilistic/deterministic analysis are also briefly mentioned.

1. Well Log Analysis
Sanjan Das

2. Overview
Log Analysis
Typical Approach for Clean Formations
2
Shaly Sand Analysis
Decisions on Productive Capability
Log Quality Control

3. Formation Evaluation & Log Analysis
Formation Evaluation – Practice of determining both the physical & chemical
properties of rocks & fluids they contain
Wireline logging is one of the evaluation methods
Decision to plug or complete a well is based upon the log response. Hence,
proper analysis of log data is a must
3

4. Log Analysis Indications
Rock type – Sandstone , Shale , Limestone, Dolomite
Rock properties – Porosity, Permeability, Resistivity
Fluids – Water / Oil / Gas, Saturation
4

5. Archie’s Equation
Determination of water saturation is the
primary objective of log analysis
Sw = water saturation of the uninvaded
zone
n = saturation exponent
a = tortuosity factor
m = cementation exponent
Rw = formation water resistivity
Rt = true resistivity of the uninvaded zone
Ф = porosity
n w
w m
t
Ra
S
R
 
5

6. Archie Water Saturation Variables
a : tortuosity factor.
m: cementation exponent.
n: saturation exponent. usually 2.
6

7. Resistivity
Fundamental of all measurements in logging
Resistivity is the physical measurement of resistance
Water bearing zones have lower resistivity than hydrocarbon bearing zones
If for a given porosity, the measured resistivity is significantly greater than the
wet resistivity, then the presence of hydrocarbon is indicated
7

8. Porosity
Porosity represents fluid storage capacity of a reservoir. Its defined as ratio of
pore space volume to the total bulk volume of the rock
Porosity (%) = (pore space volume/bulk space volume)X100
Two kinds of porosity –
a) Absolute or Total porosity
b) Effective porosity
8

9. Permeability
Ability of a formation to transmit the fluids it
contains through the existing pore network
Fundamental requirement of a productive
reservoir
Empirical law to quantify permeability
expressed as:
Q L
k
P A



k is related to:
Amount of pore space
Diameter of pore throats
Type of fluid
9

10. Typical Approach for Clean Formation
1
•Locate potential water bearing zones on logs & determine their lithology
2
•Select depths at which Rw is to be determined & perform environmental corrections on these data
3
•Determine Rw
4
•Locate potential hydrocarbon bearing zones and determine their lithology
5
•Select depth at which Sw is to be calculated and perform environmental corrections on these data
6
•Correct Rw for temperature at zone of interest
7
•Calculate Sw of the potential hydrocarbon bearing zone by Archie’s equation
10

11. Potential Water Bearing Zones
Qualitatively assess intervals in terms of
their porosity and resistivity
Permeability indicators – Microlog, SP
Negative deflections of the SP curve are used
as qualitative indicator of permeability
Wet zone – high porosity & low resistivity
11
Apply environment corrections to all data
to be used in Rw calculation

12. Rw Calculation
Inverse Archie method
Assumed that Water Saturation (Sw) is
100%
Lithology assumptions made for values of
‘m’ and ‘a’ from GR, Density and Pe curves
m
t
wa
R
R
a
 

12

13. Rw Calculation
Rw by SP method
Procedure
Determine Tf
Determine Rmf
Determine SSP
Determine SP
deflection
Differentiate between
SSP and SP
Plot ΔSP
Determine Rw
13

14. Potential Hydrocarbon Bearing Zone
Qualitatively asses the porosity and
resistivity of zones
Consider permeability indicators
Hydrocarbon presence – high porosity &
high resistivity
Priority with which a certain zone will be
evaluated is determined by its permeability
Before calculating Sw of a zone, Rw should
be temperature corrected at zone of interest
Calculate Sw by Archie’s equation
14

15. Essential Calculations
Single measured or calculated values of resistances applied over a wide range of
depths
Resistivity varies with temperature
Geothermal Gradient,   100msBHT T
G TDg 
 
Formation Temperature,
100
f ms G
D
T T g
  
    
  
Correcting resistivity for temperature, 1
2 1
2
T K
R R
T K
 
  
 
15

16. Log Indications
Log Indications
Caliper -Mudcake presence indicates permeable formation
-Caliper log is a permeability indicator
SP -Negative deflections of SP indicates permeability
-However, absence of deflection doesn’t indicate absence of
permeability
-Rw determination
-Shale volume calculation
-Well to well correlation
GR -Helps finding permeable layers
-Indicates degree of shaliness of a formation.
-Lithology identification
-Shale volume calculation
-Determine bed boundaries
-Well to well correlation
16

17. Log Indications
Log Indications
Resistivity -To determine the water saturation using Archie’s equation
-Determine hydrocarbon and water bearing zones
-Indicate permeable zones
Density -Porosity calculation
-Lithology identification
-True density of rock matrix
-Gas detection
Neutron -Porosity calculation
-Lithology identification
-Gas detection
17

18. Environmental Corrections
Logging tools are calibrated to operate in specific conditions
Factors affecting tool responses – hole size, mud weight, bed thickness, depth of
invasion, and other properties of logging environment
Depending on tool type & environmental conditions, appropriate charts are
used for corrections
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19. Corrections
Corrections
Needed
Reasons
SP Bed Thickness
(SP-1a, SP-1b)
-In relatively thin beds, deflection of SP curve is suppressed
because of the influence of underlying and overlying
formations
-Which chart to use depends on diameter of invasion
GR Borehole
Corrections (GR-1)
-Distance between the tool and the borehole wall
-Density of the mud filling the borehole
Density (POR-1) -SDL calibrated to read correct bulk density in an 8-inch
borehole filled with fresh water
-These corrections are performed real time by the logging
software
Neutron Porosity
(POR-4a, POR-4b)
-Borehole diameter correction applied real time when any
tool with caliper is used in combination.
19

20. Sand-Shale Model (with Hydrocarbons)
20

21. Complete Model
21

22. Modes of Occurrence of Clay
Type Occurrence Impact on reservoir
Laminated shale -Thin layers of clay minerals
-Interbedded with thin layers of
sandstone
- Tends to reduce
porosity & permeability
of reservoir
Structural clay - Exists as individual grains along
with framework grains of a
reservoir
-Little impact
-Doesn’t restrict or block
pore throats
Dispersed clay -Exists within the pore space of a
reservoir
-Replaces fluid volume
-Blocks pore throats
-Reduces effective
porosity and
permeability
22

23. Shale/Clay effects on log responses
Log Type Effects
Sonic
Porosity
-Porosity too high, because TT (shale) > TT (matrix of clean
formation)
Density -Actual density of formation is volumetric combination of density of
clay and sand grains
-When p (clay)> p (sand) – overestimated
-When p (clay)< p (sand) – underestimated
Neutron
porosity
-Clay minerals are hydrated & contain structurally bound OH-
-Reflects additional hydrogen even though not part of pore space
-Porosity increases
Resistivity -Adds conductance
-Clay contain saline water & other ions that decreases resistivity
23

24. Combined effect of Shale/Clay
Decreased resistivity & typically increased porosity
Results in erroneously high values of Sw by Archie Equation
Shaly sand analysis corrects these effects & reduce Sw
24

25. Procedure of Shaly Sand Analysis
1
• Determining volume of shale (Vsh) in the zone of
interest
2
• Correcting porosity for the presence of clays
(determining effective porosity of the zone of interest)
3
• Determining effective water saturation of the zone of
interest (water saturation of the effective pore network)
25

26. Volume of Shale (Vsh)
Determine the amount of clay minerals present in the formation.
Estimates of the volume of clay minerals.
Do not consider type or distribution.
Vsh < 15% of bulk rock volume – assumed clay minerals not having effect on log
response – Archie equation
Vsh > 15% of bulk rock volume – shaly shale analysis should be performed
Many types of logs used either alone or in combination.
26

27. Vsh from Gamma Ray
First, calculate GR Index (Igr)
log min
max min
GR
GR GR
I
GR GR



Second, select an equation based on type of distribution
Linear, Vsh = Igr
Clavier, Vsh = 1.7 – [3.38 – (Igr + 0.7)^2]^0.5
Steiber, Vsh = 0.5x[Igr/(1.5 – Igr)]
Choice of equation mainly depends on local knowledge
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28. Vsh from Spontaneous Potential
In water bearing sandstones of low to medium resistivity
PSP is the amount of SP deflection in the zone of interest
SSP is the maximum SP deflection in a clean formation
1.0
PSP
Vsh
SSP
 
28

29. Vsh from Neutron-Density Logs
Combination of porosity measurements from neutron and density logs is used
( )
( )
n d
sh
nsh dsh
V
 
 



Comparison of Vsh results
Vsh should be determined from as many methods as possible
Lowest resulting value should be used in determining effective porosity
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30. Determining Effective Porosity
Correcting porosity measurements for the presence of clay minerals
Фe=ф(1-Vsh)
From Sonic logs,
From Density logs,
From Neutron-Density combinations,
log 100ma sh ma
e sh
fl ma sh fl ma
t t t t
V
t t t t t

      
               
logma ma sh
e sh
ma fl ma fl
V
   

   
    
           
 
 
   
0.52 2
2
2
n corrected n cl nsh
d corrected d cl dsh
n corrected d corrected
e
n corrected d corrected
e
V
V
  
  
 

 



 
 
  
  
 
  
 
 
  
 
 
30

31. Crossplot Method
Used to find effective porosity фe and
Vsh from neutron-density logs
Vsh=0 at quartz point
Vsh=100% at shale point
Lines of equal Vsh are parallel to the clean
sand line
31

32. Crossplot Method
Shale line is фe = 0
Lines with equal фe are parallel to the
shale line
Effective porosity lines originate at the
фe=фn=фd points on sand line
32

33. Effective Water Saturation
It’s the percentage of effective porosity occupied by water
33
/2
(1 ( /2))
1
cl
tn
we mV
ecl
cl w
R
S
V
R a R





34. FRS Software
FRS
Petrophysics
model
Open Hole
(Volumetric)
Deterministic
SASHA
LARA
CORAL
Probabilistic
FAME
Cased Hole
ACE
CASE
BIS Acoustic
34

35. FRS Software
35

36. FRS Software
36

37. Decisions on Productive Capability
Moveable Hydrocarbon Index (MVI)
Bulk Volume Water (BVW)
Irreducible Water Saturation
Calculation of Reserves
37

38. Moveable Hydrocarbon Index (MVI)
Ratio of Water Saturation of uninvaded zone to Flushed Zone Water Saturation
w
xo
S
MHI
S

If MHI<1, then hydrocarbons were likely moved during invasion and the
reservoir will produce
If MHI≥1, then hydrocarbons were not moved during invasion
38

39. Irreducible Water Saturation
Value of Sw at which water in reservoir is either adsorbed onto grain surface or
bound within pore network by capillary pressure.
If a reservoir is at Swirr, water present will be immovable and production will be
water free hydrocarbons
39

40. Bulk Volume Water
Fraction of rock volume occupied by water
wBVW S  
If values of BVW remain constant throughout a reservoir, it indicates reservoir
is at Swirr
40

41. Calculation of Reserves
Barrels of oil in place,
Recoverable oil,
Cubic Feet of
gas in place,
Recoverable
gas,
 7758 1 wN S h A    
  7758 1 wRF S h A
N
SF
    

 43560 1 wG S h A    
   
 
6
1.54 10 1
460
w
f
RF S h A Pf
G
GD T
      


41

42. Log Quality Control
Log header
Calibration
Logging speed
Repeat section
Correlation with near by well
Depth control
Log quality control must be performed to ensure that data is suitable for analysis
42

43. Thank you
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