This document provides information about neutron porosity logs. It begins with an introduction to porosity estimation using neutron logs and comparing the mass of hydrogen to neutrons. It then provides examples of how porosity measurements would differ based on the fluid (water vs gas) and content (increased porosity means more hydrogen). The rest of the document discusses various aspects of neutron logs like slowing of neutrons, tools used, effects of shale and chlorine, and how investigation depth varies with porosity. It concludes with a case study example from the Volve oil field to identify water and oil bearing zones from well log data.

1. -Yash Bansal
BTech. Applied Petroleum Engineering with Specialization
in Upstream(2016-2020)
UPES, Dehradun
Email: byash706@gmail.com
Mob: +91 7599799991
Neutron Porosity Log

2. Matrix
Solid
Matrix
Pore
Spaces
(Pore
Fluids)
Reservoir
Limestone
CaCO3
Sandstone
SiO2
Dolomite
CaMg(CO3)2
Project
Objectives
Implementation
Plan
Schedules
Resources
Gas
CH4
C2H6
C3H8
Water
H2O
Oil
C4H10
C5H12
………..
For porosity estimation, we basically estimate the hydrogen content present
in the pores, as in the pore fluid we are having hydrogen as a common
element.

3. Masses
Comparable
Masses
Mass of Hydrogen
1.6735575 x 10-27 Kg or 1.00784 u
H
Mass of Neutron
1.674929 x 10-27 Kg or 1.008664 u
N
Due to comparable mass, there will be a maximum loss of energy
e- e-
e-e-

4.  Due to the comparable size of Hydrogen and Neutron, the best slowing down
process can be seen in the case of collision between Neutron and Hydrogen
atoms.
 This process can be understood by some practical examples like Carrom-
board.
Slowing Down

5. Slowing Down
Intermediate
Neutrons
Initial
Neutrons
with 4 Mev
Thermal
Neutrons
Epithermal
Neutrons

6. Example 1
Porosity=10%
H2O
Porosity=20%
H2O
Source: 1000 Source: 1000
Receiver=? Receiver=?

7. Solution 1
Porosity=10%
H2O
Porosity=20%
H2O
Source: 1000 Source: 1000
Receiver=800 Receiver<800
More the porosity, more will be the fluid present in the pores, more will the Hydrogen
present in it, and more will be the slowing down of the Neutron particles. Therefore, a less
neutron count at the receiver.

8. Example 2
Porosity=10%
H2O
Porosity=10%
Gas
Source: 1000 Source: 1000
Receiver=? Receiver=?

9. Solution 2
Porosity=10%
H2O
Porosity=10%
Gas
Source: 1000 Source: 1000
Receiver=800 Receiver>800
This leads to the underestimation of the porosity, when gas is present within the pore spaces
because gas molecules are having a high spacing between their molecules. That ultimately
results hydrogen index per unit volume to be lesser and this result is observed.

10. Neutron Beam
 Initial Energy level: 4 Mev
 Initial Velocity: 2800 cm/microsec
 Sources:
Plutronium-beryllium (PuBe)
Americum-beryllium (Am-Be)
log(Porosity)=((a*N)+b)
where,
a and b are the constants
N is the neutron tool reading
*Calibrations are done on the basis of Limestone
Formation only

11. Density-Neutron Cross-plot
 For limestone with 0%
Porosity, the Density log
reads 2.7 gm/cm3.
 For lime stone with 100%
Porosity, the Density log
reads 1 gm/cm3.

12. Gas Effect
 Due to the spacing between the gas atoms, the Neutron log
underestimates the porosity of the gas bearing formations.
*Dotted line is of
Neutron porosity plot

13. Shale Effect
 Not possible to investigate Shaley Beds
 Absorbed water Is present
 Overestimation occurs
 Correction is done by the use of Gamma Ray logs
*Dotted line is of Neutron porosity plot
SHALE EFFECT (large
positive separation)
SHALE EFFECT (large
positive separation)

14. Shale Volume Vsh
 Size of separation is the best
measure to estimate the volume off
shale
where,
Ωlog is the separation at the point of interest in the log
Ωsand is the separation when 100% sandstone formation is there
Ωsh is the separation when 100% shale formation is there

15. Chlorine Effect
 Neutron absorption takes place
 Overestimation occurs
 Chlorine ion present in mud filtrate, drilling muds and
in formations.

16. Tools
Gamma Ray/Neutron Tool(GNT)
1. A typical traditional Neutron tool
2. Sensitive to high capture gamma ray
and Thermal Neutrons
3. Effected by Chlorine effect
Sidewall Neutron Porosity Tool(SNP)
1. Sensitive to Epithermal Neutrons
2. Tool is pressed against the borehole
wall
Compensated Neutron Log(CNL)
1. Sensitive to Thermal Neutrons
2. Contains two receivers for better
results

17. Variable Formation

18. Depth of Investigation
% Porosity 90% of Signal
0 60 cm
10 34 cm
20 23 cm
30 16.5 cm
0
10
20
30
40
50
60
70
0 5 10 15 20 25 30 35
Depth(incms)
Depth

19. Practical Example Of Volve Field
 This study is done on the basis of the
available log data on the equinor
website.
 This case study is done to segregate
different formations and as a result to
find out the water and oil bearing zones.

21.  Petrophysics MSc. Course Notes by Dr. Paul Glover
References
 LQA and Basic Interpretation Manual by Hls Asia Limited
 Logging Notes by Mr. Gaurav Gairola (Assistant Professor, UPES)
 The Geological Interpretation of Well Logs by Malcom Rider

22. Thank You