BASIC LOG INTERPRETATION COCEPTS.

Porosity (φ) : The void space between grains that is generally

filled with liquids or gases.

Effective porosity: the volume of rock that connected pores

Water Saturation (Sw) : the percentage of the pore space filled

with water (as opposed to hydrocarbons or air).

Permeability (K ): the ability of the rock to pass fluids through it

Porosity is the measure
of the total volume
between the rock grains.

Pore
Space

Rock
Grains
 BULK DENSITY LOG
001) BONANZA 1
GRC ILDC RHOb DT
0 150 0.2 200 1.95 2.95 150 us/f 50
SPC SNC CNLLC
-160 MV 40 0.2 200 0.45 -0.15
ACAL MLLCF
6 16 0.2 200
RHOb
10700
1.95 2.95

10800

Bulk Density
10900
Log
We should distinguish between:-
Bulk Density :-. (e.g. : Sandstone) which decreases with increasing
the porosity.

Porous rock density increases with increasing water saturation

(compared to dry rock)

Density of the solid matrix material :-. e.g. : Qz

Density of the pore fluid :-. e.g. : water

Density Tools Can Run in open and

cased Hole & Measure Total Porosity
Equipment: 1.95 2.95
gm/cc
A radioactive source. This is usually caesium-137
or cobalt-60, and emits gamma rays of medium
energy (in the range 0.2 – 2 MeV). For example,
caesium-137 emits gamma rays with energy of
0.662 MeV.
A short range detector. This detector is very similar
to the detectors used in the natural gamma ray
tools, and is placed 7 inches from the source.
A long range detector. This detector is identical to
the short range detector, and is placed 16 inches
from the source.
Gamma ray enters the formation, then scattering &
looses some of its energy then absorbed by a
formation. Then, the detectors detect γ ray which
emitted from excited atoms which related to the
formation.
•The formation density log is a porosity log
Eccentric Density Tool
that measures electron density of formation.
 A chemical source (137Cs) emits gamma rays into
the formation

Source

As the gamma rays collide with formation

materials their energy is reduced or attenuated

Two gamma detectors measure the radiation

received in counts per second

The count rate is proportional to the porosity of

the formation
Gamma
Detectors
Densities of Typical Minerals and Fluids
Material Formula Density g/cm3
Quartz SiO2 2.65
Calcite CaCo3 2.71
Dolomite CaCo3 .MgCo3 2.87
Anhydrite CaSO4 2.96
Gypsum CaSO4 . 2H20 2.32
Halite NaCI 2.165
Sylivte KCI 1.98
Anthractic Coal 1.40-1.80
Bituminous Coal 1.20 – 1.50
Lignite 0.70 – 1.50
Water H20 1.00
Saltwater (100.000 PPM) 1.07
Saltwater (200.000 PPM) 1.146
Oil Cn (CH2) 0.80
Gas CnH2n + 2 0.20
Vertical resolution
 The vertical resolution at the typical logging speed (1300 ft/hr) is good
(about 26 cm, 10 inches),which is defined by the distance between the two
detectors.

 Even better resolutions are possible with slower logging speeds. For
example, thin (5 –10 cm thick) layers of calcareous nodules.

The high vertical resolution means that the log is useful for defining
formation boundaries.

Depth of Investigation

• The depth of investigation of the tool is very shallow.

•For Schlumberger’s FDC tool 90% of the response comes from the first 13 cm
(5 inches) from the borehole wall for a 35% porosity sandstone (which has
low density compared with most reservoir rocks). In higher density rocks the
depth of investigation is even less, and a value of about 10 cm (4 inches) can be
taken as an average value for reservoir rocks.
Identification of Evaporites:
 Evaporites are often found in a very pure state, and have clearly defined
densities.
 0 10
Lithologic Density Tool Barns/electrons

The Pe, or litho density log, run with the litho density tool
(LDT), is another version of the standard formation Material Pe
density log. In addition to the bulk density (rb), the tool
also measures the photoelectric absorption index (Pe) of
the formation. This new parameter enables a lithological Sand 1.81
interpretation to be made without prior knowledge of
porosity. Shale 3–4

Uses of the Litho-Density Log Limestone 5.08

Determination of Lithology Dolomite 3.14

 The litho-density log is one of the two most useful Salt 4.65
approaches to lithological determination down hole.
Anhydrite 5.05
This lithology may then be checked against the
other tool readings for consistency
Depth of Investigation

 PEF measurement from the litho-density tool have a depth of investigation of 50

to 60 cm.

Vertical Resolution
 The litho-density tool has a vertical bed resolution of 50 to 60 cm, which is slightly
better than the formation density tool. The enhanced vertical resolution results from
the shorter distance between the short and the long spacing detectors.

As with the formation density log, it is possible to enhance the vertical resolution of
this log by slowing down the logging speed and using modern digital data processing.
 Factors affecting Density Logs

•Borehole and mud filtrate.

–Density is a pad tool affected by washouts.

•Shale and Clays

–Shale reduce the effective porosity.
–Vsh and shale density can be calculated from log.

•Hydrocarbon
–Light HC results in low density and overestimated porosity
APPLICATIONS:
The Formation Density log has a number of applications:

 Measuring density of the formation.

 Calculation of porosity.

 When combined with sonic travel times, the density data gives the
acoustic impedance, which is important for calibration of seismic data.

 Identification of Evaporites.

 Gas detection in reservoirs when used in combination with the neutron

log.

 The Pe curve is a good lithology indicator.

 Density
1.95 G/cc 2.95

Shale

Sand

Shale

Sand

Shale
 Density Porosity
1.95 G/cc 2.95 Observed Bulk Density b=

Volume Fluid x Fluid density

+
Shale Volume Rock x Rock density

Sand
=( f x )+( ma x (1- ))

Shale ρmat  ρb
ØD 
ρmat  ρf
2.15
Sand
Sand density = 2.65
Water density = 1.0

Shale Density 2.15 -> Porosity = 30%

 POROSITY FROM NEUTRON LOG
001) BONANZA 1
GRC ILDC RHOC DT
0 150 0.2 200 1.95 2.95 150 us/f 50
SPC SNC NPHI
-160 MV 40 0.2 200 0.45 -0.15
ACAL MLLCF
6 16 0.2 200
10700 NPHI
0.45 -0.15

10800

Neutron
10900
Log
 0.45 -0.15
NEUTRON TOOLS %

 Neutron tools were the first logging instruments to use radioactive sources for determining
the porosity of the formation.

Neutron tool is a porosity log that Measures the concentration of hydrogen atoms in the
formation

 In clean reservoirs containing little or no shale, the neutron log response will provide a good
measure of formation porosity if liquid-filled pore spaces contain hydrogen, as is the case
when pores are filled with oil or water (hydrogen index =1, ). By contrast, when logging shaly or
gas-bearing formations, a combination of Neutron and Density readings will often be required
for accurate porosity assessment.

 Neutron Tools Measure Total Porosity.

The amount of energy lost at each collision depends on the relative mass of the target
nucleus, and the scattering (of gamma ray) cross section.

CNL :- Compensated Neutron Log.

SNP:- Sidewall Neutron Porosity Log
GNT:- Gamma ray / Neutron tool.
 More Fewer High
hydrogen counts porosity

Less More Low

hydrogen counts porosity

For a given formation, amount of hydrogen in the formation (I.e. hydrogen

index) impacts the number of neutrons that reach the receiver. A large
hydrogen index implies a large liquid-filled porosity (oil or water).
Summary:
The neutrons that are emitted from a neutron source have a high energy of several million
electron volts (MeV). After emission, they collide with the nuclei within the borehole fluid and
formation materials. With each collision, the neutrons loose some of their energy.

The largest loss of energy occurs when the neutrons collide with hydrogen atoms. The rate at
which the neutrons slow-down depends largely on the amount of hydrogen in the formation.

With each collision the neutrons slow down, until the neutrons reach a lower (epithermal)
energy state and then continue to lose energy until they reach an even lower (thermal) energy
state of about 0.025 eV.

At this energy the neutrons are in thermal equilibrium with other nuclei in the formation.

Porosity (or the hydrogen index) can be determined by measuring thermal neutron,
or by measuring capture gamma rays, or any combination.
Fluid Shale Matrix

SS. L.S Dol. Anhydrite

0.1 0.4-0.5 4 0 -4 -1
Accelerator Porosity Sonde (APS):-
 Makes thermal and epithermal neutrons measurements to
determine formation hydrogen content with minimal
influence from formation atom density.

 Accelerator (ELECTRONIC) neutron source,

instead of a chemical source.

 This construction produced a stable

epithermal neutron tool, which can be run
at logging speeds that are compatible with
the density
Chlorine effect:-
 Some types of neutron tool measure the thermal neutrons and gamma rays produced during the
capture of neutrons.

 There are only two elements that are found in reservoirs that contribute significantly to neutron
absorption hydrogen and chlorine.

 The presence of hydrogen in the fluids is what we want to measure, so this is not a problem.

 If the drilling mud, mud filtrate or formation fluids contain a significant amount of dissolved
chloride ions, as is often the case, the tool will measure a lower flux of neutrons.

 Hence overestimate the porosity. This is called the chlorine effect.

Shale effect:-
 Shale contain clays that have a significant amount of surface absorbed (bound) water.
Hence shales can contain a significant proportion of hydrogen's despite being low
porosity.
 The apparent porosity read from the neutron tool in shale formations is therefore
always significantly higher than it really is. This is called the shale effect or the
bound-water effect.
Depth of Investigation
• The depth of investigation of the CNL tool in a water saturated formation of 35% porosity is about
12 inches, and that of the SNP tool in the same formation is about 8 inches.

Vertical resolution
 As with most tools the vertical resolution is defined by the source detector spacing for
single detector tools and the spacing between the two detectors for dual detector tools.

 The vertical resolution of neutron tools is a little greater than these spacings.

 For the GNT tool the vertical resolution is 16 inches or 20 inches depending upon
which of the two source-detector spacings possible for this tool are used.

 The vertical resolution of the SNP tool is 16 inches, and for the CNL tool is 10
inches.
 Lithological Identification using the
Neutron-Density Combination

 Both the density log and the neutron log give a direct measurement of Total
porosity.

 Note that the compatible scale here is Density (1.95 to 2.95 g/cm3) and Neutron (-15
to 45% limestone porosity units). This is the most commonly used scale range.

 The cross-plot that for density and neutron logs plotted on compatible scales, there
will be a separation of the density and the neutron logs for sandstone and dolomite,
but no separation for limestone.

 The sandstone separation is called negative separation and the dolomite separation is
in the other direction and slightly larger, and is called positive separation.
Clean Formations
 There is no separation for pure limestones, and the porosity value that the log gives
is accurate.
 There is a small negative separation for clean sandstones.
 There is a moderate positive separation for pure dolomites.
Example of Porosity Log
 Neutron
45 Porosity -15

Shale

Sand

Shale

Sand

Shale
 Density and Neutron
1.95 Density 2.95
45 Neutron -15

Shale

Sand

Shale

Gas
Sand

Liquid

Shale
Neutron Logging Applications:-
 Porosity, usually in combination with the density tool.

 Gas detection, usually in combination with the density tool, but also
with a sonic tool.

 Calculate shale volume, in combination with the density tool.

 Lithology indication, again in combination with the density log and/or

sonic log.

 formation fluid type.

 Can be run in both open and cased holes

• Combinable with most other logging tools

 SONIC LOG
001) BONANZA 1
GRC ILDC RHOC DT
0 150 0.2 200 1.95 2.95 150 us/f 50
SPC SNC CNLLC
-160 MV 40 0.2 200 0.45 -0.15
ACAL MLLCF
6 16 0.2 200
DT
10700
150 us/f 50

10800

Sonic
Log

10900
 SONIC TOOLS
OVERVIEW
Acoustic tools measure the speed of sound waves in subsurface
formations. While the acoustic log can be used to determine
porosity in consolidated formations.

Ta Acoustic Borehole compensated sonic tools have two

Transmitter acoustic transmitters and four acoustic receivers
Acoustic
The transmitters emit compressional sound waves
R1 into the formation

t R4 The receivers measure the time it takes for the

wave to travel through the formation to the
receiver
R2 t Acoustic
Receivers
Travel time or t is the time difference of the
R3 wave as it is received at both receivers

Travel time depends on formation lithology,

Tb Acoustic
porosity, and pore fluid
Transmitter
 Basic Tool Theory
• Sound energy is focused into the formation
– The waveform received contains all the information about
how this energy is dissipated.
T Values

Material DTma(msec/ft)
CALCITE (limestone) 47.5
QUARTZ (sandstone) 55.5
DOLOMITE 43.5
ANHYDRITE 50.0
Shale 80-120
STEEL CASING 57.0
FRESH MUD 189
OIL 240
GAS 666
 Acoustic
140 us/ft 40

Shale

Sand

Shale

Gas
Sand

Liquid

Shale
 Acoustic Porosity
140 us/ft 40
Observed Delta Time DT =

Shale Volume Fluid x Fluid density

+
Volume Rock x Rock density
Sand
∆tlog = Ф ∆tf + ( 1-Ф ) ∆tmatrix

Shale

Δtlog  Δtma
Øs 
Δtf  Δtma
80 Sand
Sand DT = 55
Water DT = 189

Shale DT 80 -> Porosity = 18.5%

Sonic Logging Applications:-
 Indicating lithology (using the ratio of compressional velocity over
shear velocity)

 Determining integrated travel time (an important tool for

seismic/wellbore correlation)

 Correlation with other wells

 Detecting fractures and evaluating secondary porosity

 Determining acoustic impedance (in combination with the density log).

• Indicate formation gas

• Can be run in both open and cased holes