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Schlumberger
(05/96)
Contents
C1.0 POROSITY MEASUREMENTS ....................................................................................................1
C2.0 POROSITY MEASUREMENTS FROM THE BHC SONIC TOOL...................................................3C2.1 INTRODUCTION .....................................................................................................................3C2.2 POROSITY DETERMINATION................................................................................................4C2.3 FACTORS AFFECTING SONIC INTERPRETATION:................................................................7
C3.0 POROSITY MEASUREMENTS FROM THE LITHO-DENSITY TOOL...........................................11C3.1 INTRODUCTION ...................................................................................................................11C3.2 PRINCIPLE...........................................................................................................................11C3.3 POROSITY FROM A DENSITY LOG.....................................................................................13C3.4 LITHOLOGY FROM THE PE MEASUREMENT......................................................................17C3.5 FACTORS AFFECTING DENSITY LOG:................................................................................20
C4.0 POROSITY MEASUREMENTS FROM THE COMPENSATED NEUTRON TOOL.........................21C4.1 INTRODUCTION....................................................................................................................21C4.2 PRINCIPLE ...........................................................................................................................21C4.3 FACTORS AFFECTING CNL LOGS.......................................................................................23
C5.0 TOTAL POROSITY DETERMINATION .......................................................................................29
C6.0 GR LOG.....................................................................................................................................31C6.1 INTRODUCTION ...................................................................................................................31C6.2 PROPERTIES OF GAMMA RAYS ........................................................................................31C6.3 NATURAL GAMMA RAY SPECTROMETRY TOOL...............................................................34
C7.0 BOREHOLE GEOMETRY BY CALIPER MEASUREMENT .........................................................37C7.1 PHYSICAL PROPERTIES.....................................................................................................37
Single-Arm Caliper Configuration................................................................................................40Two-Arm Caliper Configurations .................................................................................................40Three-Arm Caliper Configurations...............................................................................................41Four-Arm Caliper Configuration ..................................................................................................41
C8.0 WORK SESSION.......................................................................................................................43
Introduction to Openhole Logging
(05/96)
Schlumberger
(05/96) C-1
C1.0 Porosity Measurements
C1.1 INTRODUCTIONTotal porosity may consist of primary and
secondary porosity. Effective porosity is thetotal porosity after the shale correction is ap-plied. Rock porosity can be obtained from thesonic log, density log or neutron log. For allthese devices, the tool response is affected bythe formation porosity, fluid and matrix. If thefluid and matrix effects are known or can bedetermined, the tool response can be deter-mined and related to porosity. Therefore, thesedevices are usually referred to as porosity logs.
All three logging techniques respond to thecharacteristics of the rock immediately adjacentto the borehole. Their depth of investigation isshallow—only a few centimeters or less—andtherefore generally within the flushed zone.
As well as porosity, the logs are affected by- volume and nature (lithology) of ma-
trix material- amount and nature of pore space con-
tents (pore geometry, water, hydrocar-bons)
- volume and nature of shales.
For example, the formula for a density logmeasurement including all these variables canbe written as
ρb = φ
e × S
w × ρ
f + φ
e(1 – S
w) ρ
hy + V
shρ
sh +
(1 – φe – V
sh) ρ
ma.
Solving for porosity in this case would notbe easy because there are several unknownsand only one measurement. However, whenwe compare other porosity and log measure-ments, we can solve for these unknowns.
Introduction to Openhole Logging
(05/96) C-2
Schlumberger
(05/96) C-3
C2.0 Porosity Measurementsfrom the BHC Sonic Tool
C2.1 INTRODUCTIONIn its simplest form, a sonic tool consists of
a transmitter that emits a sound pulse and areceiver that picks up and records the pulse asit passes the receiver.
The sound emanated from the transmitterimpinges on the borehole wall. This estab-lishes compressional and shear waves withinthe formation, surface waves along the bore-hole wall and guided waves within the fluidcolumn.
The sonic log is simply a recording versusdepth of the time, t
comp, required for a compres-
sional sound wave to traverse 1 m of forma-tion. Known as the interval transit time, transittime, ∆t or slowness, t
comp is the reciprocal of
the velocity of the sound wave. (For the re-mainder of this document, t
comp is known as
∆t.) The interval transit time for a given for-mation depends upon its lithology and poros-ity. This dependence upon porosity, when thelithology is known, makes the sonic log usefulas a porosity log. Integrated sonic transit timesare also helpful in interpreting seismic records.The sonic log can be run simultaneously withmany other services.
The borehole-compensated (BHC) tool trans-mitters are pulsed alternately, and ∆t values areread on alternate pairs of receivers. The ∆t val-ues from the two sets of receivers are averagedautomatically by a computer at the surface forborehole compensation.
The computer also integrates the transit timereadings to obtain total traveltimes (see FiguresC1 and C2).
Figure C1: Schematic of BHC sonde, showingray paths for the two transmitter-receiver sets.
Averaging the two ∆t measurements cancels er-rors from the sonde tilt and hole-size charges.
Introduction to Openhole Logging
(05/96) C-4
Sometimes the first arrival, although strongenough to trigger the receiver nearer the trans-mitter, may be too weak by the time it reachesthe far receiver to trigger it. Instead, the far re-ceiver may be triggered by a different, laterarrival in the sonic wave train, and the traveltime measured on this pulse cycle will then betoo large. When this occurs, the sonic curveshows an abrupt, large excursion towards ahigher ∆t value; this is known as cycle skip-ping. Such skipping is more likely to occurwhen the signal is strongly attenuated by un-consolidated formations, formation fractures,gas saturation, aerated muds or rugose or en-larged borehole sections.
9.8 m
2.25 m
Figure C2: BHC Sonic—GR tool distances
The sonic log is run with ∆t presented on alinear scale in tracks 2 and 3 with a choice oftwo scales:
500–100 and 300–100 µsec/m.
A three-arm caliper curve representing theaverage borehole diameter and a gamma ray(GR) curve are recorded simultaneously intrack 1 (See Figure C3).
The gamma ray curve measures the naturalradioactivity of potassium, uranium and tho-rium in the formation and is usually represen-tative of the amount of shale present. This isbecause radioactive elements tend to concen-trate in clays and shales. Later, we will use theGR to compute volume of shale (V
sh).
C2.2 POROSITY DETERMINATIONa) Wyllie Time-Average Equation
After numerous laboratory determinations,M.R.J. Wyllie proposed, for clean and con-solidated formations with uniformly distrib-uted small pores, a linear time-average orweighted-average relationship between poros-ity and transit time (see Figure C4):
tLOG
= φtf + (1 – φ)t
ma(C1)
tLOG
– t
ma
or φ = (C2) t
f –
t
ma
wheret
LOG is the reading on the sonic log in
µsec/mt
ma is the transit time of the matrix mate-
rial
Schlumberger
(05/96) C-5
600
GR
150.00000.0000 (GAPI)
CALI
375.0000125.0000 (MM)
BS
375.0000125.0000 (MM)
DT
100.0000500.0000 (US/M)
FILE 2
BS
BOREHOLE COMPENSATED SONIC
Figure C3 : Borehole-Compensated Sonic Log
Introduction to Openhole Logging
(05/96) C-6
tf is the transit time of the saturating fluid
(about 620 µsec/m for freshwater mud sys-tems)
φ is the porosity or volume occupied bypores
1 − φ is the volume of the matrix.
Typical Values:
Sand ∆tmatrix
= 182 µsec/mLime ∆t
matrix = 156 µsec/m
Dolomite ∆tmatrix
= 143 µsec/mAnydrite ∆t
matrix = 164 µsec/m
When the formations are not sufficientlycompacted, the observed ∆t
values are greater
than those that correspond to the porosity ac-cording to the time-average formula, but the φversus t relationship is still approximately lin-ear. In these cases, an empirical correctionfactor, C
p, is applied to Equation 2 to give a
corrected porosity, φSVcor
(Equation 3):
Figure C4: Components of the Wyllie Time-Average Equation
t - t
ma 1
φSVcor
= × (C3)tf -
t
ma C
P
The value of Cp is given approximately by
dividing the sonic velocity in nearby shale bedsby 328. However, the compaction correctionfactor is best determined by comparing φ
SV, as
obtained from Equations 1 and 2, with the trueporosity obtained from another source.
b) Raymer-HuntOver the 25 years since acoustic velocity
well logging was introduced, deficiencies havebeen noted in the transform of transit time ∆tto porosity φ.
Based on extensive field observations oftransit times versus porosity, the new empiri-cal Raymer-Hunt transform was derived. Thenew transform equation is too complicated tobe presented in this course. An approximationof the transform is given in Equation C4 andthe exact transform is presented in the chartbooks as the red lines on all sonic charts.
tLOG
- t
ma
φsv = C (C4)
tLOG
The value of the constant C has a range of0.625 to 0.7 depending upon the investigator.Chart Por-3m (Figure C6) uses 0.7 for C: thiswas the value originally proposed. However,more recent transit time-to-porosity compari-sons indicate that a value of 0.67 is more ap-propriate.
Schlumberger
(05/96) C-7
For the case of a gas-saturated reservoir rock,C becomes 0.6. It should be used when therock investigated by the sonic tool contains anappreciable amount of hydrocarbon in thegassy (vapor) phase. Because of the shallowdepth of investigation, this condition normallyexists only in higher porosity sandstones(greater than 30%).
From the example sonic log (Figure C3) at593 m we read 352 µsec/m. Given ∆t
ma =182
µsec/m we can solve for φ:
Wyllie:
352 - 182 φ = ≅ 39%
620 - 182
Vma
(m/sec)
∆tma
(µ sec/m)
Vma
(m/sec)Range ofValues
Sandstone
Limestone
Dolomites
Anhydrite
Salt
Casing (iron)
5486
6400
7010
6096
4572
5334
182
156
143
164
219
187
5486–5944
6400–7010
7010–7925
6100
4566
5348
Fluid Transit Time: V1 = 1615 m/sec
∆tf = 620 microsec/m for fresh muds = microsec/m for salt muds
Figure C5: Chart showing values used for commonreservoir rocks
Raymer-Hunt (approximation):
5(352 - 182) φ = ≅ 30%
8(352)
Chart Por-3m (Figure C6) solves this equa-tion graphically. Enter t
log of 352 µsec/m on
abscissa and project upward until the appropri-ate ∆t
ma line is reached (V
ma= 5500 m/sec). If
different values of Vma
are used, we get differ-ent values of φ.
With a ∆tlog
= 250µsec/m we would get
Raymer-Wyllie Hunt
Vma F F
Sandstone (5500 m/sec) 16% 18.5%Limestone (6400 m/sec) 21% 24%Dolomite (7010 m/sec) 26% 28.5%
C 2.3 FACTORS AFFECTING SONICINTERPRETATION
LithologyLithology must be known to obtain the cor-
rect Vma
. An incorrect choice of Vma
will pro-duce erroneous calculations.
ShaleShale content generally causes ∆t to read too
high for a porosity calculation because of thebound water in the shale. The sonic reads pri-mary porosity, which may be affected byshale.
Introduction to Openhole Logging
(05/96) C-8
Porosity Evaluation from SonicSvf = 1615 m/s
100 150 200 250 300 350 400
t, interval transit time (µsec/m)
vf = 1615 m/sec50
40
30
20
10
0
50
40
30
20
10
0
φ, p
oros
ity (
p.u.
)
φ, p
oros
ity (
p.u.
)
Time average Field observation
1.1
1.2
1.3
1.4
1.5
1.6
Dolomite
Calcite
Quartz
sandsto
ne
8000
7000
6400
5500
5950
vma(ft/sec)
Bcp
Dol
omite
Cal
cite
Qua
rtzsa
ndst
one
Cem
ente
dqu
artz
sand
ston
e
EXAMPLE: t = 76 µs/ft (249 µs/m)SVma = 19,500 ft/s (5950 m/s) - SandstoneThus, φ = 18% (by either weighted average or empirical transform)
SVma (ft/S) tma (µs/ft) SVma (m/s) tma (µs/m)SandstonesLimestonesDolomites
18,000 - 19,50021,000 - 23,00023,000 - 26,000
55.5 - 51.347.6 - 43.543.5 - 38.5
5486 - 59446400 - 70107010 - 7925
182 - 168156 - 143143 - 126
Por-3m
Figure C6
Schlumberger
(05/96) C-9
Fluid TypeThe depth of investigation of the sonic is
shallow; therefore, most of the fluid seen bythe sonic will be mud filtrate.
OilOil usually has no effect.
WaterThere is usually no effect from water except
where the drilling fluid is salt saturated, andthen a different V
f should be used, usually 607
µsec/m.
GasResidual gas causes ∆tlog to read too high
when the formation is uncompacted. The gasbetween the sand grains slows down the com-pressional wave resulting in a long ∆t. Incompacted sands, the wave will travel fromone sand grain to another and the gas effectwill be reduced.
CompactionThe value of ∆t
log will read too high in un-
compacted sand formations. Compaction cor-rections can be made if the compaction factor(B
cp) is known.
An approximate Bcp
is obtained from the sur-rounding shales (B
cp = ∆
tsh/328). B
cp can also
be obtained by comparing the porosity ob-tained from another source (core, density log,neutron log, computed log porosity) to thatobtained from the sonic log in a clean waterzone. (For example, if the neutron log in aclean water zone reads 20% and the sonic logreads 25%, then B
cp = 25%/20% = 1.25.)
Secondary PorosityThe sonic generally ignores secondary po-
rosity. For example, in vugular porosity, thetraveltime through the formation matrix isfaster than the time through fluid in the vugs,because ∆t
f is about 3 to 4 times the value of
∆tma
.
Borehole EffectThe compensated sonic is unaffected by
changing hole size except in the case of ex-tremely rough, large holes where the formationsignal is severely affected by the noise of themud signal and formation damage.
MudcakeMudcake has no effect on the BHC sonic be-
cause the traveltime through the mudcake iscompensated.
Introduction to Openhole Logging
(05/96) C-10
Schlumberger
(05/96) C-11
C3.0 Porosity Measurements from theLitho-Density Tool
C3.1 INTRODUCTIONLitho-Density logs are primarily used for po-
rosity and lithology measurements. Other usesinclude the identification of minerals inevaporite deposits, detection of gas, determi-nation of hydrocarbon density, evaluation ofshaly sands and complex lithologies, determi-nation of oil-shale yield and calculation ofoverburden pressure and rock mechanicalproperties.
C3.2 PRINCIPLEA radioactive source, applied to the borehole
wall in a shielded sidewall skid (Figure C7),emits medium-energy gamma rays (662 keV)into the formation.
Figure C7: Schematic Drawing of the Dual SpacingLitho-Density Logging Device
Classical GR interactions by energy level areshown in Figure C8. Because of the medium-energy GR emission, only points 2 and 3 oc-cur with respect to Litho-Density operation.
These gamma rays may be thought of as high-velocity particles that collide with the electronsin the formation. At each collision, a gammaray loses some, but not all, of its energy to theelectron and then continues with diminishedenergy. This type of interaction is known asCompton scattering. The scattered gamma raysreaching the detector, at a fixed distance fromthe source, are counted as an indication offormation density.
The number of Compton-scattering colli-sions is related directly to the number of elec-trons in the formation. Consequently, the re-sponse of the density tool is determinedessentially by the electron density (number ofelectrons per cubic centimeter) of the forma-tion. Electron density is related to the true bulkdensity ρ
b, which, in turn, depends on the den-
sity of the rock matrix material, formation po-rosity and density of the fluids filling thepores.
(GR energy > 1.02 MeV)
(over entire GR energy range)
(ρe)
(low-energy GR)
(Z)
Figure C8: Classical GR— Matter Interactions by Energy Level
Introduction to Openhole Logging
(05/96) C-12
In addition to the bulk density measurement,the tool also measures the photoelectric ab-sorption index of the formation, P
e. Photelec-
tric absorption can be related to lithology;whereas the ρ
b measurement responds primar-
ily to porosity and secondarily to rock matrixand pore fluid, the P
e measurement responds
primarily to rock matrix (lithology) and secon-darily to porosity and pore fluid.
At a finite distance from the source, such asthe far detector, the energy spectrum may lookas illustrated in Figure C9. The number ofgamma rays in the higher energy region(region of Compton scattering) is inverselyrelated only to the electron density of the for-mation (i.e., an increase in the formation den-sity decreases the number of gamma rays).The number of gamma rays in the lower en-ergy region (region of photoelectric effect) isinversely related to both the electron densityand the photoelectric absorption. By compar-ing the counts in these two regions, the pho-toelectric absorption index can be determined.
E (keV)
Figure C9: Variations in Spectrum forFormation with Constant Density but Different Z
The gamma ray spectrum at the near detectoris used only to correct the density measure-ment from the far detector for the effects ofmudcake and borehole rugosity.
7 m
4.5 m
Figure C10: Basic SGT- CNT- LDTTool Configuration
Schlumberger
(05/96) C-13
ρma ρf
(1 – φ) φ
ρbFigure C11: Components of Density
Porosity Calculation
C 3.3 POROSITY FROM A DENSITYLOG
For a clean formation of known matrix den-sity ρ
ma, with a porosity φ that contains a fluid
of average density ρf,, the formation bulk den-
sity ρb, will be (Figure C11):
ρb = φρ
f + (1 – φ) ρ
ma (clean wet zone)
where:ρ
b is the measured bulk density (from
Litho-Density tool)ρ
ma is the density of the matrix
ρf is the density of the fluid
φ is the percent volume of pore space(1 – φ) is the percent volume of matrix.
This can be written as
ρma
– ρb
φD =
ρma
– ρfl
where:ρ
ma depends on lithology
ρb is measured by the density log
ρfl depends on fluid type in pore
volumes.
The equation for ρb can be proven mathe-
matically, unlike the sonic equation, which isan empirical relationship. Values of ρ
b are used
for common reservoir rocks (zero porosity)(Figure C12).
From the example Litho-Density log (FigureC13) at 593 m we read ρ
b = 2180 kg/m3.
Given ρf = 1000 kg/m3, ρ
ma = 2650 kg/m3, we
can solve for φD:
2650 − 2180φ
D = = 28.5%
2650 − 1000
Chart Por-5 (Figure C14) solves this equa-tion graphically. For ρ
b = 2180 kg/m3 solving
for porosity using other matrix values gives:
ρma
= 2710 kg/m3 φD = 31%
ρma
= 2870 kg/m3 φD = 36.9%
Introduction to Openhole Logging
(05/96) C-14
ρb Values for Common Reservoir Rocks and Fluids
Compound FormulaActualDensity
ρ
ρa
(as seen bytool)
QuartzCalciteDolomiteAnhydriteSylviteHalite
SiO2
CaCO3
CaCO3MgCO3
CaSO4
KCINaCI
265427102870296019842165
264827102876297718632032
Compound FormulaActualDensity
ρ
ρa
(as seen bytool)
Fresh WaterSalt WaterOilGas
H2O200,00ppm
n(CH2)C1.1 H4.2
10001146850ρg
10001135850
1.325 ρg-0188
Figure C12
Schlumberger
(05/96) C-15
600
GR
150.00000.0000 (GAPI)
CALI
375.0000125.0000 (MM)
BS
375.0000125.0000 (MM)
RHOB
3000.00002000.0000 (K/M3)
DRHO
250.0000-250.0000 (K/M3)
FILE 2
BS
LITHOLOGY DENSITY
Figure C13
Introduction to Openhole Logging
(05/96) C-16
Formation Density Log Determination of Porosity
40
30
20
10
0
2.8 2.6 2.4 2.2 2.02.31
1.0 0.9 0.8
1.1
1.2
φ, p
oros
ity, (
p.u.
)
ρb, bulk density (g/cm3)
ρ ma
=2.
87(d
olom
ite)
ρ ma
=2.
71(c
alcite
)
ρ ma
=2.
65(q
uartz
sand
ston
e)
ρ ma
=2.
83ρ m
a=
2.68
ρma – ρb
ρma – ρfφ =
ρf
Bulk density, ρb, as recorded with the FDC* or LDT density logs, is converted to porosity with this chart. Touse, bulk density, corrected for borehole size, is entered in abscissa; go to the appropriate reservoir rock typeand read porosity on the appropriate fluid density, ρf. scale in ordinate. (ρf is the density of the fluid saturat-ing the rock immediately surrounding the borehole - usually mud filtrate.)
EXAMPLE:ρb = 2.31 Mg/m3 in limestone lithologyρma = 2.71 (limestone)ρf = 1.1 (salt mud)
Therefore φD = 25 puPor-5
Figure C14
Schlumberger
(05/96) C-17
C3.4 LITHOLOGY FROM Pe
MEASUREMENTThe P
e curve is a good matrix indicator. It is
slightly influenced by formation porosity andthe presence of gas, but responds mainly tolithology (Figure C15). Hence, a safe interpre-tation of matrix lithology can be made forsimple lithologies (one-mineral matrix). Inconjunction with other log data, more complexmineral combinations can be analyzed.
Typical Litho-Density responses for com-mon minerals are presented in Figure C16.
The Pe
measurement is used
1. alone as a matrix indicator (the lithol-ogy curve)
2. in combination with density ρb to ana-
lyze two-mineral matrices and deter-mine porosity
Pe φt
0.5 0.4 0.3 0.2 0.1 0
Figure C15: Photoelectric Absorption Index as a Function of Porosity and Fluid Content
Introduction to Openhole Logging
(05/96) C-18
3. In combination with the density andneutron to analyse more complexlithologies (solutions to three-mineralmatrices and porosity).
A direct benefit from the more accurate de-scription of the matrix is a more reliable dis-tinction between gas and oil.
In this section of the course, we use the Pe
curve as a matrix indicator in simple litholo-gies. Using P
e for more advanced applications
(complex lithology identification and heavymineral-detection) is covered in Section H,Porosity in Complex Lithologies.
Examples of the direct use of the Pe curve
for lithology identification are shown in FigureC17. In the case of an anhydrite, P
e is equal to
that of limestone. Anhydrite is positively iden-tified by the bulk density or density porosityvalues.
Pe ρb ρe
0 0
00
0
Figure C16: Typical Litho-Density Responses for Common Sedimentary Rocks
Schlumberger
(05/96) C-19
Figure C17: Lithology Identification with the CNT, Litho-Density and Pe
Introduction to Openhole Logging
(05/96) C-20
C3.5 FACTORS AFFECTING THEDENSITY LOG
LithologyThe correct ρ
ma must be known to get correct
porosity.
ShaleThe density of shale in sands can range from
2200 to 2650 but is usually close to 2650, thesame as sandstone. In shaly sands, the densityusually gives a good value of effective porosityregardless of the shale content. The shale ap-pears as matrix to the density tool.
ρb = ρ
f φ
e + ρ
ma (1 – φ
e – V
sh) + ρ
shV
sh
collecting terms:
ρb = ρ
f (φ
e) + ρ
ma(1 – φ
e) + V
sh (ρ
sh – ρ
ma)
if ρsh
= ρma
, the last term is zero.
Fluid TypeThe depth of investigation is quite shallow:
usually most of the formation fluid is flushedaway from the wellbore and the density toolsees drilling fluid or filtrate in the pore space.Hence, the values of ρ
f to use is that of the
drilling mud filtrate rather than the formationwater density.
OilResidual oil will make density porosities
slightly high, because oil is lighter than drillingmud filtrate.
WaterWater density is proportional to the amount
of salt content. The value of ρf is selected in the
computer for porosity determination.
GasThe ρ
f of gas is 100–300 kg/m3. Porosity
determination in gas zones may be high ifthere is residual gas near the borehole. Usuallymost of the gas is flushed and little effect isseen on the density log.
CompactionThe density tool is unaffected by lack of
compaction.
Secondary PorosityThe density reads intercrystalline, vugular
and fractured porosity. The porosity measuredis therefore total porosity.
Borehole EffectDensity gives good values for smooth holes
up to 381 mm in diameter. The tool compen-sates for minor borehole rugosity, but a roughhole causes the density to read too low densi-ties (high porosities) because the skid-to-for-mation contact is poor.
MudcakeFor normal mudcake thickness, there will be
no effect because the tool automatically com-pensates for mudcake.
However for a ∆ρ correction of 100 kg/m3
and greater (i.e., ∆ρ > 100 kg/m3), the toolcompensation may be insufficient and the ρ
b
no longer representative of the formation den-sity. In this case, the density should obviouslynot be used for porosity calculations.
Schlumberger
(05/96) C-21
C4.0 Porosity Measurements from theCompensated Neutron Tool
C4.1 INTRODUCTIONNeutron logs are used principally for the de-
lineation of porous formations and determina-tion of their porosity. They respond primarilyto the amount of hydrogen in the formation.Thus, in clean formations that have pores filledwith water or oil, the neutron log reflects theamount of liquid-filled porosity.
Gas zones can often be identified by com-paring the neutron log with another porositylog or a core analysis. A combination of theneutron log with one or more other porositylogs yields even more accurate porosity valuesand lithology identification—even an evalua-tion of shale content.
3 3/8-in. DIAMETER
Figure C18: Schematic Drawing of the Dual Spacing Compensated Neutron Tool
C4.2 PRINCIPLENeutrons are electrically neutral particles,
each with a mass almost identical to the massof a hydrogen atom. High-energy (fast) neu-trons are continuously emitted from a radioac-tive source in the sonde. These neutrons collidewith the nuclei of the formation materials inwhat may be thought of as elastic billiard-ballcollisions. With each collision, the neutronloses some of its energy.
The amount of energy lost per collision de-pends on the relative mass of the nucleus withwhich the neutron collides. A greater energyloss occurs when the neutron strikes a nucleusof practically equal mass (i.e., a hydrogen nu-cleus). Collisions with heavy nuclei do notslow the neutron much. Thus, the slowing ofneutrons depends largely on the amount of hy-drogen in the formation.
Within a few microseconds, the neutronshave been slowed by successive collisions tothermal velocities, corresponding to energiesof about 0.025 eV. They then diffuse ran-domly, without losing more energy, until theyare captured by the nuclei of atoms such aschlorine, hydrogen or silicon.
The capturing nucleus becomes intensely ex-cited and emits a high-energy gamma ray ofcapture.
Introduction to Openhole Logging
(05/96) C-22
When the hydrogen concentration of thematerial surrounding the neutron source islarge, most of the neutrons are slowed andcaptured within a short distance of the source.On the contrary, if the hydrogen concentrationis small, the neutrons travel farther from thesource before being captured. Accordingly, thecounting rate at the detector increases for de-creased hydrogen concentrations and viceversa. Thus, the neutron tool responds to thehydrogen index of the formation. The hydro-gen index is a measurement of the amount ofhydrogen per unit volume of formation (HI ofwater = 1).
Neutron logging tools include the GNT(Figure C19) tools series (no longer in use),
sidewall neutron porosity (SNP) tools (in lim-ited use) and the CNL tool series, which in-cludes the compensated neutron and DNL*Dual-Energy Neutron Log. The current toolsuse americium-beryllium (AmBe) sources toprovide neutrons with initial energies of sev-eral million electron volts.
1) SNP- detects epithermal neutrons- utilizes a skid mounted single detector- can be run in open hole only, either liq-
uid-filled or empty- most corrections are automatically ap-
plied during logging- limited availability.
0
Figure C19: Neutron Energy Travel History
Schlumberger
(05/96) C-23
2) CNL tooldetects thermal neutrons- The CNL tool uses a two-detector sys-
tem that depth and resolution matcheseach count rate before the ratio is com-puted. The ratio value is then convertedto porosity on a linear scale (FigureC20), based on the matrix selected forthe computation (limestone, sandstoneor dolomite).
- Conversion from one porosity assump-tion to another can be done using ChartPor-13b (Figure C22). Por-13b con-verts curves labelled "NPHI" that arenot environmentally corrected and alsoconverts for curves labelled "TNPH"and "NPOR," which are environmen-tally corrected.
- The CNL tool is especially designed foruse in combination with other devices.
- The CNL tool can be run in liquid-filledholes, either open or cased, but notempty holes (i.e., air- or gas-filledholes.)
3) DNL tooldetects thermal and epithermal neutrons- The DNL tool incorporates two
epithermal neutron detectors in additionto the two thermal neutron detectors.Two separate porosity measurementsare obtained, one from each pair of de-tectors.
- Improves the response to gas and en-hances interpretation in the presence ofthermal neutron absorbers.
- In shaly formations containing a largenumber of thermal neutron absorbers,the porosity measured by the epithermal
detectors reads lower and agrees moreclosely with density-derived porosity.
- As with the CNL tool, the DNL tool isespecially designed for use in combina-tion with other devices. In addition, theDNL tool can be run in liquid-filledholes, air/gas-filled holes (epithermalporosity only) and open or cased holes.
C4.3 FACTORS AFFECTING CNL LOGS
LithologyA single known matrix must be present to
accurately determine porosities. Large errorscan occur if the matrix selection is incorrect.
ShaleThe presence of hydrogen in chemically
bound water causes the CNL/DNL tool to readhigh porosities in shales or shaly formations.
Fluid TypeWater: Fresh water has no effects. Saline
water has a reduced hydrogen content and theCNL/DNL tool will read low porosity; thecorrection is in the chart book.
Liquid Hydrocarbons: If the hydrogen con-tent is close to that of water, there is little or noeffect.
Gas: If the hydrogen concentration is low,the CNL/DNL tool reads low porosity.
CompactionAll neutron logs are unaffected by compac-
tion.
Introduction to Openhole Logging
(05/96) C-24
600
GR
150.00000.0000 (GAPI)
CALI
375.0000125.0000 (MM)
BS
375.0000125.0000 (MM)
NPHI
0.0000(V/V)
(K/M3)
0.6000
DPHI
0.6000 0.0000
FILE 2
BS
COMPENSATED NEUTRON LITHODENSITY (NO PEF CURVE)
Figure: C20
Schlumberger
(05/96) C-25
Secondary PorosityAll neutron equipment measures total poros-
ity (including primary and secondary).
Borehole EffectThe effects of rough hole are minimized by a
large depth of investigation obtained by the useof a high-yield source and the two-detectorsystem.
When run in combination with the densitytool, an automatic caliper correction system isaccurate to [356 mm]. Normally there is zerostandoff correction.
MudcakeCorrections for mudcake, fluid (mud and
formation) salinity, mud weight, pressure andtemperature are in Charts Por-14(a) and 14(b),in the Log Interpretation Chart Book, but arenot discussed in this course.
The average net correction is usually betweenone and three porosity units. Hence, for calcu-lations by hand, the correction is usually notdone.
Introduction to Openhole Logging
(05/96) C-26
Neutron Porosity Equivalence CurvesSidewall Neutron Porosity (SNP), Compensated Neutron Log (CNL*)
Sands
tone
Limes
tone
Dolomite
40
30
20
10
00 10 20 30 40
φSNPcor, Apparent Limestone Neutron Porosity (p.u.) φCNLcor, Apparent Limestone Neutron Porosity (p.u.)
φ, T
rue
Por
osity
for
Indi
cate
d M
atrix
Mat
eria
l
SNP
CNL
©Schlumberger
When the SNP or CNL log is recorded in limestone porosity units, this chart is used to find porosity in sandstonesor dolomites. For the SNP log, first correct for mudcake thickness. (Chart Por-15 is used for SNP mudcakecorrections.)
For the CNL log, simply enter the chart in abscissa with the apparent limestone neutron porosity; go to the ap-propriate matrix line, and read true porosity on the ordinate. (Chart Por-14 is used for CNL environmentalcorrections.)
EXAMPLE: Sandstone bed Giving, hmc = 1/4 in.øSNP = 13 pu (apparent limestone porosity) øSNP = 11 pu (corrected for mudcake)Bit Size = 77/8 in. And, øSNP (sandstone) = 14 puSNP caliper = 75/8 in.
This chart can also be used to find apparent limestone porosity (needed for entering the various CP-crossplotcharts) if the SNP or CNL recording is in sandstone or dolomite porosity units. This chart should be used for CNLvalues labeled NPHI—it should not be used for CNL values labeled TNPH or NPOR.
Por-13a
Figure C21
Schlumberger
(05/96) C-27
Neutron Porosity Equivalence CurvesCompensated Neutron Log (CNL*)
40
30
20
10
00 10 20 30 40
φCNLcor, apparent limestone neutron porosity (p.u.)
φ, tr
ue p
oros
ity fo
r in
dica
ted
mat
rix m
ater
ial
Qua
rtzsa
ndst
one
Calcite
(limes
tone
)
Dolomite
Formation salinity
TNPH NPHI
0 kppm
250 kppm
*Mark of Schlumberger
Por-13b
Figure C22
Introduction to Openhole Logging
(05/96) C-28
Schlumberger
(05/96) C-29
C5.0 Total Porosity Determination
We have seen that porosity measurementsare inferred from measurements of bulk den-sity, hydrogen index and acoustic traveltimes.We have also seen that each measurementprovides the necessary input to calculate po-rosity under the following conditions:
– Porosity type is intergranular, not frac-tured or secondary (vuggy, moldic,etc.).
– Matrix type is known and constant.– Rock is clean, (i.e., no shale present).– Porosity is filled with fluid.
Violations of any of these conditions willcause the different porosity measurements todisagree in one fashion or another. This can beused to determine lithology, primary and sec-ondary porosity and gas vs. liquid content. Thequestion to be answered here is: Which poros-ity measurement should be used?
In a sand-shale sequence, for initial compu-tations,
a) if φD is available, use φ
TOTAL = φ
D
b) if φN and ∆t are available, use φ
TOTAL
= φS with compaction corrections
applied.
In a carbonate, for initial computations(limestone matrix),
a) if φN and φ
D are available in sandstone
and limestone units, then use φTOTAL
:
φN
+ φD
φT =
2b) if only ∆t is available, use φ
TOTAL:
φT = φ
S + estimate φ
VUGS.
If gas is present in the reservoir, additional cor-rections to φ
N and φ
D must be applied, as dis-
cussed in Section F.
Porosity calculations in complex lithologies shallare discussed in Section H.
Introduction to Openhole Logging
(05/96) C-30
Figure C23: Porosity Comparison between the LDT, CNT and SLT
Schlumberger
(05/96) C-31
C6.0 GR Log
6.1 INTRODUCTIONThe GR log is a measurement of the natural
radioactivity of the formations. In sedimentaryformations the log normally reflects the shalecontent of the formations. This is because theradioactive elements tend to concentrate inclays and shales. Clean formations usuallyhave a very low level of radioactivity, unlessradioactive contaminant such as volcanic ashor granite wash is present or the formationwaters contain dissolved radioactive salts.
"Clean" Formation GR Reading Sands 15 to 30 APILimestones 10 to 20 APIDolomites 8 to 15 API
The GR log can be recorded in cased wells,which makes it very useful as a correlationcurve in completion and workover operations.It is frequently used to complement the SP logand as a substitute for the SP curve in wellsdrilled with salt mud, air or oil-base muds. Ineach case, it is useful for the location of shalesand nonshaly beds and, most importantly, forgeneral correlation.
6.2 PROPERTIES OF GAMMA RAYSGamma rays are bursts of high-energy elec-
tromagnetic waves that are emitted spontane-ously by some radioactive elements. Nearly allthe gamma radiation that occurs in the earth isemitted by the radioactive potassium isotope ofatomic weight 40 (K40) and by the radioactiveelements of the uranium and thorium series.
Each of these elements emits gamma rays,the number and energies of which are distinc-tive for each element. Figure C24 shows theenergies of the emitted gamma rays: potas-sium (K40) emits gamma rays of a single en-ergy at 1.46 MeV, whereas the uranium andthorium series emit gamma rays of variousenergies.
Figure C24: Gamma Ray Emission Spectraof Radioactive Minerals
Introduction to Openhole Logging
(05/96) C-32
In passing through matter, gamma rays ex-perience successive Compton-scattering colli-sions with atoms of the formation material,losing energy with each collision. After thegamma ray has lost enough energy, it is ab-sorbed, by means of the photoelectric effect,by an atom of the formation. Thus, naturalgamma rays are gradually absorbed and theirenergies degraded (reduced) as they passthrough the formation. The rate of absorptionvaries with formation density. Two formationswith the same amount of radioactive material
per unit volume, but with different densities,will show different radioactivity levels; the lessdense formations will appear slightly moreradioactive. (Figure C25).
GR uses:1. definition of shale beds2. indicator of shale content3. detection of radioactive and non-
radioactive minerals4. identification of formation tops.
Schlumberger
(05/96) C-33
Figure C25: Relative GR Response for Various Rocks/Formations
Introduction to Openhole Logging
(05/96) C-34
6.3 NGS NATURAL GAMMA RAYSPECTROMETRY TOOL
Like the GR log, the NGS Natural GammaRay Spectrometry tool measures the naturalradioactivity of the formations. Unlike the GRlog, which measures only the total radioactiv-ity, this log measures both the number ofgamma rays and the energy level of each andpermits the determination of the concentrationsof radioactive potassium, thorium and uraniumin the formation rocks (Figure C27).
Physical PrincipleMost of the gamma ray radiation in the earth
originates from the decay of three radioactiveisotopes: potassium (K40), uranium 238 (U238)and thorium 232 (Th232).
Potassium-40 decays directly to the stableargon-40 with the emission of a 1.46-MeVgamma ray. However, uranium-238 and tho-
rium-232 decay sequentially through a longsequence of various daughter isotopes beforearriving at stable lead isotopes. As a result,gamma rays of many different energies areemitted and fairly complex energy spectra areobtained, as Figure C26 shows. The charac-teristic peaks in the thorium series at 2.62MeV are caused by the decay of thallium-208and bismuth-214 respectively.
It is generally assumed that formations are insecular equilibrium; that is, the daughter iso-topes decay at the same rate as they are pro-duced from the parent isotope. This means thatthe relative proportions of parent and daughterelements in a particular series remain fairlyconstant; so, by looking at the gamma raypopulation in a particular part of the spectrumit is possible to infer the population at anyother point. In this way, the amount of parentisotope present can be determined.
Figure C26: Potassium, Thorium and Uranium Response Curves (NAl Crystal Detector)
Schlumberger
(05/96) C-35
2025
CGR
TENS---
---SGR
---URAN
---THOR
---POTA
---POTA
2000
1/240
61 02-JUN-1992 15:15 INPUT FILE(S) CREATION DATE
CP 32.6 FILE 3 00- -1941 00:39
---CGR
-10.00 30.000
URAN(PPM )
0.0 .10000
POTA
0.0 40.000
THOR(PPM )
0.0 150.00
CGR(GAPI)
0.0 .09370
POTA
0.0 150.00
SGR(GAPI)
50000. 0.0
TENS(N )
POTASSIUM
THORIUM
NATURAL GAMMA SPECTROMETRY
ACCUMULATED INTEGRATION VALUES SUMMARY:
Integrated Hole Volume: 2.07418 M3 FROM 209.87 M TO 1995.07 M
Figure C27
Introduction to Openhole Logging
(05/96) C-36
Once the parent isotope population is known,the amount of nonradioactive isotope can alsobe found. The ratio of potassium-40 to totalpotassium is stable and constant on the earth,whereas, apart from thorium-232, the thoriumisotopes are rare and so can be neglected. Therelative proportions of the uranium isotopesdepend somewhat on their environment, andthere is also a gradual change because of theirdifferent half-lives; at present, the ratio of ura-nium-238 to uranium-235 is about 137.
Applications:- identification of radioactive sands that
may be misinterpreted as shales- identification of different types of
shales/clays (see Figure C28)- depth correlation (same as GR)- complex lithology analysis.
Figure C28: Classification of Radioactive Minerals as a Function of the Th and K Values
Schlumberger
(05/96) C-37
C7.0 Borehole Geometryby Caliper Measure
C7.1 PHYSICAL PROPERTIESThe hole diameter is usually recorded in
conjunction with the following surveys:
- Sonic logs (BHC versions, ASI ArraySeismic Imager, DSI Dipole ShearSonic Imager)
- Microresistivity logs (microlog, Micro-SFL, EPT Electromagnetic Propagationlogs)
- Litho-Density logs- Dipmeter logs (Dual Dipmeter Forma-
tion MicroScanner, FMI FormationMicroImager tools)
- Borehole geometry log
The readings given by different calipers inthe same hole may be different depending onthe caliper design and the hole cross section.
Figure C29 shows the characteristics of thedifferent calipers:
Caliper toolNo. ofArms
Phasing of theArms (Degrees) Maximum Diameter Remarks
Sonic tool 3 120 16 in. [406 mm]3 arms coupled1 reading
Microlog tool 1 0 20 in. [508 mm]1 arm1 reading
Micro-SFL tool(option A)
1 0 16 in. [406 mm]1 arm1 reading
Micro-SFL tool(option B)
4 90 22 in. [558 mm]4 arms coupled 2 × 22 paired readings
Density tool 1 0Short Arm 16 in. [406 mm]Long Arm 21 in. [533 mm]
1 arm1 reading
Dipmeters 4 90 FMS/FMI 22 in. [558 mm]4 arms coupled 2 × 22 independent readings
Borehole Geometrytool
4 90Standard 30 in. [762 mm]Special 40 in. [1016 mm]
4 arms coupled 2 × 22 independent readings
Dual Axis 2 180 16 in. [406 mm]2 arms coupled1 reading
Figure C29: Caliper Specifications for Different Devices Statedon the Logs
Introduction to Openhole Logging
(05/96) C-38
1) Mudcake is a good reason to have dif-ferent calipers reading different values:- If the arm of the caliper is the blade
type, it will cut into the cake andthis arm will ignore the thicknessof the mudcake.
- If the arm is of the pad type, it willskid over the cake and the mudcakethickness will be taken into account.
2. Assuming no mudcake, the readings ofdifferent calipers in a perfectly roundhole will be identical.
But holes are not always round. Inclearly ovalized holes, two- three- andfour-arm calipers will read differenthole diameter values, mostly because ofthe way these arms are coupled to-gether.
If the logging tool is fairly free to rotateinside the hole:- Two-arm calipers will ride using
the larger diameter of the hole.- Four-arm calipers will ride with
one pair of coupled arms using thelarger diameter of the hole.
3) In deviated wells, calipers may par-tially collapse under their own weightand give readings that are too low.
The following example (Figure C30)shows different calipers in an ovalizedhole:
- The sonic caliper (three arms linkedtogether) shows an average hole di-ameter.
- The density caliper (one arm) is ap-plied on the wall with strength. Itsback-up arm will cut into the mud-cake. If no small-axis hardware isused, it will orient itself to read thelargest diameter. If small-axishardware is used, the Litho-Densitytool tracks the smoother, short axisof the hole (if ovality exists).
- The microlog caliper (one arm) willprobably orient itself to read thelarger diameter. Its pad will skid onany mudcake. This is the case in theupper part and lower part of thissection.
- Most calipers are designed to rec-ord accurate hole diameters in cy-lindrical boreholes. When bore-holes are noncylindrical anddepending on caliper configura-tions, a tool string will orient itselfin some preferential direction. Thiscan effect both caliper readings andlog responses.
Using Figure C31, consider the caliperresponses in a 200- × 400-mm ovalborehole for the various caliper types,configurations and preferred tool orien-tations. 100 m of 200- × 400-mm holehas a volume of 6.28m3.
Schlumberger
(05/96) C-39
Figure C30: Comparison of Various Caliper Responses
Introduction to Openhole Logging
(05/96) C-40
Single-Arm Caliper Configuration:• records one borehole diameter = 400
mm• calculated 100 m hole volume = 12.57
m3 (+100% error)• tool examples:
- Litho-Density log(No short-axis hardware)
- MicroSFL tool (option A)- EPT Electromagnetic Propa-
gation tool.
Two-Arm Caliper Configurations:a. Unidirectional
• records one borehole diameter = 400mm
• calculated 100 m hole volume = 12.57m3 (+100% error)
• tool example:- MicroSFL tool (option B).
b. Bidirectional Long Axis• records one borehole diameter = 195
mm• records a second diameter = 195 mm• calculated 100 m hole volume = 2.9 m3
( −53%).
c. Bi-directional Short Axis• Records one borehole diameter = 273
mm• Records a second diameter = 273 mm• Calculated 100m hole volume = 5.85m3
(−7%).
Figure C31: Caliper Responses Under Various Hole Conditions
Schlumberger
(05/96) C-41
Three-Arm Caliper Configurations:a. Centered
• records one borehole diameter = 260mm
• calculated 100 m hole volume = 5.31m3
(−15%)• tool example:
- sonic log.
b. 90- Degree Offset• records one axis diameter = 200 mm• records a second diameter = 382 mm• calculated 100m hole volume = 6.00 m3
(−4%)• tool examples:
- CNL Compensated Neutronlog
- Litho-Density log (short-axishardware applied).
Four-Arm Caliper Configuration:• records one-axis diameter = 200 mm• records a second diameter = 400 mm• calculated 100-m hole volume = 6.28
m3 (0%)• tool examples:
- borehole geometry log- Dual-Dipmeter tool- Formation MicroScanner- FMI Formation MicroImager.
Figure C31 (Continued)
Introduction to Openhole Logging
(05/96) C-42
Schlumberger
(05/96) C-43
C8.0 Work Session
1a. For the example logs of Figures C32 – C34, calculate the following:
(Formation = Sandstone) 581 m 600 m
a. RILD
b.Rt
c. ∆t
d.φS
e. φD
f. φN
2. Using the sonic log of Figure C34, calculate the sonic porosity at 586 m.
∆tf = 620 µsec/m
∆tma
= 182 µsec/m
∆t - ∆tma
φs = =
∆tf - ∆t
ma
5(∆t - ∆tma
)φ
s = =
8∆t
b. Using Chart Por-3m (Figure C6)
φs Wyllie Time-Average =
φs Field Observation =
Introduction to Openhole Logging
(05/96) C-44
3a. On the CNT–Litho-Density log of Figure C35, what effect is seen at 1941 to 1946 m?
b. Using the Pe, what is the lithology in this zone?
c. Convert the log readings (φN and φ
D) to equivalent sandstone values.
d. Explain the effect identified in question 3a.
Schlumberger
(05/96) C-45
600
SP
0.0000-150.0000 (MV)
SFLU
2000.00000.2000 (OHMM)
ILD
2000.00000.2000 (OHMM)
ILM
2000.00000.2000 (OHMM)
FILE 2
ILM
DUAL INDUCTION - SP/SFL
Figure C32
Introduction to Openhole Logging
(05/96) C-46
600
GR
150.00000.0000 (GAPI)
CALI
375.0000125.0000 (MM)
BS
375.0000125.0000 (MM)
NPHI
0.0000(V/V)
(V/V)
0.6000
DPHI
0.6000 0.0000
FILE 2
BS
COMPENSATED NEUTRON LITHODENSITY (NO PEF CURVE)
SANDSTONE
Figure C33
Schlumberger
(05/96) C-47
600
GR
150.00000.0000 (GAPI)
CALI
375.0000125.0000 (MM)
BS
375.0000125.0000 (MM)
DT
100.0000500.0000 (US/M)
FILE 2
BS
BOREHOLE COMPENSATED SONIC
Figure C34
Introduction to Openhole Logging
(05/96) C-48
NPHI---
---PEF
DRHO---
1925
1/240
1 05-JUN-1992 08:58 INPUT FILE(S) CREATION DATE
CP 32.6 FILE 4 05-JUN-1992 11:42
.45000 -.1500
DPHI(V/V )
.45000 -.1500
NPHI(V/V )
0.0 10.000
PEF
-250.0 250.00
DRHO(K/M3)
0.0 150.00
GR(GAPI)
125.00 375.00
CALI(MM )
125.00 375.00
BS1
125.00 375.00
C2(MM )
CP 32.6 FILE 4 05-JUN-1992 11:42
LIMESTONE
COMPENSATED NEUTRON - LITHO DENSITY (WITH PE)
1950
DPHI---
---BS1
---CALI
---GR
Figure C35
Recommended