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POROSITY DETERMINATIONFROM LOGS
Most slides in this section are modified primarily from NExT PERF Short Course Notes, 1999.However, many of the NExT slides appears to have been obtained from other primarysources that are not cited. Some slides have a notes section.
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Well LogSP Resistivity
OPENHOLE LOG EVALUATION
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Oil sand
Gammaray
Resisitivity Porosity
Increasingradioactivity Increasingresistivity Increasingporosity
Shale
Shale
POROSITY DETERMINATION BY LOGGING
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POROSITY LOG TYPES3 Main Log Types
Bulk density
Sonic (acoustic) Compensated neutron
These logs do not measures porosity directly. To
accurately calculate porosity, the analyst mustknow:Formation lithology Fluid in pores of sampled reservoir volume
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DENSITY LOGS Uses radioactive source to generate
gamma rays
Gamma ray collides with electrons information, losing energy Detector measures intensity of back-
scattered gamma rays, which is relatedto electron density of the formation Electron density is a measure of bulk
density
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DENSITY LOGS
Bulk density, b, is dependent upon:
Lithology Porosity
Density and saturation of fluids in pores
Saturation is fraction of pore volumeoccupied by a particular fluid (intensive)
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GR API 0 200
CALIX IN 6 16
CALIY IN 6 16
RHOB G/C3 2 3
DRHO G/C3 -0.25 0.25
4100
4200
DENSITY LOG
Caliper
Density
correction
Gamma ray Density
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Formation ( b)
Long spacingdetector
Short spacingdetector
Mud cake( mc + h mc )
Source
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BULK DENSITY
f mab 1Matrix Fluids in
flushed zoneMeasures electron density of a formationStrong function of formation bulk density
Matrix bulk density varies with lithology
Sandstone 2.65 g/cc
Limestone 2.71 g/cc Dolomite 2.87 g/cc
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POROSITY FROM DENSITY LOG
Porosity equation
xohxomf f S1S
f ma
bma
Fluid density equation
We usually assume the fluid density ( f ) is between 1.0 and 1.1. If gas is present, theactual f will be < 1.0 and the calculated porosity will be too high.
mf is the mud filtrate density, g/cc
h is the hydrocarbon density, g/cc
S xo is the saturation of the flush/zone, decimal
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DENSITY LOGS
Working equation (hydrocarbon zone)
mashshsh
hcxomf xob
V1V
S1S
b = Recorded parameter (bulk volume)
S xo mf = Mud filtrate component
(1 - S xo) hc = Hydrocarbon component Vsh sh = Shale component
1 - - V sh = Matrix component
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DENSITY LOGS If minimal shale, V
sh 0
If hc mf f , then
b
= f - (1 - )
ma
f ma
bmad
d = Porosity from density log, fractionma = Density of formation matrix, g/cm 3
b = Bulk density from log measurement, g/cm 3
f = Density of fluid in rock pores, g/cm 3
hc = Density of hydrocarbons in rock pores, g/cm 3
mf = Density of mud filtrate, g/cm 3
sh = Density of shale, g/cm 3
Vsh = Volume of shale, fractionSxo = Mud filtrate saturation in zone invaded by mud filtrate, fraction
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GRC 0 150
SPC MV -160 40
ACAL 6 16
ILDC 0.2 200
SNC 0.2 200
MLLCF 0.2 200
RHOC 1.95 2.95
CNLLC 0.45 -0.15
DT us/f 150 50
001) BONANZA 1
10700
10800
10900
BULK DENSITY LOG
Bulk DensityLog
RHOC1.95 2.95
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NEUTRON LOG
Logging tool emits high energyneutrons into formation
Neutrons collide with nuclei offormations atoms
Neutrons lose energy (velocity) witheach collision
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NEUTRON LOG
The most energy is lost when collidingwith a hydrogen atom nucleus
Neutrons are slowed sufficiently to becaptured by nuclei
Capturing nuclei become excited andemit gamma rays
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NEUTRON LOG Depending on type of logging tool either gammarays or non-captured neutrons are recorded
Log records porosity based on neutronscaptured by formation
If hydrogen is in pore space, porosity is relatedto the ratio of neutrons emitted to those countedas captured
Neutron log reports porosity, calibratedassuming calcite matrix and fresh water in pores,if these assumptions are invalid we must correctthe neutron porosity value
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NEUTRON LOG
Theoretical equation
Nmashshsh
NhcxoNmf xoN
V1V
S1S
N = Recorded parameter
S xo Nmf = Mud filtrate portion
(1 - S xo) Nhc = Hydrocarbon portion
Vsh Nsh = Shale portion
(1 - - V sh ) Nhc = Matrix portion where = Trueporosity of rock
N = Porosity from neutron log measurement, fraction
Nma = Porosity of matrix fraction
Nhc = Porosity of formation saturated with
hydrocarbon fluid, fraction
Nmf = Porosity saturated with mud filtrate, fraction
Vsh = Volume of shale, fraction
S xo = Mud filtrate saturation in zone invadedby mud filtrate, fraction
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GRC 0 150
SPC MV -160 40 ACAL
6 16
ILDC 0.2 200
SNC 0.2 200 MLLCF
0.2 200
RHOC 1.95 2.95
CNLLC 0.45 -0.15
DT us/f 150 50
001) BONANZA 1
10700
10800
10900
POROSITY FROM NEUTRON LOG
NeutronLog
CNLLC0.45 -0.15
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Uppertransmitter
Lower
transmitter
R1 R2
R3 R4
ACOUSTIC (SONIC) LOG
Tool usually consists ofone sound transmitter(above) and two receivers(below)
Sound is generated,travels through formation
Elapsed time betweensound wave at receiver 1
vs receiver 2 is dependentupon density of mediumthrough which the soundtraveled
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sec50
T0 E2
E1 E3
Mud wavesRayleigh
wavesCompressional
waves
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Lithology Typical Matrix Travel
Time, tma , sec/ftSandstone 55.5Limestone 47.5Dolomite 43.5Anydridte 50.0Salt 66.7
COMMON LITHOLOGY MATRIX
TRAVEL TIMES USED
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ACOUSTIC (SONIC) LOG
Working equation
mashshsh
hcxomf xoL
tV1tV
tS1tSt
tL = Recorded parameter, travel time read from log
S xo tmf = Mud filtrate portion
(1 - S xo) thc = Hydrocarbon portion
Vsh tsh = Shale portion
(1 - - V sh ) tma = Matrix portion
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ACOUSTIC (SONIC) LOG If V
sh = 0 and if hydrocarbon is liquid
(i.e. tmf tf ), then
tL = tf + (1 - ) tma
or
maf
maLs
tt
tt
s = Porosity calculated from sonic log reading, fraction
tL = Travel time reading from log, microseconds/ft
tma = Travel time in matrix, microseconds/ft
tf = Travel time in fluid, microseconds/ ft
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DT
USFT 140 40
SPHI % 30 10
4100
4200
GR API 0 200
CALIX IN 6 16
ACOUSTIC (SONIC) LOG
Sonic travel time
Sonicporosity
Caliper
GammaRay
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SONIC LOG
The response can be written as follows:
f malog t1tt
maf
ma
tt
tt
log
tlog = log reading, sec/ft
tma = the matrix travel time, sec/ft
tf = the fluid travel time, sec/ft
= porosity
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GRC 0 150
SPC MV -160 40 ACAL
6 16
ILDC 0.2 200
SNC 0.2 200 MLLCF
0.2 200
RHOC 1.95 2.95
CNLLC 0.45 -0.15
DT us/f 150 50
001) BONANZA 1
10700
10800
10900
SONIC LOG
Sonic
Log
DT150 50us/f
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EXAMPLE
Calculating Rock PorosityUsing an Acoustic Log
Calculate the porosity for the following intervals. The measured travel times from thelog are summarized in the following table.
At depth of 10,820, accoustic log reads travel time of 65 s/ft.
Calculate porosity. Does this value agree with density and neutronlogs?
Assume a matrix travel time, tm = 51.6 sec/ft. In addition, assume the formation issaturated with water having a tf = 189.0 sec/ft.
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GRC 0 150
SPC MV -160 40 ACAL
6 16
ILDC 0.2 200
SNC 0.2 200 MLLCF
0.2 200
RHOC 1.95 2.95
CNLLC 0.45 -0.15
DT us/f 150 50
001) BONANZA 1
10700
10800
10900
SPHI ss 45 -15
EXAMPLE SOLUTION SONIC LOG
SPHI
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FACTORS AFFECTING SONIC
LOG RESPONSE
Unconsolidated formations
Naturally fractured formations
Hydrocarbons (especially gas)
Rugose salt sections
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RESPONSES OF POROSITY LOGS
The three porosity logs: Respond differently to different matrix
compositions Respond differently to presence of gas or
light oils
Combinations of logs can : Imply composition of matrix Indicate the type of hydrocarbon in pores
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GAS EFFECT
Density - is too high
Neutron - is too low Sonic - is not significantly
affected by gas
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ESTIMATING POROSITY FROMWELL LOGS
Openhole logging tools are the most common methodof determining porosity:
Less expensive than coring and may be lessrisk of sticking the tool in the hole
Coring may not be practical in unconsolidatedformations or in formations with high secondaryporosity such as vugs or natural fractures.
If porosity measurements are very important, bothcoring and logging programs may be conducted sothe log-based porosity calculations can be used tocalibrated to the core-based porosity measurements .
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Influence Of Clay-Mineral DistributionOn Effective Porosity
Dispersed Clay Pore-filling Pore-lining Pore-bridging
Clay Lamination
Structural Clay (Rock Fragments,
Rip-Up Clasts, Clay-Replaced Grains)
e
e
e
Clay Minerals
Detrital Quartz Grains
e
e
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FlowUnitsGamma RayLogPetrophysicalDataPoreTypesLithofaciesCore
1
2
3
4
5
CorePlugsCapillary Pressure vs k
GEOLOGICAL AND PETROPHYSICALDATA USED TO DEFINE FLOW UNITS
S h ti R i L i P fil
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Schematic Reservoir Layering Profilein a Carbonate Reservoir
Baffles/barriers
3150
SA -97A SA -251
SA -356 SA -71 SA -344 SA -371 SA -348
SA -346 SA -37
3200
3250
3300
3350
3100
3150
3250
3300
3250
3150
3200
3100
3150
3200
3250
3200
3250
3250
3350
3300
3150
3200
3250
3300
3100
3200
3250
3300
3350
3150
3200
3250
Flow unit