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POROSITY DETERMINATION

FROM LOGS

Most slides in this section are modified primarily from NExTPERF Short Course Notes, 1999.

However, many of the NExT slides appears to have been obtained from other primary

sources 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

Gamma

ray

Resisitivity Porosity

Increasingradioactivity

Increasingresistivity

Increasingporosity

Shale

Shale

POROSITY DETERMINATION BY LOGGING

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POROSITY LOG TYPES

3 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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GRAPI0 200

CALIXIN6 16

CALIYIN6 16

RHOBG/C32 3

DRHOG/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+ hmc)

Source

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BULK DENSITY

fmab 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/ccDolomite 2.87 g/cc

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POROSITY FROM DENSITY LOG

Porosity equation

xohxomff S1S

fma

bma

Fluid density equation

We usually assume the fluid density (f) is between 1.0 and 1.1. If gas is present, the

actual 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

Sxo is the saturation of the flush/zone, decimal

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DENSITY LOGS

Working equation (hydrocarbon zone)

mashshsh

hcxomfxob

V1V

S1S

b = Recorded parameter (bulk volume)

Sxomf = Mud filtrate component

(1 - Sxo) hc = Hydrocarbon component

Vshsh = Shale component

1 - - Vsh = Matrix component

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DENSITY LOGS

If minimal shale, Vsh

0

If hcmff, then

b

= f

- (1 - ) ma

fma

bmad

d = Porosity from density log, fractionma = Density of formation matrix, g/cm

3

b = Bulk density from log measurement, g/cm3

f = Density of fluid in rock pores, g/cm3

hc = Density of hydrocarbons in rock pores, g/cm3

mf = Density of mud filtrate, g/cm3

sh = Density of shale, g/cm3

Vsh = Volume of shale, fraction

Sxo = Mud filtrate saturation in zone invaded by mud filtrate, fraction

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GRC0 150

SPCMV

-160

40

ACAL

6

16

ILDC0.2 200

SNC0.2

200

MLLCF

0.2

200

RHOC1.95 2.95

CNLLC0.45

-0.15

DTus/f150 50

001) BONANZA 1

10700

10800

10900

BULK DENSITY LOG

Bulk Density

Log

RHOC

1.95 2.95

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NEUTRON LOG

Logging tool emits high energy

neutrons into formation

Neutrons collide with nuclei of

formations atoms

Neutrons lose energy (velocity) witheach collision

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NEUTRON LOG

The most energy is lost when colliding

with a hydrogen atom nucleus

Neutrons are slowed sufficiently to be

captured 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

NhcxoNmfxoN

V1V

S1S

N = Recorded parameter

Sxo Nmf = Mud filtrate portion

(1 - Sxo) Nhc= Hydrocarbon portion

VshNsh = Shale portion

(1 - - Vsh) Nhc = Matrix portion where = True

porosity 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

Sxo = Mud filtrate saturation in zone invaded

by mud filtrate, fraction

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GRC0 150

SPCMV

-160

40

ACAL

6

16

ILDC0.2 200

SNC0.2

200

MLLCF

0.2

200

RHOC1.95 2.95

CNLLC0.45

-0.15

DTus/f150 50

001) BONANZA 1

10700

10800

10900

POROSITY FROM NEUTRON LOG

Neutron

Log

CNLLC

0.45 -0.15

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Upper

transmitter

Lower

transmitter

R1R2

R3R4

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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sec

50

T0E2

E1E3

Mud waves

Rayleigh

waves

Compressional

waves

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Lithology Typical Matrix Travel

Time, tma, sec/ftSandstone 55.5Limestone 47.5Dolomite 43.5

Anydridte 50.0Salt 66.7

COMMON LITHOLOGY MATRIX

TRAVEL TIMES USED

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ACOUSTIC (SONIC) LOG

Working equation

mashshsh

hcxomfxoL

tV1tV

tS1tSt

tL = Recorded parameter, travel time read from log

Sxotmf= Mud filtrate portion

(1 - Sxo) thc = Hydrocarbon portion

Vshtsh = Shale portion

(1 - - Vsh) tma = Matrix portion

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ACOUSTIC (SONIC) LOG

If Vsh

= 0 and if hydrocarbon is liquid

(i.e. tmftf), 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

USFT140 40

SPHI

%30 10

4100

4200

GR

API0 200

CALIX

IN6 16

ACOUSTIC (SONIC) LOG

Sonic travel time

Sonic

porosity

Caliper

Gamma

Ray

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SONIC LOG

The response can be written as follows:

fmalog 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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GRC0 150

SPCMV

-160

40

ACAL

6

16

ILDC0.2 200

SNC0.2

200

MLLCF

0.2

200

RHOC1.95 2.95

CNLLC0.45

-0.15

DTus/f150 50

001) BONANZA 1

10700

10800

10900

SONIC LOG

Sonic

Log

DT

150 50us/f

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EXAMPLE

Calculating Rock Porosity

Using an Acoustic Log

Calculate the porosity for the following intervals. The measured travel times from the

log 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 is

saturated with water having a tf= 189.0 sec/ft.

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GRC0 150

SPCMV

-160

40

ACAL

6

16

ILDC0.2 200

SNC0.2

200

MLLCF

0.2

200

RHOC1.95 2.95

CNLLC0.45

-0.15

DTus/f150 50

001) BONANZA 1

10700

10800

10900

SPHIss

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 matrixcompositions

Respond differently to presence of gas orlight 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 significantlyaffected by gas

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ESTIMATING POROSITY FROM

WELL LOGS

Openhole logging tools are the most common method

of determining porosity:

Less expensive than coring and may be less

risk of sticking the tool in the hole

Coring may not be practical in unconsolidatedformations or in formations with high secondary

porosity such as vugs or natural fractures.

If porosity measurements are very important, both

coring and logging programs may be conducted so

the log-based porosity calculations can be used to

calibrated to the core-based porosity measurements.

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Influence Of Clay-Mineral DistributionOn Effective Porosity

Dispersed ClayPore-filling

Pore-lining

Pore-bridging

Clay Lamination

Structural Clay(Rock Fragments,

Rip-Up Clasts,Clay-Replaced Grains)

e

e

e

ClayMinerals

Detrital QuartzGrains

e

e

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FlowUnits

Gamma RayLog

PetrophysicalData

PoreTypes

LithofaciesCore

1

2

3

4

5

CorePlugs

CapillaryPressure

vs k

GEOLOGICAL AND PETROPHYSICALDATA USED TO DEFINE FLOW UNITS

Schematic Reservoir Layering Profile

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Schematic Reservoir Layering Profile

in a Carbonate Reservoir

Baffles/barriers

3150

SA -97ASA -251

SA -356 SA -71 SA -344 SA -371SA -348

SA -346SA -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