porosity determination

8/10/2019 Porosity Determination

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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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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


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

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3350

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3100

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3250

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3100

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3250

Flow unit

porosity determination

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