accura poro ation

7/24/2019 Accura poro ation

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RALpH E. JENKINS

CORE LABORATORIES, INC.

The opinions expressed in this paper are those of the author and are not

necessarily the opinions of the Society of Professional Well Log Analysts or its

members.


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ACCURACYOF POROSITYDETERMINATIONS

RalphE. Jenkins

CoreLaboratories,nc.

ABsTWcT

Porosity values of reservoir rocks may be obtained directly by measure-

ments on core samples, or may be calculated indirectly from data provided by

various types of downhole logs. Calculation of porosity values by the in-

direct methods requires the use of certain factors which are characteristic

of the formation and its contained fluids.

These factors can be determined

best by

calibration of the log data with directly measured porosity data.

The direct methods most commonly used for determining porosity by core

analysis are presented.

The advantages, limitations and errors of each are

discussed and the methods are evaluated.

In the usual porosity ranges, the

acceptable methods for determining porosity are accurate to within

~ 1/2 of a

porosity per cent. An understanding of the methods will assist the log analyst

in evaluating the porosity data available for use in determining the factors

to be used in log interpretations.

INTRODUCTION

The problem of quantitative determination of the porosity of producing

formations has become of ever increasing importance to the well log analyst.

This is reflected by the emphasis that has been placed over the past ten to

fifteen years upon the development of logging tools which are sensitive to, or


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rock material,

regardless of whether or not they are interconnected; whereas,

effective porosity encompasses only the interconnected pores.

This convention

also has shortcomings which are apparent when it is realized that even fused

glass has low but measurable permeability.

It would be necessary to establish

a limiting permeability between isolated pores and an interconnected pore

system in order to well define total and effective porosity.

This paper will

consider effective porosity to encompass the pores from which, or into which,

a gas can flow so that pressure equilibrium can be obtained in a reasonable

length of time, perhaps three or four hours.

For reservoir evaluation purposes, two basis approaches may be made to

porosity determinations:

1. Cores may be cut and analyzed,

with the porosity values determined

by direct measurement.

2.

Certain down hole logs may be analyzed, and porosity values

calculated from the log data.

The well log analyst is primarily concerned with the latter method.

The micro-contact resistivity logs, the density logs, including the gamma

and velocity or sonic logs,

and the neutron logs have been widely used for

calculating formation porosity.

Equations have been developed for each of the

logging methods to enable the log analyst to calculate porosity values from

the log data. These equations have factors sometimes fixed within narrow

limits) to compensate for changes in characteristics of the formation and its

contained fluids, hole conditions,

and characteristics of the logging instru-

mentation.

For best results these factors are determined by calibrating the

log data with core analysis data.

When log calibrations made on one or a relatively few wells are projected


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dimensions of a specifically shaped sample and applying the proper geometric

formula; by measuring the displacement of a non-wetting fluid such as mercury;

by the application of Archimedes principle, e.g. ,

the sample is weighed in

air and weighed again suspended in a wetting liquid; or, by adding directly

and independently measured grain volume and pore volume values.

Where the bulk

volume is measured by displacement of a wetting liquid, the sample must be

saturated with the displaced liquid to an extent that no displaced liquid will

be imbibed into the sample.

The technique used for the determination of bulk volume may vary in any

one laboratory depending upon the size and type of sample analyzed and other

factors such as the surface texture.

Calipering samples for calculation by a

geometric formula is the least accurate method resulting in routine errors of

1 , and not uncommonly they are 2 and 3 of the bulk volume value.

The dis-

placement techniques are normally the best,

yielding routine errors of less

than 0.5 of the bulk volume.

The accuracy of the summation of grain and pore

volumes method is dependent upon the accuracy of the grain and pore volume

measurements,

PORE VOLUME AND GRAIN VOLUME DETERMINATIONS

The pore volumes and grain volumes of core

variety of methods.

These methods have come to

determination methods even though separate bulk

necessary in most of them.

samples are obtained by a

be known as the actual porosity

volume measurements are also

Boyles Law Method

Gas is compressed into or expanded from the pores of a core sample.

Measure-

ments of the volumes and pressures of the gas allow the calculation of either the

pore volume or grain volume by the use of Boyles Law.

Many Boyles Law poro-


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method, but can more accurately be described as a vacuum extraction method.

The air initially in the pores of the core sample at atmospheric pressure is

evacuated from the sample, collected,

and returned to atmospheric pressure.

The volume of air thus collected is measured.

Routine accuracy of measurement

of the pore volume to within ~ 5 of the pore volume is claimed for this method.

A scrupulously clean instrument and careful laboratory technique will allow

more accurate measurements.

The Washburn-Bunting method yields effective porosity.

This method is

somewhat slow and tedious,

particularly when low permeability samples are

involved and several evacuations must be made.

A dirty instrument will cause

high pore volumes to be measured and the instrument is difficult to keep clean

on a routine basis.

Tiletechnique is poor for measuring low permeability and

porosity samples.

Resaturation or Gravimetric Method

An extracted and dried sample is weighed, then saturated with a liquid

of known density, either water or ilydrocarbon, then reweighed. The weight

increase divided by the density of the saturating liquid yields the pore volume

of the sample.

This technique also yields effective porosity. The use of the technique

must be restricted to samples whose saturated weights can be determined.

Samples with solution cavities on the surface cannot be handled. Some core

* 2 of the pore

nalysts feel that the pore volume can be measured within

volume, but most analysts feel that the extracted and dried samples can seldom

be completely resaturated.

Pore volumes determined with this technique common-

ly are 10 lower than pore volumes determined on the same samples with Boyles

Law or other techniques.

Tinetechnique requires considerable time, but many

samples can be handled simultaneously.

Grain Density Method


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Summations of Fluids Method

Since the pores of a core sample contain gas,

the pore volume can be determined by measuring the

water, and

volumes of

possibly oil,

gas, water, and

oil in a sample and adding them.

The gas volume is determined by measuring the

amount of mercury that can be injected into a portion of the sample at 750 or

1000 psi. The oil and water contents of the remainder of the sample are dis-

tilled from the sample, recovered, and measured.

Although the pore volume

could be determined,

it is customary in this procedure to determine the gas,

water,

and oil content individually as a percentage of the bulk volume of the

same,

and the summation of these three values gives porosity expressed as a

percentage of bulk volume without further need to refer to the bulk volume

~ 270of its value. m-

easurements. The gas volume is

determined to within

cept where the pore water and water of hydration or absorbed water are very

difficult to distinguish, the water content is determined within

i 2 . This

method, then,

gives pore volume values essentially accurate to within ? 2 of

the correct values.

In some modifications the summation of fluids method yields effective

porosity, However, when a high temperature retort is used to distill the oil

and water from the samples, the pore volume is closer to being total pore

volume than effective pore volume.

This technique enjoys the advantage of

utilizing a larger sample than the other methods.

It is a simple and rapid

method, which lends itself very well to routine laboratory determinations. It

has a disadvantage in that a distinction in the recovered water must be made

between pore water and water of hydration,

and a correction must be made to

the observed oil volume.

SAMPLE PREPARATION

All the techniques cited except the last require a prepared sample, The


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error each in determining pore volume and bulk volume directly and independent-

ly as in equation l),

the maximum error in the porosity measurement would be a

constant 2 , regardless of the porosity value.

The line for equation 2)

where bulk volume is determined by adding pore volume and grain volume is

similar to that for equation l),

showing a slight improvement as porosity

increases.

The relatively flat line is due to the numerator being a directly

determined pore volume value.

Equation 3), however,

shows a sharp increase

in the percentage of error in the porosity determination as porosity decreases.

At 10 porc)sity, the error in the porosity determination would be 18 for a

unit error each in the bulk volume and grain volume determinations,

This is

due to the numerator consisting of the difference of two large numbers.

Figure No. 2 shows the bands of percentage of errors in the determination

of a porosity of 30 as a function of the errors in measuring pore volume,

grain volume,

and bulk volume, when using equations 1) and 3).

The lines

describing the errors for equation 3) have much greater slope, again showing

the sensitivity of this equation to the errors in measurements of its variables.

Figure No. 3 shows how much greater the slope is for the condition of 10

porosity.

Note that the bands in Figures No. 2 and No. 3 are only half of the

total band,

showing errors in bulk volume from O to + 2 only.

Translated into approximate error in porosity per cent the Boyles Law,

summation of fluids,

and grain density methods yield porosity values accurate

to i 0.5 porosity per cent.

The Washburn-Bunting method normally yields values

+ 1.0 porosity per cent.

ccurate to .

Tiieresaturation technique routinely

yields porosity values lower by 2 to 10 of the value than do the other methods.

COMPARISON OF POROSIIW DATA

It is extremely difficult to prove the absolute accuracy of a porosity

method when analyzing a natural sample which has an unknown density.

By com-


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values determined by the other method,

even in this low porosity range.

This

sampling is representative of a much larger group of data, and indicates that

the differences between total and effective porosity are usually academic.

It is recognized that by definition total porosity cannot be lower than

effective porosity. Larger effective values are frequently found in actual

measurements because both total and effective porosity values generally fall

within the limits of accuracy of the measurements.

Table IV is a tabulation of porosity values obtained by all the methods

discussed, The number of samples in each group is much larger than in the

prior tables. The data were taken from 80 formations from the Gulf Coast, the

Ark-La-Tex area, the West Texas and the Mid-Continent area, the Rocky Mountain

area, Canada, and Iran.

The number of samples per formation varied from 3 to

380, with the average number being 40 per formation.

The data in Table IV

includes some of the data in prior tables,

and conform to the patterns already

stated.

The resaturation porosity values are consistently lower than the other

values by 10 or more, except for the carbonate samples.

All the resaturation

data presented here were obtained with the use of water as the resaturant, but

other reports have shown similar data when using a hydrocarbon as the saturant.

Note that the summation of fluids porosity values and values measured by

other methods cannot always be made on the same sample.

This introduces a

statistical problem, and it is necessary to use a large number of samples for

comparison in order to compensate for the effect of variations in actual

porosity of adjacent samples.

Comparison of large sample groups shows good

correlation between Boyles Law and summation of fluids porositites. The

Boyles Law values are typically higher than the summation of fluids values

in clean sands. The sorption phenomena present in the Boyles Law method

may account for this. However, in the shaly or clayey samples, the summation

of fluid values are typically higher, probably due to the distinction made


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resistivity, mud filtrate resistivity, and the resistivity of the completely

flushed formation.

Some of these factors are subject to significant change

in relatively short distances,

which would seem to require relatively frequent

or calibration points. Then too, a gamma log is often used to pick relatively

shale-free sections in which to calculate porosity.

This is satisfactory most

of the time,

but the gamma logging of cores at the surface in recent years has

shown that it is not infallible.

Several cases have been found where the

relatively high gamma radiation came from the more porous sections of the core.

Resolution of porosity variations in highly porous zones and their beds can

also be a problem.

This may be particularly important in reservoir evaluation

work.

A knowledge of the grain density of the formation being evaluated is

necessary in order to calculate porosity from a gamma-gamma density type log.

Since this calculation is quite sensitive to the grain density value, it is im-

portant that an accurate value be available or determined.

The metihanics of

bombarding a formation with neutrons and measuring backscattered neutrons is

very complex.

Rock matrix composition has a considerable effect on the capture

and scattering of neutrons so that the calibration of a neutron log against

core analysis data can seldom be accurately used over a wide area. Resolution

of porosity values is sometimes poor, and the drag effect of these delayed

response type instruments should be kept in mind.

The velocity or sonic log has undoubtedly become the most popular log for

porosity evaluation in recent years. In the past the sonic log has been primarily

calibrated against core analysis, and many log analysts still feel this is the

best approach. It is recognized that the velocity of sound waves in a formation

is affected by the lithologic characteristics of the formation, the density and

pressure of the contained fluids, the rock stresses, and by such factors as

fractures,

vugs and gas-filled pores when they are present.

Since these factors

can be expected to vary throughout a field,

several calibrations may be nec-

essary.

It is generally believed that the overburden pressure is consistently


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4.

Resaturation porosity values are generally low due to difficulty in

completely resaturating an extracted and dried sample.

5.

Total and effective porosity values generally show good agreement.

6.

Pore volume and grain volume should be measured directly when possible.

7.

Log analysts should carefully consider factors affecting log values

as well as the core analysis values.


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1

03

F

40

35

30

25

20

I

I

5

0

5

0(

\

\

I

I

I

10

1

20

30 40

50

POROSITY, %

-MAXIMUM 9 . ERROR VS POROSITY MAGN

MEASURED VOLUMES IN ERROR 176

TUDE


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ERROR IN SAND GRAIN VOLUME, %

.

w

3

:

>

l

iii

a

0

n

u-

0

Oxo

>

1

G

a

n

z

a

0

a

a

LIJ

+12

2

-1

8

=30%

10

IN

OF

121

2

-1

ERROR

G.2-RELATIONSHIP

o +1

PORE VOLUME, %

MEASUREMENT

+2

ERRORS


13/17

a

0

a

a

Id

+60

+50

+40

+30

+20

+10

o

lo

20

30

40

50

ERROR IN SAND GRAIN VOLUME, %

2

-1

0

+1

+2

I I I

I

1

(p=lo%

601

,

I

I

I

I

2

I

-1

ERROR IN

G.3-RELATIONSHIP OF

o +1

+2

PORE VOLUME, %

MEASUREMENT ERRORS


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TABLE I

COMPARISON OF POROSITY MEASUREMENTS

WASHBURN-

SUMMATION

BOYLES

SAMPLE BUNTING

OF FLUID

LAW

NUMBER POROSITY,%

POROSITY,%

POROSITY, %

5.5 2.0 1.67

2

4.2

1.9

2.47

3 6.8 5.5 6.17

4

4.0 I .9

1.20

5 5.3

1.6

1.53

6 4.4

1.9

1.64

7

5.8 1.6

2.58

8

5.5 2.0 I .86

9

4.3

1.7

I .68

10

5.6 3.6

2.90

AVERAGE

5.1

2.4

2.37


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TABLE H

COMPARISON OF POROSITY MEASUREMENTS

WASHBURN-

BUNTING

POROSITY,%

RESATURATION

(WATER)

POROSITY,%

BOYLES

LAW

POROSITY,%

2.56

SAMPLE

NUMBER

I

2.66

2.03

2.76

2.41

2.15

3

4

3.33 3,08 3.32

4.00.54 3.04

5

4.54 4.39

5.05

5.68

8.66

12.2

}2.5

6 4.98

.19

7

9.10 7.45

8 12.20 8.96

9 12.5

10.4

10

12.8 11.1

13.3

AVERAGE

6.93

5.?6 7.00


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TABLE III

SON OF POROSITY MEASUREMENTSOMPAR

I

BOYLES

GRAIN DENSITY

I

SAMPLE

LAW- (TOTAL)

NUMBER

POROSITY, %

PO ROSITY,%

+

2.19

.45

4.07

5.89

1.42.60

2.68

1.99

5.41

.13

3.302.56

1.47

I .47

3.69

4.05

I

4.43

5.49

2.58

0

1.84

3.12

VERAGE

3.24


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TABLE ISZ

COMPARISON OF POROSITY VALUES

MEASURED BY ALL METHODS

UMBER

WASli

OF

POROSITY

BUNT

hMPLES TYPE

RANGE POR.

232 ICLEAN sANO I 8 35 I

I 15.7 I 17.6 I

85 lSL. SHALY

I 4 28 [ I 17.5 I 19.5 I

29 ICONGLOMERATE[ 6 I 5 I

70 lCARBONATE [ 6 25 [ I 18.9 I 20.0 I

95 lCLEAN sANO I 8 35 1 117.81

18.6 I

390 [SL. SHALY

I 4 28 I

I 5 lcoNGLoMERATE/ 6 I 5

I 10.2 I 110.91

I I 2

lcARBONATE

I

320 1

I

14.8 1

137 ISAND

I

1 22 1 I 5.55 I 5.42

210 lSAND 11.3

12.8

3220 IALL TypEs

1 2 351

14.6 I

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