SPE-1463-PA_Measurement of Some Mechanical Properties of Rocks and Their Relationship to Rock...

8/12/2019 SPE-1463-PA_Measurement of Some Mechanical Properties of Rocks and Their Relationship to Rock Drillability

1/6

PETROLEUM

TR NS CTIONS

~

~ t : ~

t').l

Measurement of Some Mechanical Properties of Rocks

And

Their Relationship to Rock Drillability

ABSTRACT

S. GSTALDER

J.

RAYNAL

MEMBERS

A ME

Consideration was given to simple tests which could be

performed on

rocks

to give a measure of rock drillability.

Various methods of breaking rocks were considered and

the hardness test developed by Schreiner was selected for

this study. The test involves application of an increasing

load

on

the rock face through a flat-faced cylindrical

punch until rupture

OCClirs

Test results

show

that hardness is a

good

measure of

the breaking strength

of

rocks.

Useful

relationships are

shown to exist between hardness and other physical quan

tities such as specific disintegration, Young s modulus and

sonic velocity.

Specific

disintegration

(volume

of rock bro

ken

per unit of work input provides a possible

means

of

comparing the effectiveness of other methods of rock

breakage.

The

relationship between hardness

and

sonic velocity

may be very significant for it may be possible to deduce

rock drillability from sonic log data, provided that a

mineralogical factor is taken into account.

To

test further the results

of

the work, laboratory dril

ling tests were performed which closely simulated down

hole pressure conditions. Correlations of drilling results

with

rock

hardness

measurements

were quite good.

It

was

concluded that rock hardness, as measured in the labora

tory or as deduced froll1 sonic logs, could be used in re

lations for predicting rock drilling performance.

INTRODUCTION

Drillability of rocks cannot be defined in an absolute

manner

by

a single

quantity

or

measured

by a single test.

Resistance of

rock

to drilling depends to a large extent

upon

the

means

used

for rock

destruction.

This

compli

cates drill ability classification of rocks for there may be

as many classifications as

there

are methods of rock break

age without known functional relations between them.

The objective of this work was to develop, in terms of

rock destruction behavior, means

of

selecting

the

best

drilling

method

for a given rock

or

the

conditions for

optimum output from a given drilling method.

Original

manuscr ipt

received in Society

of

Petroleum Engineers of -

fice April 19. 1966. Revised

manuscr ipt

received .Ttilv

1.

1966. Papel

was

presented

at

A.R.T.F.P.

Colloquium. held in

Paris, France, June.

1965.

Discussion of this and all

following technical

articles

is invited. Dis-

cussion

in writing

(three copies)

may be sent t the

office

of the

Journal

of

Petroleum

Technology. Any discussion offered

after De


2/6

difficulty

of

obtaining correlations based

on

this quantity.

However, these problems

can

be overcome to a reasonable

degree by taking a sufficient

number of

test results and

reporting the arithmetic mean plus the standard deviation

from the mean.

For

practical application of results from the previously

mentioned tests, it is necessary to have a functional re

lationship between the measured quantity of the test and

the specific disintegration.

n

the final analysis, it will then

be necessary to develop relations between specific disinte

gration and actual rock drilling parameters. Such relations

have

been

studied in the present work.

SPECIFIC DISINTEGRATION BY

HARDNESS

TEST

SELECTION OF METHOD

The

Schre iner hardness test was selected for the pres

ent

investigation

of

rock drill ability because it

is

believed

to be a fairly good measure of the destructive work done

by

the tooth of

a rock bit.

The

method is simple, the re

sults are quite reproducible and the tests can be carried

out

fairly rapidly

and at

small cost. Although the method

is a static test

made on

rocks

at

atmospheric pressure,

good correlations with drilling results were obtained.

DESCRIPTION OF METHOD

The

Schreiner hardness test consists

of

loading a rock

surface with a flat-faced cylindrical

punch

at

increasing

loads until failure

of

the rock occurs. The loading head

used

in

this work is shown

in

Fig. 1 and the control and

recording assembly

are

shown in Fig.

2.

Load is applied

to the rock by means of a universal testing machine having

a maximum capacity of 10 tons with four sensitivity

levels (0 to

O.

3, 1,3 and 10 tons).

The

punches are of var

ious sizes and are

made

of

either high speed tool steel

or

tungsten carbide.

The

displacement-load curve

is

recorded on an

XY

re

corder. Displacements

up

to 1116 in. are given by variable

induction transducers mounted so as to exclude all ex-

FIG. I-HARDNESS

TEST

LOADING

HEAD.

992

traneous distortions which might be caused by the press.

Loads

are

measured by a pressure transducer (250 psi). A

recorder is also used to monitor the load curve and to

control the speed with which the load is applied.

The

volume

of

rock broken by the loading is deter

mined by weighing the cuttings and applying the rock

density which has been previously determined. Results of

all measurements show a mean accuracy

of

about 5 per

cent, which is far less than the value corresponding to the

scattering due to rock heterogeneity.

METHOD OF DATA ANALYSIS

Fig. 3 shows typical results obtained from the hardness

test. After a small correction for

punch

deformation

is

made, the following quantities can be determined from the

test record.

1

Hardness:

n .

= F /A

kg/mm

FIG.

2-CONTROL

ND RECORDING

ASSEMBLY

FOR HARDNESS TEST.

/

0>

Fr LOAD AT RUPTURE /

----------- -

lJ

Fe EL STIC LIMIT

/

I g TOTAL WORK Wr

u

o

a:

z

o

o

o

...J

ELASTIC WORK

W

~

0

/

he

PUNCH

DISPL CEMENT

h mm

FIG. 3-LoAD-DISPLACEMENT CURVE FROM HARDNESS TEST.

JOURNAL OF PETROLEUM TECHNOLOGY


3/6

Fr

= load

at

which failure occurs, kg

A

=

cross-sectional

area of punch, mm'

2

Specific disintegration: v, =

V/W

mm/kg

mm

V = volume of broken rock, mm'

W,

= work required to break

rock, kg mm

3. Plasticity coefficient: K =

Wr/We

W .

=

total

work to break rock

(total

area under

load-displacement curve), kg mm

W, =

elastic

work

(cross-hatched

area

in Fig. 3),

kgmm

4. Young's modulus: E = (1 -

p,2

Fe D he, kg/mm'

p, =

Poisson's

ratio

Fe

=

load

at

elastic limit,

kg

D = diameter of punch, mm

he

=

displacement

of punch at

elastic unit, mm.

Extensive tests were made to determine the influence

of the punch on

results

obtained, The

first three quan

tities

above

were

not

affected

by

size as long as

the punch

measured

a few

square

millimeters

in area

(much larger

than the

size of individual grains or pores). Young's mod

ulus

calculated

by Boussinesq's equation is affected by

variation in

section dimensions.

For

results which agree

with values

obtained by

uniaxial compression tests,

the

punch

section needs to be several tens of square milli

meters.

In making the

tests it seemed advisable to select

a

standard punch shape and

size so

that

this would

not

be a variable in the correlations.

PUNCH TEST RESULTS

Fig. 4 shows the relation between hardness and specific

disintegration

for

various rocks, including limestones, dolo

mites

and

granites

from

quarries

and

limestones

and

sand

stones

from

wells.

Each

point represents

the

average of

at least 10 tests. A least-squares fit

of

the data gives the

following relation:

nr =

70.5 X V;O.5S.

Although the spread

of

data

seems

to

be large (observe

the

20

per cent

deviation lines),

the

correlation is valid

-

'

(I)

(I)

w

z

0

c::


4/6

of sonic waves

through the

rock as previously shown gives

hope that sonic logging may provide a means of deducing

in

situ mechanical properties of rock. Although other

properties are involved in

the

relation

and

should be

thoroughly studied for a given formation, it should be pos

sible to determine relative variations in hardness from a

sonic log.

To

test this supposition, sonic logs were run in

two

wells in the Paris basin

from

which core samples

were available. Lithological characteristics of the sections

In the wells are as follows:

Well Cramaille 102 (ACrl02) 1,595 to 1,670 m -

clay, soft sandy limestone; 1,670 to 1,808

m-oolitic

limestone, granular limestone.

Well Villeblain 101 (AVblOl) 1,415.5

to

1,487 m

dark grey to black clay, thin sand stringers; 1,487

to 1,497.7 m-grey oolitic limestone.

Results of hardness measurements as function of depth

and sonic log travel time vs

depth

are

compared

for

both

wells in Fig. 7 with very good correlations shown. Al

though there appears to be a larger variation in hardness

results

than

in sonic log data, consideration must be given

to

the averaging characteristics of the sonic log.

COMPARISON

WITH

DRILLING

PERFORMANCE

LABORATORY DRILLING TESTS

The

question now arises as to

what

relations

may

exist

between the three quantities (hardness, specific disintegra

tion

and

sonic velocity)

and

actual rock drilling perform

ance. To examine this question, drilling tests were per

formed on several rock samples using the drilling machine

shown in Fig. 8.

The

drilling machine is designed to simulate down-hole

conditions as closely as possible. Confining pressure is

N

150

::;:

::;:

"-

'

'

lao

f)

f)

w

z

0

a:

50


5/6

sonic velocity (Figs. 10c and cl). Figures on the left are

for the soft-medium formation hit while the right-hand

figures are for the medium-hard rock hit. Curves for the

two constant weights

2

and 4 tonnes) are shown.

It is apparent from

the

figures that any correlation be

tween drilling performance and rock characteristics would

have to take into account the type of hit and the load on

the hit. Within the limits of the present tests, drilling pene

tration rate is linearly proportional to weight on the bit.

A relatively larger amount of scatter

of

the data occurs

for the harder rocks and is probably a result of the load

being too light for effective drilling

of

these

hard

rocks.

It

should be further noted that penetration rate does not in

crease with decreased

mck

hardness as rapidly for the hard

rock bit as it does for the soft rock bit. In fact, at the

SOFT

ROCK

BIT SMF-T

8 ~ ~ = = = = = = = = = = ~ ~

0)

WEIGHT ON BIT:

4

z

..J

..J

a:

0

z

..J

..J

a:

0

8

4

0.2

0.4

HARDNESS Kg/mm

2

8 ~ ~

(e) (d)

4T

4

_ 0 _ 4T

~ c I

.

~ tt>6b 0

L-______ ______

L _ _ _ _ _ _ _L

0.6 0.8 0 0.2

0.4

0.6

0.8

SPECIFIC DISINTEGRATION

mm

3

/Kg mm

FIG. 9-CORRELATTON

OF DRILLING PERFORMA

WITH ROCK

CHARACTF.RISTTCS.

SOFT

ROCK

BIT SMF-T

8 - - - ~ = = = = = = = = = = = = ~ - - - - ~

MED. HARD

BIT

W4T

8 - - - ~ = = = = = = = = = = = - - - - - ~

WEIGHT ON

BIT:

(0) (b)

4

_ 4T

__ ____ ~ ________L

o

2000 4000

6000

YOUNG S MODULUS Kg/mm

2

8

8 r ~

(e)

(d)

4T

4

-

2 T ~

_

0

o

o

O ~ ______

L

____ ____

4T

2 ~

L-

______

L

. . J

______

~ = _

__

o

2000

4000

6000

8000

o

2000 4000

6000 8000

SONIC VELOCITY

m/sec

FlG. IO-CORRELATION

OF

DRlLLlNG

PERFORMANCE

WITH ROCK CHARACTERISTICS.

AUGUST.

1966

995


6/6

were

made

at atmospheric pressure conditions while drill-

ing tests were

run

with confining and circulating fluid

pressures. Efforts have been made to perform hardness

tests

under

pressure

but

the difficulty is

that

sudden rup-

ture

of the rock

no

longer occurs in excess of about 300

psi (20 bars) pressure. Thus, results

of

hardness tests under

pressure are not sufficiently conclusive to permit reason-

able correlations. Hardness tests at atmospheric pressure

do

give conclusive results. Since these results correlate

with drilling performance, atmospheric pressure hardness

tests are considered useful

for

rock drillability classifica-

tion.

In

regard to sonic velocity measurements, it

is

gener-

ally

known that

sonic velocity increases with applied pres-

sure. Thus, higher velocities would be expected for in situ

sonic log measurements than

for

laboratory sonic velocity

measurements

at

atmospheric pressure. This in no way

detracts from the possible use

of

a sonic log for prediction

of

rock drillability.

CONCLUSIONS

Rock hardness, as determined by a modified Schreiner

test, is shown to be a useful quantity with which to pre-

dict rock drilling performance.

From

hardness test data,

996

specific disintegration (a quantity having greater physical

significance) may be determined. This latter term, which

is a quantitative measure of rock damage done per unit

of

work. expended, should be useful in comparing the

effectiveness

of

different rock breakage methods.

Rock hardness can be deduced from sonic velocity

measurements, provided mineralogical characteristics of

the rocks are considered. Thus, the possibility exists that

the sonic log may be useful in predicting rock drilling

performance.

In the possible use of rock hardness measurements for

drilling performance prediction, account will have to be

made for the fact that drilling

is

dependent on

other

parameters such as bit type, weight on the bit, rate of

rotation and related quantities.

REFEREN(:ES

1. Cauthier, G. and Baron, G.: Classifieation of Roc:b Previous

to Any Drillability Investigation , evue de l lnstillli Francais

dlt Petroie (March, 1965)

XVIII

No. :3

34, 3468.

2.

Sehreiner,

L A. et

ai.: MechanilOal and Abrasive Properties of

Ro('b , Traduction Neyr/or,

Mo ,ow, U.S.S.R. (l9511) T. 1531.

3. Timoshenko,

S.:

Theory

0

Elasticity, McGrawIIill Book Co.,

New York, N.Y. (}934).

JO IJRN A L O F

PETROLEIJM

TECH N O LO G Y

SPE-1463-PA_Measurement of Some Mechanical Properties of Rocks and Their Relationship to Rock...

Documents

Transcript of SPE-1463-PA_Measurement of Some Mechanical Properties of Rocks and Their Relationship to Rock...