Rock Mechanics Laboratory Tests for Petroleum Applications

Rob MarsdenReservoir Geomechanics AdvisorGatwick

Summary• A wide range of well established and proven laboratory tests are available for petroleum rock

mechanics studies. Not only can these provide fundamental rock properties for characterization purposes or for well designs, the more specialist tests can provide information on how formations might behave when they are subjected to complex stress-paths around wellbores or within reservoirs at various stages during the field-life.

• Laboratory studies also provide methods for obtaining data that can not otherwise be derived from log data or history matching. For example, whilst strength properties can only be inferred empirically from log data, they may be determined directly from laboratory measurements. Similarly, whilst logs can provide data on dynamic elastic properties, their static (i.e., mechanical) elastic and non-elastic deformation parameters can only be derived from history matching or laboratory tests. Continuous log data combined with laboratory tests on selected samples can therefore provide complementary tools for use in petroleum geomechanics.

• When coupled with petrophysical studies, the simulation of realistic mechanical loading on a laboratory sample can also significantly enhance the reliability and usefulness of what might otherwise be routine petrophysical analyses.

• Laboratory techniques also exist for obtaining in situ stress data and, since these methods are based on fundamentally different processes to those employed in hydraulic fracturing etc., they provide useful and complementary adjuncts to rig-site methods.

• This presentation describes the type of rock mechanics tests available, including some of the more specialist studies as well as routine measurements. The types of equipment employed are introduced, and the data obtained from such laboratory programmes are explained in the context of their end-use for well engineering and reservoir engineering applications.

Rock Mechanics Tests

….. routinely involve measurements of:• Strength & deformation characteristics• Pore pressure responses under un-drained

loading• Porosity and bulk volume changes• Fluid transport characteristics• Influences of temperature, effective stress path

etc.

and measurements of:• Ultrasonic velocities• AE

Essential Capabilities:• Simulate deviatoric stresses at depth or in

proximity to a wellbore• Measure deformations and bulk/pore volume

changes

Systems typically incorporate :• Triaxial or confining cell• Load or reaction frame• Closed-loop loading, confining & pore pressure

systems• Heating systems (for HT applications)• Devices for measuring deformations, pore volume

changes, flow, ultrasonic velocities

Closed-Loop Controli.e., programmed rate of displacement ∆fp/∆t is selected

actual displacement fa is measureddifference fp-fa feeds back to the system controlsystem responds, correcting fp-fa to zero

Confining pressurecontroller LoggerProgram

fp

Amp fa-fp fa

Hydraulicpower

Servo-valveAmp

fa-fp

t

fp

t

f1 f2

Logger

Triaxialframe & cell

Program

Hydraulicpower

Piston displacement monitored

Pressureintensifier

Axial displacement

LoadSelect

feed-back

Correctingsignal

Correctingsignal

Axial loadcontroller

Cellpressure

Fluid reservoir

2000kN capacity system(configured for triaxial tests)

1600kN capacitydynamic system

Examples of Advanced Test Systems

3000kN capacityHPHT system

Uniaxial Compression Testing

010

2030

4050

0 2 4 6 8 10 12Axial strain (mstr)

Axi

al s

tress

(MP

a)

01

23

45

Mea

n di

amet

ral s

train

(mst

r)

Axial stress Mean diametral strain

Indirect methods:• Schmidt hammer• Brinell hardness• Penetrometer• Point load

• Static Young’s modulus (E) @ σ3‘ = 0• Static Poisson’s ratio (v) @ σ3‘ = 0• Uniaxial yield strength (σY)• Uniaxial compressive strength (UCS)

Triaxial Testing

Configuration forcoupled HPHT

Conventional triaxial compression:• Lateral stresses are σ2=σ3• Axial stress σ1 is increased

Triaxial extension:• Lateral stresses are σ1=σ2• Axial stress σ3 is reduced

Data obtained:• Triaxial yield strength (σY)• Peak strength• Residual strength (σR)• Static Young’s modulus (E) at confinement • Static Poisson’s ratio (v) at confinement

Shearstress

Normal stress

Curvedfailure envelope

Axial deviatorstress (σ1-σ3)

Axial strain

Increasingconfinement

Individualtest data

Minor ef fect ive st ress σ3’

Majoref fect ivestress σ1’

Uniaxial compressivestrength UCS

Tensile st rength

Possible states

Impossible state

Triaxial Deformation & Peak Strength Data

Multi-stage triaxial compression(or extension)

0 5 10 15 200

5

10

15

20

25

Axial strain, millistrain

Con

finin

g st

ress

, MPa

e.g., σ3 =12510

20 MP

0 5 10 15 200

50

100

150

0

5

10

15

Axial strain, millistrain

Axia

l stre

ss, M

Pa

Dia

met

ral s

train

, milli

stra

inAxial stress Diametral strain 1 Diametral strain 2

Multiple Measurements on Single Plug

σ1

Time

σ3

KGrain

KBulk

E, v,C, & φ

Pore pressureu

σ1 =3 MPaσ3 =3 MPau =1 MPa

σ1 =12 MPaσ3 =12 MPau =10 MPa

σ1 =22 MPaσ3 =22 MPau =10 MPa

Measuresqueeze-out

volume

5-stage triaxial@ σ3 =22, 25, 30, 35, 40 MPa

u =10 MPa

Flush with kerosene then measure kKero@ σ1 =3 MPa, σ3 =3 MPauUp =1 MPa, u Down=0 MPathen set uUp =u Down =1 MPakKero

01

23

45

6

0 2 4 6 8 10Effective stress (MPa)

Bulk

vol

umet

ric s

train

(mst

r)

16.5

16.6

16.7

16.8

16.9

17.0

Poro

sity

(%)

Volumetric strain Porosity

010

2030

4050

60

0 100 200 300 400 500Effective stress (Bar)

Bul

k m

odul

us (k

Bar

)

0.6

0.7

0.8

0.9

1B

iot's

con

stan

t alp

ha

Bulk modulus Biot's constant alpha

040

8012

016

0

0 4 8 12 16 20 24Axial strain (mstr)

Axi

al s

tress

(MP

a)

02

46

8M

ean

diam

etra

l stra

in (m

str)

Axial stress Mean diametral strain

010

2030

4050

6070

0 2 4 6 8 10 12 14 16 18 20Time (minutes)

Flow

vol

ume

(cc)

E, ν, C, φ,σR & UCS

Kbulk & αwith σ’

kfluid K & σ’-porosity

Stress- & strain-path triaxial tests… stresses/strains replicate a specific burial or engineering condition

q=σ1-σ3 (MPa)

34 36 38 40 420

2

4

6

8

10

12

14Uniaxial compaction

(Ko triaxial)(i.e. axial compaction

with total lateral restraint)

p’ =(σ1+2σ3)/3 - pore pressure (MPa

Conventional Ko Test

0 0.5 1 1.5 2 2.5 330

35

40

45

50

55

• Constant pore pressure• σaxial is increased• σlateral adjusted so εlateral = 0

εaxial (millistrains)

σaxial(MPa)

30 35 40 45 50 5530

32

34

36

38

40σlateral(MPa)

σaxial (MPa)

Ko triaxial by Depletion/Repressuring

20 30 40 50 6010

20

30

40

50

Pore pressure (MPa)

Late

ral s

tress

(MP

a)

0 2 4 6 8 10 12 1410

20

30

40

50

60

Axial compaction (mstr)

Pre

ssur

e or

stre

ss (M

Pa)

Lateral stress Pore pressure

0 200 400 600 800 1000 120010

20

30

40

50

02468101214

Time (mins)

Por

e pr

essu

re (M

Pa)

Com

pact

ion

(mst

r)

Pore pressure Axial compaction

•Elastic compression•Onset of pore collapse•Inelastic compaction•Reservoir stress-path

Coupled TestsAll these mechanical measurements can be undertake in conjunction with:• Ultrasonic measurements• Poro-perm (axial and lateral)• Electrical properties• Rel-perm• Cap pressure• Temperature• Fluid interactions (swelling & core-flood)

allowing the investigation of:• Coupled properties and behaviour • Stress-path and effective stress effects

Coupled Ultrasonic/Mechanical(VP, VS1 & VS2, & also transverse transmission)

Triaxialcell

Pulser

Digitaloscilloscope

Switch

Switch

T’ducer

T’ducer

Sleeve

Pre- amp

PC

Printer

Typicallyfrequencyresponse

200kHz-1MHz

VP, VS & dynamic elastic moduli• at confinement• under deviatoric loading,• at varying degrees of saturation etc.

σ1 > σ2 > σ3

Primarily a research tool

True Triaxial Testing

Coupled rock mechanical/petrophysical measurement

Poro-perm Rel-perm Stress-path etc.Ultrasonic (VP, VS1 & VS2) AEStrength Deformation Electrical

Wellbore stability Radial flowCavity completions Fracture treatmentsThermal fractures PerforationsFluid interactions Plasticity corrections

Overburden

Confinement

Porefluid

Wellpressure

TWC Studies

Applications :• wellbore stability • fracture treatment design• cuttings reinjection

Directtensile

test

Brazil indirecttensile test

Cylinderrupture

test Chevron bend test(also short-rod &

notched Brazil tests)

Tensile Strength and Fracture Toughness

Fracture Properties• Stress/pore pressure effects on fracture conductivity• Shear and normal stiffnesses• Shear strength• Residual strength

Franklin shear box

HPHTtriaxial cellwith fluid-flow alonga fracture

CoringElastic recovery +

anelastic deformation

Gauged sample withstrain measuring elements at

0o , 90o , 45o & 135o +ve Z

+ve Y

+ve X1

10

2 3

9

4

5

611

7

8

12

DSA samplewith respect

to original core

Differential Strain Analyses (DSA)for in situ stress determination

In situ coreσA

σB

Confiningpressure

DSA Response

Strain

Crack closure &elastic compression

Elasticcompression

Analyses of the crack-closurestrains yield estimates of:• Principal stress ratios • Principal stress orientations• Relies on existence of stress-relief microcracks

• If ASR works, DSA will work• Useful adjunct to other techniques• Still requires mechanical measurements …. • Shear-wave splitting, acoustic attenuation etc.

Pressure

Round-Up• Experimental measurements are valuable

for all studies, i.e., wellbore to far-field, surface to TD

• Lab measurements provide data that can not otherwise be obtained (i.e., by logs)

• Coupled rock mechanics/petrophysicsstudies supplements + complements + enhances SCAL and log derived data

• Coupled geomechanics and dynamic reservoir simulation means such studies are increasingly needed and are becoming morecommon