Hydraulic Fractures, Acoustic Emissions and Shearing

University of OklahomaMewbourne School of Petroleum and Geological Engineering

Norman, OK, USA

C. Moreno, Y. Chitrala, C. Sondergeld and C. Rai

Norman, July 21, 2011

2Warpinski (2009)

Hydraulic Fracturing

3

Experimental setup•16 piezoelectric sensors

•Frequency response:50 KHz- 2 MHz

•Sampling rate: 5 MHz

Field setup

4

Waveforms

sandstone: event # 4Upward first motion: Compressional waveform

Downward first motion: Dilatational waveformSPE-138441

5

AE Calibration

Hsu and Breckenridge (1981)

3700

4000

4300

4600

4900

0 60 120 180 240 300 360

Vp

, m/s

Angle (degrees)

Limestone

3700

4000

4300

4600

4900

0 60 120 180 240 300 360

Vp, m

/s

Angle (degrees)

Sandstone

3700

4000

4300

4600

4900

0 60 120 180 240 300 360

Vp, m

/s

Angle (degrees)

Pyrophyllite

P-wave CVA

SampleMineralogy,

wgt% Model VP (m/sec) |error | (mm)

Indiana Limestone

Calcite, 95Constant velocity

model3900 ±3.00

Lyons Sandstone

Quartz, 85Constant velocity

model4335 ±3.22

Syn-ShaleAnisotropic model (Berryman 2008)

4300 ±6.00

Anisotropic parameters:ε=0.25, γ= 0.74, δ=0.40

Indiana Limestone: k= 5 md; 3-samps

• Maximum activity during pressure build-up

• 38% of the recorded events were locatable 6

Pump rate=15 cc/min

Pbdwn = 1529 psi

Total 244 events

• Fracture aligned with stress direction

• Spatial and temporal propagation of fracture observed (color coding)

• Well developed nearly planar fracture

7

Indiana Limestone: isotropic velocity model

Perpendicular Parallel

0

30

60

90

120

-50 -30 -10 10 30 50

Z (

mm

)

X (mm)

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Z (

mm

)

Y (mm)

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Z (

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)

X (mm)

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Z (

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)

Y (mm)

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Z (

mm

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X (mm)

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30

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90

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-50 -30 -10 10 30 50

Z (

mm

)

Y (mm)

Perforation

Sensor

Early

Intermediate

Late

Time

X

Y(mm)

(mm)

4000 psi

SPE-138441

0

20

40

60

80

100

120

-50 -30 -10 10 30 50

Z (m

m)

X (mm) -60

-50

-40

-30

-20

-10

0

10

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-60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60

Y-A

xis

(mm

)

X- Axis (mm)

8

Limestone- Spatial evolution of fractures

2“

1“

Fracture

Lyons Sandstone: k=20 md; ; 8-samps

Pumping rate=10 cc/min

Pbdwn= 4038 psi

Total events =2700

9

• Events occur during pressure build-up and at pump shut-off

• 28% of the recorded events were locatable

• Fracture aligned with applied stress

• More diffuse distribution of events compared to limestone

• 765 events located

10

Lyons Sandstone: isotropic velocity model

Perpendicular Parallel

Perforation

Sensor

Early

Intermediate

Late

Time

4000 psi

X

Y(mm)

(mm)

0

20

40

60

80

100

120

-50 -30 -10 10 30 50

Z (m

m)

X (mm) -60

-50

-40

-30

-20

-10

0

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40

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60

-60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60

Y-A

xis

(mm

)

X- Axis (mm)

11

Lyons Sandstone- Spatial evolution of fractures

Fracture highlighted with

black lines

Fracture through going in

the center, origin and tapers

upward away from injection

consistent with hypocenters

in previous slide.

1“

OILOIL

OILOIL

OILOIL

Fracture

12

Field scale mine-backs

A.

Warpinski et al., 1982

Warpinski and Teufel, 1987

1 cm

Syn-Shale: k=5 nd ; 4-samps

Stress applied parallel to beddingPumping rate=5 cc/min

Pbdwn= 1700 psi

Total events= 102

13

• 42% of the recorded events were locatable

• Most of the AE distributed during initial stage

• Fracture subparallel to stress applied

• 47 events were located

• 42% of events recorded were locatable

• Limited development of fracture plane

• Fairly narrowly confined to a plane

14

Syn-Shale- Stress applied parallel to bedding

4000 psi

ParallelPerpendicular

0

25

50

75

100

-50 -30 -10 10 30 50

Z (

mm

)

X (mm)

0

25

50

75

100-50 -30 -10 10 30 50

Z (

mm

)

Y (mm)

Perforation

Sensor

Early

Intermediate

Late

Time-50

-25

0

25

50

-50 -25 0 25 50Y (

mm

)

X (mm)

X

Y(mm)

(mm)

-60

-50

-40

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-20

-10

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xis

(mm

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X- Axis (mm)

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m)

X (mm)

15

Syn-Shale- Stress applied parallel to bedding

Fractures are

consistently

through going in all

core

samples

2“2“

1“

Fracture

Syn-Shale- Stress applied perpendicular to bedding

Pumping rate=5 cc/min

Pbdwn =2300 psi

Total events= 302

16

• Majority of events before breakdown

• 22% of the recorded events were locatable

4-samps

• A local cluster developed around the borehole

• Deviation of the fracture direction from maximum stress orientation is 25 , consistent with calculations

• 66 events were located

17

Syn-Shale- Stress applied perpendicular to bedding

PerpendicularParallel

4000 psi

0

25

50

75

100

-50 -30 -10 10 30 50

Z (

mm

)

X (mm)

0

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Z (

mm

)

Y (mm)

Perforation

Sensor

Early

Intermediate

Late

TimeX

Y(mm)

(mm)

0

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60

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-50 -30 -10 10 30 50

Z (m

m)

Y(mm)

18

Syn-Shale- Stress applied perpendicular to bedding

-60

-50

-40

-30

-20

-10

0

10

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Y-A

xis

(mm

)

X- Axis (mm)

1“

Fracture

19

Plan View Syn-Shale

Stress distribution-Anisotropic reservoirs• Hoop stress

symmetry different from isotropic materials

• Deviation depends on anisotropy and far field stress

• Fracture direction deviates from the maximum stress direction by 20

20

Aadnoy, 1987

Focal Mechanism

Studies

FOCAL MECHANISMS

Lyons Sandstone

55%

33%

5%7%

Total events = 548

Sandstone

time, sec

Cu

mu

lati

ve A

E e

ven

ts

SEM

Observations

of

Hydraulic Fractures

-50

-25

0

25

50

-50 -25 0 25 50Y (

mm

)

X (mm)

Lyons SandstonePerpendicular

4000 psi

0

25

50

75

100

-50 -30 -10 10 30 50

Z (

mm

)X (mm)

Perforation

Sensor

Early

Intermediate

Late

Time

2“

Slice width = 2 mm

Fracture

slice #1slice #10

1“Fracture propagation

Slice-6

Fracture

SEM along Fracture front propagation is toward viewer

Shear fracture

SEM

SEM: Scanning Electron Microscope

Shear fracture

3 mm

5 mm

Processzone

bifurcated fracture

bifurcation

8 mm

Process zone

10 mm

12 mm

pore

14 mm

Conchoidal

fractures 0.5 “Sandstone

Tensilefracture

17 mm

Conchoidal

fractures

Quartz

slice-1

fracture

Fracture after propagating 1” from wellbore

2 mm

bifurcation

4 mm

7 mm

8 mm

very

thin fracture

10 mm

Fracture

disappears

12 mm

Large process

Zone

thin fracture

13 mm

Process zone ?

Slice

10

Hydraulic fracturing• Physical location of fractures agrees with hypocenter locations

• Isotropic materials fractures controlled by applied stress direction

• Anisotropic materials, initiation direction controlled by directionof applied stress and anisotropy

• Shear failure dominates the hydraulic fracture propagation

• MS events correlate with pressure buildup, secondary activity withpump stoppage for brittle lithologies

• Fractures are discontinuous and bifurcate like natural systems

• Fractures are not planar surfaces…controls proppant placement

• Process zone is discontinuous

47

48

Triaxial Hydraulic Fracture Tests

We thank Devon Energy for their continued support

Mr. Gary Stowe for his laboratory expertise

Apache and Mewbourne Oil donated the SEM used in

these studies