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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
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2Warpinski (2009)
Hydraulic Fracturing
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3
Experimental setup•16 piezoelectric sensors
•Frequency response:50 KHz- 2 MHz
•Sampling rate: 5 MHz
Field setup
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4
Waveforms
sandstone: event # 4Upward first motion: Compressional waveform
Downward first motion: Dilatational waveformSPE-138441
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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
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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
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• 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
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Perforation
Sensor
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X
Y(mm)
(mm)
4000 psi
SPE-138441
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0
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m)
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xis
(mm
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8
Limestone- Spatial evolution of fractures
2“
1“
Fracture
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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
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• 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)
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m)
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(mm
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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
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12
Field scale mine-backs
A.
Warpinski et al., 1982
Warpinski and Teufel, 1987
1 cm
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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
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• 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
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Z (
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-25
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mm
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Y(mm)
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m)
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15
Syn-Shale- Stress applied parallel to bedding
Fractures are
consistently
through going in all
core
samples
2“2“
1“
Fracture
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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
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• 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
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Sensor
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TimeX
Y(mm)
(mm)
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18
Syn-Shale- Stress applied perpendicular to bedding
-60
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1“
Fracture
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19
Plan View Syn-Shale
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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
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Focal Mechanism
Studies
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FOCAL MECHANISMS
Lyons Sandstone
55%
33%
5%7%
Total events = 548
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Sandstone
time, sec
Cu
mu
lati
ve A
E e
ven
ts
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SEM
Observations
of
Hydraulic Fractures
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-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
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2“
Slice width = 2 mm
Fracture
slice #1slice #10
1“Fracture propagation
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Slice-6
Fracture
SEM along Fracture front propagation is toward viewer
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Shear fracture
SEM
SEM: Scanning Electron Microscope
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Shear fracture
3 mm
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5 mm
Processzone
bifurcated fracture
bifurcation
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8 mm
Process zone
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10 mm
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12 mm
pore
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14 mm
Conchoidal
fractures 0.5 “Sandstone
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Tensilefracture
17 mm
Conchoidal
fractures
Quartz
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slice-1
fracture
Fracture after propagating 1” from wellbore
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2 mm
bifurcation
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4 mm
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7 mm
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8 mm
very
thin fracture
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10 mm
Fracture
disappears
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12 mm
Large process
Zone
thin fracture
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13 mm
Process zone ?
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Slice
10
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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
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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