Barnett Shale
Fracture Overview
Julia F. W. Gale
&
Robert M. Reed
Permian Basin
Geological Synthesis Project
Fracture Research and Application Consortium
Julia F. W. Gale,
PBGSP Annual Meeting
2/27-28/2006
Barnett Shale Fracture Overview
• Natural opening-mode fractures
– Core observations
• Distinguishing natural from induced fractures
• Orientation, intensity, openness, height, aperture, connectivity
– Fracture clustering
• Geomechanical modeling and subcritical crack index measurements
– Fracture porosity and storage capacity
• Microfractures and fracture attribute scaling
• Faults
• Hydraulic fracture treatments
– Microseismic observation of propagating fractures
– Interaction with natural fractures
• In situ stress
• Conclusions
Fracture Classification
Extension
(Mode I)
Twiss and Moores, 1997
Shear
(Mode II)
Julia F. W. Gale,
PBGSP Annual Meeting
Shear
(Mode III)
2/27-28/2006
Barnett Shale Core
Fracture Description
Texas United Blakely #1
Mitchell Energy Thomas P. Sims #2
Julia F. W. Gale,
PBGSP Annual Meeting
2/27-28/2006
Natural fracture at 6485’
Core piece
missing – upper
termination
indeterminate
Natural fracture approx 1ft high
(length indeterminate, aperture >0.05 mm )
Inset: Cement growth on fracture surface
Natural
fracture
Fracture
surface with
mineral growth
(likely calcite)
Lower
termination
indeterminate
Other features distinct
from natural fractures
Core breakup along
shaley partings –
stress release effect
Core handling
fractures at slabbed
core edges (no
mineral growth on
surface)
Fragments of thin
walled brachiopods –
parallel to bedding
Natural
fracture
En Echelon Fractures
Fracture tips are
mostly straight.
En echelon fractures
do not indicate shear
in this case.
They arise when
stress intensity at a
flaw rises above the
level required for
failure (probably
subcritical growth)
as other fractures
propagate with
elevated stress
intensity at their tip.
Note both right and
left stepping
examples.
En Echelon Fractures
A few fracture
tips curve
towards each
other.
Might indicate
moderate local
stress
anisotropy.
More common
in horizontal
plane if SHmax
and Shmin are
close in
magnitude.
Fractures in carbonate concretions
Local to concretions only
Multiple phases
seal fractures
5 cm
Julia F. W. Gale,
PBGSP Annual Meeting
2/27-28/2006
Aperture vs height
Aperture
vs. height
1000
Height (mm)
100
10
T.P. Sims unconstrained
T.P. Sims constrained
Blakely unconstrained
Blakely constrained
1
0.01
0.1
Kinematic aperture (mm)
1
10
Natural Fracture Observations in Cores
• Many narrow sealed opening-mode fractures
– >24 in 103 ft (Blakely #1)
– 20 in 14 ft (T.P. Sims #2)
• Several groups of en-echelon fractures
• All fractures are sealed
– Widest 1.15 mm; narrowest < 0.05 mm; Tallest 68 cm
• Concretions commonly fractured
– Fractures local
• Pale, dolomite-rich layers
– Fracture intensity not greater than other lithologies
(exception is Forestburg)
– Number of sets may be higher
• Vertical fracture terminations: gradual taper or abrupt at
bedding planes with greater mud content
Julia F. W. Gale,
PBGSP Annual Meeting
2/27-28/2006
Rose diagrams of natural fracture orientations
T.P. Sims #2
R. E. Hill 1992 GRI topical report
Core fractures
FMS fractures
Julia F. W. Gale,
PBGSP Annual Meeting
2/27-28/2006
Comparison of fractures in
Barnett Shale & Austin Chalk
(fine-grained mudrock with carbonate
layers & chalk with marl layers)
Sealed
fractures
Large open fractures
Austin Chalk outcrop
100
10
1
Barnett Shale
Narrow sealed
fractures
Austin Chalk
0.1
0.01
0
50
100
150
200
Position along scanline (m)
250
Subcritical index & network geometry
Geomechanical modeling by Jon Olson (FRAC)
•low n, spacing < bed thickness, early subcritical growth
•high n, widely spaced clusters, late critical growth
10
10
10
n=5
n=80
n=20
8
8
8
6
6
6
4
4
4
2
2
2
0
0
0
-2
-2
-2
-4
-4
-4
-6
-6
-6
-8
-8
-8
-10
-10
-8
-6
-4
-2
0
n=5
2
4
6
8
-8
-6
-4
-2
0
n=20
2
4
6
-10
8 -8
-6
-4
-2
0
n=80
2
4
6
8
Subcritical crack index measurements
T.P. Sims #2 core samples
Depth
Specimen
n
6,432'
KB32-3a2
218
KB32-3a3
172
KB78-6a1
131
KB78-2a1
172
KB76-6a2
325
KB76-4a1
206
KB87-5a1
290
KB87-8a2
249
KB32-2a1
109
KB17-7a1
153
KB35-6a1
309
KB35-5a1
339
KB57-8a1
335
KB57-4b1
240
KB28-3a1
378
KB28-5a1
263
6,578'
6,476'
6,487'
6,617'
6,635'
6,757'
6,728'
Lithology
#1: black shale
#1: black shale
Fractures probably clustered
• High subcritical crack index
• En echelon arrays
#2: calcite rich (ls)
#2: calcite-rich (ls)
#3: silt rich black shale
#4: coarse-grain (swaley)
#5: concretion
#5: concretion
Tests by Jon Holder (FRAC)
SEM Imaging of Fractures at 7,749 ft
T.P. Sims #2 core
Imaged with Secondary Electrons, Backscattered Electrons, and
Cathodoluminescence, with EDS mapping
• Two
samples from 7,749 ft
• shale
• dolomitic layer below shale
• Both samples have multiple fracture sets
• Core oriented based on FMI log
(281)
Backscattered electron
image of 281°-trending
fracture
Six Phases of Mineral Fill in Fracture Trending
281º
Backscattered electron
image (BSE) shows
differences in atomic
number, brighter
indicates higher
number
Pyrite
Calcite
Dolomite
Albite and quartz are
not distinguishable in
BSE
Barite
Albite
False-color EDS
element map
Quartz
Red = Si; Green = S; Blue = Ca
This combination of elements best shows the 6 different phases.
After R. M. Reed, 2004
Fractures in Dolomitic Layer
(184)
calcite +
dolomite
pyrite
(190)
calcite
calcite +
dolomite
~N
horizontal thin section
Cold-cathode CL
image mosaic
(262)
~ same orientation
as 6-phase fracture
in shale
(280)
Crack-Seal Texture
NS-trending fracture in dolomitic layer (UV-blue CL)
Fracture Orientation Rose Diagram
Sample from 7,749 ft, T. P. Sims #2 Core
Oldest
Calcite+
dolomite fill
Crack-seal
Youngest
6 phases of fill
Youngest
Calcite fill
Calcite+dolomite
+pyrite
Open induced
fractures or reactivation
along natural fractures
N=13
Circle = 23%
Fracture Trends, Mudstone Sample
N=11
Circle = 27%
Fracture Trends, Dolomitic Sample
Fracture porosity, connectivity and
storage capacity
• Narrow natural fractures are sealed
– Fracture system porosity low
– If larger fractures open, permeability could be
high
• At least two fracture sets
– Improves connectivity
• Storage capacity low
Julia F. W. Gale,
PBGSP Annual Meeting
2/27-28/2006
Faults in core
•
Dip-slip faults in core with breccia and
slickensides
• One fault trending 109°/55° SSW
identified in the T.P. Sims #2
(Hill, 1992)
Julia F. W. Gale,
PBGSP Annual Meeting
2/27-28/2006
Fault at 6,623 ft
Slickenlines
indicating
dip slip
Fault zone with breccia
Calcite fill along
fault
Julia F. W. Gale,
PBGSP Annual Meeting
2/27-28/2006
Fault at 6,648 ft
Fault plane
slickenfibres
Slab face (above) and
fault plane (left) of 45°
shallow, dip slip fault
Hydraulic Fracture Treatments
• Provide permeability linked to the wellbore
• Hydraulic fractures will initially propagate parallel
to SHmax
• Waterfracs pumped at high rates (rather than gel)
• Monitored by microseismic
– Vertical wells (seismic receivers in offset well)
– Horizontal wells (need to monitor full extent of
well and fractures)
• Fracture height control – underlying Ellenburger
Julia F. W. Gale,
PBGSP Annual Meeting
2/27-28/2006
Hydraulic Fracture Treatments
Monitoring fracture growth using microseismic
detectors (Warpinski et al. 2005)
1. Waterfracs propagate parallel to SHmax (NE)
2. Reopen natural sealed NW fracs to link the
system giving a 3D network
–
Fractures pop open because the fill does not template
onto grains in the wall rock
3. Connect to and reopen NE trending natural
fractures
En echelon
natural
3
fractures
2
2
1
3
3
Hydraulic fractures
Hydraulic Fracture Treatments
•
•
In some tight naturally fractured reservoirs connecting with
the cross-trending fractures is seen as a problem
– Premature screen out
– Natural fracture damage
In Barnett Shale these problems are avoided by
– Water rather than gel
– Low proppant loadings
En echelon
natural
fractures
1
Hydraulic fractures
In situ stress
Present day in situ stress
controls hydraulic fracture
orientation
Fort Worth Basin
- in Mid-Plate Compression province
West Texas, Culberson Co.
and Reeves Co.
FWB
- along boundary between Cordilleran
Extension and Southern Great
Plains (SGP) provinces
- need to carefully establish SHmax
C/R
Map modified from Zoback and Zoback (1989)
and Laubach et al. 2004
Julia F. W. Gale,
PBGSP Annual Meeting
2/27-28/2006
Conclusions
Fort Worth Basin
• Many small sealed natural fractures trending NW
– Possible larger open fractures
– Less common sets NS and NE
– Intrinsic storage capacity low
– Reactivate during hydraulic fracturing
West Texas
• Stress province different
– Uncertain in-situ stress – need to measure
(FMI breakouts at least)
• Unknown natural fracture orientation –
evaluation required
Julia F. W. Gale,
PBGSP Annual Meeting
2/27-28/2006
Aperture size distribution
100
10
Grove Creek
Kinlaw
Power (Grove Creek)
Power (Kinlaw)
F = 0.3364b–0.6786
R2 = 0.9846
1
0.1
F = 0.1052b–0.5575
R2 = 0.979
0.01
0.001
0.01
0.1
1
10
Kinematic aperture, b (mm)
100
Threshold frequency prediction
10
Kinlaw core
Threshold frequency
1.277/m
1
0.1
Threshold frequency
0.026/m
Grove Creek
0.01
Emergent threshold
Kinlaw 0.14 mm
0.001
0.01
0.1
Emergent threshold
Grove Creek 11 mm
1
10
Kinematic aperture, b (mm)
100
1000