Julia F. W. Gale,

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Barnett Shale Fracture Overview

• Julia F. W. Gale
• Robert M. Reed

Permian Basin Geological Synthesis
Project Fracture Research and Application
Consortium
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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

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Fracture Classification
Twiss and Moores, 1997
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Barnett Shale CoreFracture Description

• Texas United Blakely 1
• Mitchell Energy Thomas P. Sims 2

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Natural fracture at 6485
Natural fracture approx 1ft high (length
indeterminate, aperture gt0.05 mm ) Inset Cement
growth on fracture surface
Core piece missing upper termination
indeterminate
Natural fracture
Fracture surface with mineral growth (likely
calcite)
Lower termination indeterminate
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Other features distinct from natural fractures
Natural fracture
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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.
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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.
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Fractures in carbonate concretions Local to
concretions only
Multiple phases seal fractures
5 cm
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Aperture vs. height
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Natural Fracture Observations in Cores

• Many narrow sealed opening-mode fractures
• gt24 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 lt 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

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Rose diagrams of natural fracture orientations
T.P. Sims 2R. E. Hill 1992 GRI topical report
Core fractures
FMS fractures
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Comparison of fractures in Barnett Shale Austin
Chalk (fine-grained mudrock with carbonate
layers chalk with marl layers)
Barnett Shale
Narrow sealed fractures
Austin Chalk
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Subcritical index network geometryGeomechanical
modeling by Jon Olson (FRAC)

• low n, spacing lt bed thickness, early subcritical
  growth
• high n, widely spaced clusters, late critical
  growth

n5 n20
n80
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Subcritical crack index measurementsT.P. Sims 2
core samples
Depth Specimen n Lithology
6,432' KB32-3a2 218 1 black shale
KB32-3a3 172
6,578' KB78-6a1 131 1 black shale
KB78-2a1 172
6,476' KB76-6a2 325 2 calcite rich (ls)
KB76-4a1 206
6,487' KB87-5a1 290 2 calcite-rich (ls)
KB87-8a2 249
6,617' KB32-2a1 109 3 silt rich black shale
KB17-7a1 153
6,635' KB35-6a1 309 4 coarse-grain (swaley)
KB35-5a1 339
6,757' KB57-8a1 335 5 concretion
KB57-4b1 240
6,728' KB28-3a1 378 5 concretion
KB28-5a1 263

• Fractures probably clustered
• High subcritical crack index
• En echelon arrays

Tests by Jon Holder (FRAC)
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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

Backscattered electron image of 281-trending
fracture
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Six Phases of Mineral Fill in Fracture Trending
281º
Backscattered electron image (BSE) shows
differences in atomic number, brighter indicates
higher number
Albite and quartz are not distinguishable in BSE
False-color EDS element map Red Si Green
S Blue Ca This combination of elements best
shows the 6 different phases.
After R. M. Reed, 2004
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Timing and Relationships Fracture in Shale

• Six phases of mineral fill in 281-trending
  fracture in shale
• Quartz albite at fracture tips calcite pyrite
  elsewhere
• Quartz albite are early, predating calcite
• Barite is late
• Dolomite probably late may be replacing calcite
  (in the dolomite layer, dolomite precedes calcite
  in fractures)
• Calcite partly predates pyrite, pyrite may be
  replacing calcite
• Sulfide and sulfate minerals both present
• indicates a change in chemical environment.

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Fractures in Dolomitic Layer
N
same orientation as 6-phase fracture in shale
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Crack-Seal Texture
NS-trending fracture in dolomitic layer (UV-blue
CL)
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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
Calcite fill
Youngest
Calcitedolomite pyrite
Open induced fractures or reactivation along
natural fractures
N11
N13
Circle 27
Circle 23
Fracture Trends, Dolomitic Sample
Fracture Trends, Mudstone Sample
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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

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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)

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Fault at 6,623 ft
Slickenlines indicating dip slip
Fault zone with breccia
Calcite fill along fault
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Fault at 6,648 ft
Fault plane
slickenfibres
Slab face (above) and fault plane (left) of 45
shallow, dip slip fault
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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

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Hydraulic Fracture Treatments

• Monitoring fracture growth using microseismic
  detectors (Warpinski et al. 2005)
• Waterfracs propagate parallel to SHmax (NE)
• 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
• Connect to and reopen NE trending natural
  fractures

En echelon natural fractures
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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
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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. - along boundary
between Cordilleran Extension and Southern
Great Plains (SGP) provinces - need to
carefully establish SHmax Map modified from
Zoback and Zoback (1989) and Laubach et al. 2004
FWB
C/R
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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

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Aperture size distribution
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Threshold frequency prediction