barnett shale fracture overview

2/27-28/2006Julia F. W. Gale, PBGSP Annual Meeting

Barnett Shale Barnett Shale Fracture OverviewFracture Overview

Julia F. W. Gale&

Robert M. ReedPermian Basin

Geological Synthesis Project

Fracture Research and Application Consortium

Barnett Shale Fracture OverviewBarnett 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


Extension (Mode I)

Shear (Mode II)

Shear (Mode III)

Fracture Classification

Twiss and Moores, 1997


Barnett Shale CoreBarnett Shale CoreFracture DescriptionFracture Description

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

Natural fracture approx 1ft high(length indeterminate, aperture >0.05 mm )Inset: Cement growth on fracture surface

Fracture Fracture surface with surface with mineral growth mineral growth (likely calcite)(likely calcite)

Natural fracture

Core piece Core piece missing – upper missing – upper termination termination indeterminateindeterminate

Lower Lower termination termination indeterminateindeterminate

Natural fracture at 6485’Natural fracture at 6485’

Natural fracture

Core breakup along Core breakup along shaley partings – shaley partings – stress release effectstress release effect

Core handling Core handling fractures at slabbed fractures at slabbed core edges (no core edges (no mineral growth on mineral growth on surface)surface)

Other features distinct Other features distinct from natural fracturesfrom natural fractures

Fragments of thin Fragments of thin walled brachiopods – walled brachiopods – parallel to beddingparallel to bedding

En Echelon FracturesFracture tips are Fracture tips are mostly straight.mostly straight.

En echelon fractures En echelon fractures do not indicate shear do not indicate shear in this case. in this case.

They arise when They arise when stress intensity at a stress intensity at a flaw rises above the flaw rises above the level required for level required for failure (probably failure (probably subcritical growth) subcritical growth) as other fractures as other fractures propagate with propagate with elevated stress elevated stress intensity at their tip.intensity at their tip.

Note both right and Note both right and left stepping left stepping examples.examples.

En Echelon Fractures

A few fracture A few fracture tips curve tips curve towards each towards each other. other.

Might indicate Might indicate moderate local moderate local stress stress anisotropy.anisotropy.

More common More common in horizontal in horizontal plane if Splane if SHmaxHmax and Sand Shminhmin are are close in close in magnitude. magnitude.


Fractures in carbonate concretions Fractures in carbonate concretions Local to concretions onlyLocal to concretions only

5 cm

Multiple phases Multiple phases seal fracturesseal fractures

Aperture vs. height

Aperture vs height

1

10

100

1000

0.01 0.1 1 10

Kinematic aperture (mm)

Hei

ght (

mm

)

T.P. Sims unconstrainedT.P. Sims constrained

Blakely unconstrainedBlakely constrained


Natural Fracture Observations in CoresNatural Fracture Observations in Cores• Many narrow sealed opening-mode fracturesMany narrow sealed opening-mode fractures

– >24 in 103 ft >24 in 103 ft (Blakely #1)(Blakely #1)– 20 in 14 ft 20 in 14 ft (T.P. Sims #2)(T.P. Sims #2)

• Several groups of en-echelon fracturesSeveral groups of en-echelon fractures

• All fractures are sealedAll fractures are sealed– Widest 1.15 mm; narrowest < 0.05 mm; Tallest 68 cm Widest 1.15 mm; narrowest < 0.05 mm; Tallest 68 cm

• Concretions commonly fracturedConcretions commonly fractured– Fractures localFractures local

• Pale, dolomite-rich layers Pale, dolomite-rich layers – Fracture intensity not greater than other lithologies Fracture intensity not greater than other lithologies

(exception is Forestburg)(exception is Forestburg)– Number of sets may be higherNumber of sets may be higher

• Vertical fracture terminations: gradual taper or abrupt at Vertical fracture terminations: gradual taper or abrupt at bedding planes with greater mud contentbedding planes with greater mud content


Rose diagrams of natural fracture orientations T.P. Sims #2

R. E. Hill 1992 GRI topical report

Core fractures FMS fractures

Comparison of fractures in Comparison of fractures in Barnett Shale & Austin Chalk Barnett Shale & Austin Chalk (fine-grained mudrock with carbonate

layers & chalk with marl layers)

Sealed fractures

Austin Chalk

Barnett Shale

Large open fracturesLarge open fracturesAustin Chalk outcropAustin Chalk outcrop

Narrow sealed Narrow sealed fracturesfractures 0.01

0.1

1

10

100

0 50 100 150 200 250Position along scanline (m)

Subcritical index & network geometryGeomechanical modeling by Jon Olson (FRAC)

- 1 0

- 8

- 6

- 4

- 2

0

2

4

6

8

1 0

- 8 - 6 - 4 - 2 0 2 4 6 8

n = 5

- 1 0

- 8

- 6

- 4

- 2

0

2

4

6

8

1 0

- 8 - 6 - 4 - 2 0 2 4 6 8

n = 2 0

- 1 0

- 8

- 6

- 4

- 2

0

2

4

6

8

1 0

- 8 - 6 - 4 - 2 0 2 4 6 8

n = 8 0

n=5 n=20 n=80

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

Subcritical crack index measurementsSubcritical 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 Tests by Jon Holder (FRAC)

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

SEM Imaging of Fractures at 7,749 ft SEM Imaging of Fractures at 7,749 ft T.P. Sims #2 core T.P. Sims #2 core

Imaged with Secondary Electrons, Backscattered Electrons, and Imaged with Secondary Electrons, Backscattered Electrons, and Cathodoluminescence, with EDS mappingCathodoluminescence, with EDS mapping

(281)

Backscattered electron Backscattered electron image of 281°-trending image of 281°-trending fracturefracture

• Two samples from 7,749 ft Two samples from 7,749 ft • shaleshale• dolomitic layer below shaledolomitic layer below shale

• Both samples have multiple fracture setsBoth samples have multiple fracture sets• Core oriented based on FMI logCore oriented based on FMI log

Six Phases of Mineral Fill in Fracture Trending Six Phases of Mineral Fill in Fracture Trending 281º281º

Pyrite

Calcite

Dolomite

Barite

Albite

Quartz

Albite and quartz are not distinguishable in BSE

After R. M. Reed, 2004

Backscattered electron image (BSE) shows differences in atomic number, brighter indicates higher number

False-color EDS False-color EDS element mapelement map

Red = Si; Green = S; Blue = CaRed = Si; Green = S; Blue = Ca

This combination of elements best shows the 6 different phases.This combination of elements best shows the 6 different phases.

horizontal thin section

Cold-cathode CL image mosaic

calcite +dolomite

calcite

pyrite

calcite +dolomite

(190)

(184)

(262)

(280)

Fractures in Dolomitic LayerFractures in Dolomitic Layer

~ same orientationas 6-phase fracturein shale

~N

NS-trending fracture in dolomitic layer (UV-blue CL)

Crack-Seal TextureCrack-Seal Texture

Fracture Trends, Mudstone Sample Fracture Trends, Dolomitic Sample

6 phases of fill

N=13Circle = 23%

N=11Circle = 27%

Calcite fill

Calcite+dolomite+pyrite

Fracture Orientation Rose DiagramFracture Orientation Rose Diagram Sample from 7,749 ft, T. P. Sims #2 CoreSample from 7,749 ft, T. P. Sims #2 Core

Open induced fractures or reactivation along natural fractures

Youngest

Calcite+dolomite fill Crack-seal

Youngest

Oldest


Fracture porosity, connectivity and Fracture porosity, connectivity and storage capacitystorage 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


Faults in coreFaults in core

• Dip-slip faults in core with breccia and Dip-slip faults in core with breccia and slickensidesslickensides

• One fault trending 109°/55° SSW One fault trending 109°/55° SSW identified in the T.P. Sims #2 identified in the T.P. Sims #2

(Hill, 1992)(Hill, 1992)


Fault at 6,623 ftFault at 6,623 ft

Fault zone with brecciaFault zone with breccia

Slickenlines Slickenlines indicating indicating

dip slipdip slip

Calcite fill along Calcite fill along faultfault

Fault at 6,648 ftFault at 6,648 ft

Fault plane

slickenfibresslickenfibres

Slab face (above) and Slab face (above) and fault plane (left) of 45fault plane (left) of 45°° shallow, dip slip faultshallow, dip slip fault


Hydraulic Fracture TreatmentsHydraulic Fracture Treatments

• Provide permeability linked to the wellboreProvide permeability linked to the wellbore• Hydraulic fractures will initially propagate parallel Hydraulic fractures will initially propagate parallel

to Sto SHmaxHmax

• Waterfracs pumped at high rates (rather than gel)Waterfracs pumped at high rates (rather than gel)• Monitored by microseismicMonitored by microseismic

– Vertical wells (seismic receivers in offset well)Vertical wells (seismic receivers in offset well)– Horizontal wells (need to monitor full extent of Horizontal wells (need to monitor full extent of

well and fractures) well and fractures) • Fracture height control – underlying EllenburgerFracture height control – underlying Ellenburger

Hydraulic Fracture TreatmentsHydraulic Fracture TreatmentsMonitoring fracture growth using microseismic Monitoring fracture growth using microseismic

detectors (Warpinski et al. 2005)detectors (Warpinski et al. 2005)1.1. Waterfracs propagate parallel to SWaterfracs propagate parallel to SHmaxHmax (NE) (NE) 2.2. Reopen natural sealed NW fracs to link the Reopen natural sealed NW fracs to link the

system giving a 3D networksystem giving a 3D network– Fractures pop open because the fill does not template Fractures pop open because the fill does not template

onto grains in the wall rock onto grains in the wall rock 3.3. Connect to and reopen NE trending natural Connect to and reopen NE trending natural

fracturesfractures

3

3

3

En echelon natural fractures

1

Hydraulic fractures

2

2

Hydraulic Fracture TreatmentsHydraulic Fracture Treatments• In some tight naturally fractured reservoirs connecting with In some tight naturally fractured reservoirs connecting with

the cross-trending fractures is seen as a problem the cross-trending fractures is seen as a problem – Premature screen out Premature screen out – Natural fracture damageNatural fracture damage

• In Barnett Shale these problems are avoided byIn Barnett Shale these problems are avoided by– Water rather than gelWater rather than gel– Low proppant loadings Low proppant loadings

En echelon natural fractures

1

Hydraulic fractures


In situ stressIn situ stress

FWB

C/R

Present day in situ stress Present day in situ stress controls hydraulic fracture controls hydraulic fracture orientationorientation

Fort Worth BasinFort Worth Basin - in Mid-Plate Compression province- in Mid-Plate Compression province

West Texas,West Texas, Culberson Co. Culberson Co. and Reeves Co.and Reeves Co. - along boundary between Cordilleran - along boundary between Cordilleran Extension and Southern GreatExtension and Southern Great Plains (SGP) provincesPlains (SGP) provinces - need to carefully establish S- need to carefully establish SHmaxHmax

Map modified from Zoback and Zoback (1989) Map modified from Zoback and Zoback (1989) and Laubach et al. 2004and Laubach et al. 2004


ConclusionsConclusionsFort Worth BasinFort Worth Basin• Many small sealed natural fractures trending NWMany small sealed natural fractures trending NW

– Possible larger open fracturesPossible larger open fractures– Less common sets NS and NELess common sets NS and NE– Intrinsic storage capacity lowIntrinsic storage capacity low– Reactivate during hydraulic fracturingReactivate during hydraulic fracturing

West TexasWest Texas• Stress province differentStress province different

– Uncertain in-situ stress – need to measure Uncertain in-situ stress – need to measure (FMI breakouts at least)(FMI breakouts at least)

• Unknown natural fracture orientation – Unknown natural fracture orientation – evaluation requiredevaluation required

Kinematic aperture, b (mm)

F = 0.1052b–0.5575

R2 = 0.979

F = 0.3364b–0.6786

R2 = 0.9846

0.001

0.01

0.1

1

10

100

0.01 0.1 1 10 100

Grove CreekKinlawPower (Grove Creek)Power (Kinlaw)

Aperture size distributionAperture size distribution

Threshold frequency predictionThreshold frequency prediction

0.001

0.01

0.1

1

10

0.01 0.1 1 10 100 1000Kinematic aperture, b (mm)

Emergent threshold Kinlaw 0.14 mm

Grove Creek

Kinlaw core

Threshold frequency1.277/m

Threshold frequency0.026/m

Emergent thresholdGrove Creek 11 mm

barnett shale fracture overview

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