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Natural Fractures in Shale Hydrocarbon Reservoirs*
Julia F.W. Gale1
Search and Discovery Article #41487 (2014)**
Posted November 17, 2014
*Adapted from 2013-2014 AAPG Foundation Distinguished Lecture. Please refer to related article by the author, Search and Discovery Article #41486 (2014).
**Datapages © 2014 Serial rights given by author. For all other rights contact author directly.
1
Bureau of Economic Geology, University of Texas at Austin ([email protected])
Abstract
Using examples from shale reservoirs worldwide, I demonstrate the diversity of shale-hosted fracture systems and present evidence for how
and why various fractures systems form. Core and outcrop observations, strength tests on shale and on fractures in core, and geomechanical
models allow prediction of fracture patterns and attributes that can be taken into account in well placement and hydraulic fracture treatment
design. Both open and sealed fractures can interact with and modify hydraulic fracture size and shape. Open fractures can enhance reservoir
permeability but may conduct treatment fluids great distances, in some instances possibly aseismically. We have addressed the challenge of
incomplete sampling of subsurface fractures through comprehensive fracture data collection in cores and image logs and careful selection of
outcrops, coupled with an understanding of how fractures and their attributes scale. We also use tested mechanistic models of how fractures
grow in tight sandstones and carbonates to interpret fractures in shale. In order to predict fracture patterns and attributes, it is helpful to
understand their mechanism of formation and timing in the context of the burial and tectonic histories of the basin in which they are forming. A
key variable is the depth of burial, and thereby the temperature, pore-fluid pressure and effective stress at the time of fracture development. For
the most part, the origin of fractures cannot be determined from their orientation or commonly-measured attributes, such as width, height and
length. The mineral fill in sealed fractures does provide an opportunity, however, and we use fluid-inclusion studies of fracture cements tied to
burial history to unravel their origin. Interaction with hydraulic fracture treatments may serve to increase the effectiveness of the hydraulic
fracture network, or could work against it. Factors governing the interaction include natural fracture intensity, orientation with respect to
reservoir stress directions, and the strength of the fracture plane relative to intact host rock. We tested the effect of calcite-sealed fractures in
Barnett Shale on tensile strength of shale with a bending test. Samples containing natural fractures have half the tensile strength of those
without and always break along the natural fracture plane. Yet in other examples the weakness is in the cement itself, partly because of retained
fracture porosity. Natural fractures in shales likely grew by slow, chemically assisted (subcritical) propagation, and we use a subcritical
propagation criterion to model the growing fractures. The subcritical crack index is a mechanical rock property that controls fracture spacing
and an input parameter for the models. We measured the subcritical crack index for several shales. The index is generally high for Barnett
Shale, in excess of 100, although it does show variability with facies. By contrast, subcritical indices in the New Albany Shale are much lower,
and also show considerable variability. Barnett Shale subcritical indices suggest high clustering, whereas New Albany Shale subcritical indices
suggest fractures are likely to be more evenly spaced, with spacing related to mechanical layer thickness. We are investigating the variability in
subcritical index in shale and how it might tie to other rock properties.
References Cited
Dewhurst, D.N., Y. Yang and A.C. Aplin, 1999, in Muds and Mudstones: Physical and Fluid Flow Properties, in A.C. Aplin, A.J. Fleet, and
J.H.S. MacQuaker, eds., Muds and Mudstones: Physical and Fluid-Flow Properties: Geological Society (London) Special Publication 158/1, p.
23–43.
Fidler, L., 2011, Natural fracture characterization of the New Albany Shale, Illinois basin, United States: M.S. thesis University of Texas at
Austin.
Gale, J.F.W., R.M. Reed, and J. Holder, 2007, Natural fractures in the Barnett Shale and their importance for hydraulic fracture treatments:
AAPG Bulletin, v. 91/4, p. 603-622.
Gale, J.F.W., and J. Holder, 2008, Natural fractures in the Barnett Shale: Constraints on spatial organization and tensile strength with
implications for hydraulic fracture treatment in shale-gas reservoirs, in Proceedings of the 42nd US Rock Mechanics Symposium. Paper no.
ARMA 08-96.
Holder, J., J.E. Olsen, and Z. Philip, 2001, Experimental determination of subcritical crack growth parameters in sedimentary rock:
Geophysical Research Letters, v. 28/4, p. 599-602.
Roth, M., R. Peebles, and T. Royer, 2012, Sweetspot mapping in the Eagle Ford with multi-volume seismic analysis: Presentation at the 18th
Annual 3D RMAG Seismic Symposium.
Natural Fractures in Shale
Hydrocarbon Reservoirs
Julia F. W. Gale
Steve Laubach, Jon Olson, Randy Marrett, Jon Holder, Peter Eichhubl, András Fall, Esti Ukar, Luke Fidler, Laura Pommer Why Worry About Fractures?
Production Variability
Barnett
Haynesville
NY Times, June 26, 2011
“…shale formations have small spots of very productive & profitable
wells, surrounded by large areas where wells produce far less gas…”
Classic symptom of fractures
Basic Questions
• Are natural fractures present?
–
–
–
–
How abundant are they?
Are there different fracture types?
What is their intensity?
How large are they; are they connected?
• Do they affect production?
– Do they interact with hydraulic fracture
treatments?
– Do they enhance permeability?
• Can we predict them?
– Enhanced permeability?
Jointed pavements, Ohio River at New
Albany, looking west.
New Albany Shale
4-inch diameter core
SW Indiana
6
Subvertical Fractures
L
Fracture
trace
T
R
Fracture
trace
5 cm
1 cm
7
Bedding-Parallel Fractures
fossil
bed-parallel fracture
detrital layers
1 cm
8
Bedding-Parallel Fractures
Examples from Smithwick Shale, San Saba Co.
Houston Oil and Minerals, Neal, R.V. #A-1-1
2⅝-inch diameter core
Compacted Fractures and
Concretion-Related Fractures
H2
fault
barite
in pores
folds
1 cm
Marcellus
H1
New Albany
1 cm
Barnett,
Delaware Basin
1 cm
10
Faults
South Texas seismic volume
Niobrara
Roth et al., 2013 Sweetspot Mapping in the Eagle Ford with
11
Multi-Volume Seismic Analysis, RMAG 3D Symposium
Quantification of Fracture
Populations
• Abundance (semi-quantitative)
– Degree to which fractures present
– Used where sample does not permit intensity
measure
• Intensity, frequency (quantitative)
– Number of fractures per unit length, area or
volume
– Requires extensive sample relative to fracture
size
Sampling Fractures in Vertical Wells
A
Well A:
No fractures in layer 1
Many fractures in layer 2
Layer 1
Well B:
A few fractures in layer 1
No fractures in layer 2
Layer 2
B
Quantifying Fracture Abundance
• Count number of fractures per 100 ft of vertical
core (include all fractures  30 µm wide)
• Apply descriptor to ranges of abundance
– Many: >10 per 100 ft
– Several: 5-10 per 100 ft
– Few:
< 5 per 100 ft
• 18 shale formations examined
• Additional data from literature
14
Subvertical Fractures
Horn River Basin
In all cores studied
Neuquen
Basin
>10/100 ft
sealed
open
5–10/100 ft
< 5/100 ft
15
Horn River Basin
Bedding-Parallel
Fractures
Neuquen
Basin
>10/100 ft
sealed
open
< 5/100 ft
16
Compacted Fractures, ConcretionRelated Fractures & Faults
Horn River Basin
Neuquen
Basin
compacted
concretionrelated
faults
Quantification of Fracture
Populations
• How can we measure frequency?
• Intensity challenging to quantify
– Fractures may be clustered
– Sampling limitation in subsurface
– Intensity must be considered relative to
fracture size
Fracture
Kinematic
Aperture
Includes cement
and opening
Measured
orthogonal to
fracture walls
Vaca Muerta Fm., Neuquen Basin, Argentina
Fracture Frequency
Austin Chalk, Grove Creek, Waxahachie, TX
Kinematic aperture (mm)
100
10
1
0.1
0.01
0
50
100
150
Position along scanline (m)
200
250
300
Aperture-Size Distribution
Austin Chalk, Grove Creek
Cumulative frequency, F (fracs/m)
1
F = 0.1052 b-0.5575
R2 = 0.979
0.1
0.01
Widest fracture
0.001
0.01
0.1
1
10
Kinematic aperture, b (mm)
100
1000
Which Fractures Are Open?
Wide,
partly open
fractures
10 cm
Narrow, calcitesealed fractures
Aperture < 1mm
Which Fractures are Open?
100
Open
Emergent threshold
Kinematic aperture (mm)
10
Sealed
1
0.1
0.01
0
50
100
150
200
Position along scanline (m)
250
300
Which Fractures are Open?
1
Sealed Open
0.1
10
≈ 30 m
F = 0.1052 b-0.5575
0.01
0.001
0.01
100
0.1
1
10
100
kinematic aperture, b (mm)
1000
1000
Average Spacing (m)
cumulative frequency, F (fracs/m)
1
Which Fractures are Open?
100
Open
Emergent threshold
Kinematic aperture (mm)
10
Sealed
1
0.1
0.01
0
50
100
150
200
Position along scanline (m)
250
300
Fracture Sealing Patterns
in Shales
Vaca Muerta
Niobrara Fm.
Wide, partly
open fractures
Calcite cement
New Albany
Shale
Woodford Fm.
Smithwick Fm.
Marcellus Fm.
Barnett Shale
Calcitesealed
fractures
<1 mm
wide
Microfractures
Widespread, Nearly Always Sealed
Eriboll Sst
Pennsylvanian
Dolostone
Open
fracture
Bridge
Cement
SEM-cathodoluminescence images
0.5 mm
Julia F. W. Gale,
Lance 9709 ft
Pottsville 4240
La Boca A-1 A
Clear Fork dol 1
Knox dolomite
Cupido Fm. dolomite
Nugget
Cozzette 9034.7
La Boca A-1 B
Clear Fork dol 4
Ellenburger 9070.9
Marcellus J1
100000
0.00001
10000
0.0001
1000
0.001
100
0.01
10
0.1
Microfractures?
1
1
0.1
10
0.01
100
0.001
1000
0.0001
0.001
10000
0.01
0.1
1
Kinematic aperture, b (mm)
10
100
Average spacing (m)
Ozona Sst. Core, W. Texas
Dakota 7181
Vizcaya C-1
Clear Fork siltstone*
Marble Falls Lst
MF5 7777
250 to 2100 m
Cumulative frequency, F (fracs/m)
Austin Chalk outcrop
Dakota 7529
Weber Deerlodge outcrop
Chinook Ridge 3483 A-J
Clear Fork dol 3
Ellenburger 9079.6
Marcellus J2
Microfractures
• Lack of sealed microfractures in most ion-milled
SEM images of shale
• Direct observation of microfractures in shales
(sealed and open) reported in literature
• Indirect, inferred evidence of fluid-filled
microfractures: anisotropy of ultrasonic and
seismic velocities
Microfractures
“The extent to which microfractures enhance
mudstone permeability, both instantaneously
and over longer periods of geological time, is
poorly constrained.”
Dewhurst, Yang and Aplin, 1999, in Muds and Mudstones:
Physical and Fluid Flow Properties, Geol Soc Spec Pub 158.
Basic Questions
• Are natural fractures present?
–
–
–
–
How abundant are they?
What is their intensity?
How many sets are there?
How large are they and are they connected?
• Do they affect production?
– Do they interact with hydraulic fracture
treatments?
– Do they enhance permeability?
• Can we predict them?
– Enhanced permeability?
Hydraulic Fracture
Treatments
Pumping Phase
N
Hydraulic fracture
resumes in SHmax
direction at
natural fracture tip
Reactivation
of natural
fractures
Trace of part
of horizontal
wellbore with
perforation
~ 500 ft
After Gale et al., 2007
Weakly Bonded Fracture Cement
New Albany Shale
Tensile Testing
Sample Preparation
Step 1 Cut horizontal discs from core
Step 2 Mark and cut specimens
Natural, calcite-filled fracture
Sample from #2 T. P. Sims, 7,611 ft
Gale and Holder (2008)
Tensile Testing
Results
• Failure along fracture,
EVEN THOUGH THESE
ARE SEALED
• Specimens with natural
fractures are half as
strong as those without
Post-test specimens
Specimen
Rupture
(kpsi)
With natural fracture
2T
2.45
5T
3.86
3B
3.29
No natural fracture
9T
6.15
11T
From Gale and Holder (2008)
6.41
Fracture Prediction
• How can we predict fractures in the
interwell volume?
–Outcrop analogs?
–Geomechanical modeling
–Seismic detection
Fractures in Outcrops
Useful Analogs for the Subsurface?
• Sometimes yes, mostly no
• Consider timing of fractures relative to burial
– Stress history
– Diagenesis of host rock
• Is there fracture cement? If not why not?
• How do lithologies of host rocks compare?
• How far away are the outcrops from the
reservoir?
Subcritical Crack Index & Network Geometry
Geomechanical modeling by Jon Olson
Map views of fracture pattern models
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
-8
-6
-4
-2
0
n=5
2
4
6
-10
8
-8
-6
-4
-2
0
2
n=20
4
6
-10
8 -8
-6
-4
-2
0
2
n=80
4
6
8
Measuring Subcritical Properties
L
P=Load
a
•
•
•
•
W
d
P/2
P/2
a
crack length
P
Wm
=displacement
Crack Guide
P/2
Holder et al. (2001)
P/2
dual torsion test
test in air, water, brine, oil, …
multiple tests per sample
sample size 20 x 60 x 1.5 mm
Low
JOINTS
Modeling
High
Subcritical Index
and Fracture
Toughness:
Effect on
Fracture Patterns
Luke Fidler, MS
thesis
New Albany Shale
Conclusions 1
• Natural fractures are abundant and have a
role, even if they are sealed
• Distinct groups of fractures are present
–
–
–
–
–
Vertical (possibly more than one set)
Horizontal
Early compacted
Concretion-related
Faults
• Fracture cement geochemistry tied to burial
history
– insight into fracture/fluid flow in reservoir
Conclusions 2
• Fractures sampled in core may only
represent part of population
– Large fractures may be missed
– Microfractures?
• Outcrops must be used carefully
• Geomechanical modeling provides testable
prediction of frequency and spacing
Acknowledgments
Funding
• Fracture Research and Application Consortium (FRAC)
• RPSEA (Funding for New Albany and Marcellus studies)
• GDL Foundation
Samples and collaboration
• Fracture Research and Application Consortium (FRAC)
• Kentucky & Indiana Geological Survey, Noble Energy,
NGas, Daugherty Petroleum, Range Resources,
Pioneer Natural Resources
• Terry Engelder