1. Natural Fracture Study
July 2005
Natural Fracture Study
Implications for development of effective drilling and completion technologies
Julia F. W. Gale
Stephen E. Laubach
Bureau of Economic Geology, Jackson School of Geosciences,
The University of Texas at Austin, USA

2. Objective
Field-verified methodology to identify areas of high productivity in the New Albany Shale
Focus on thermogenic part of play
Well experiments
Natural fractures key for drilling and completion
Reactivation during hydraulic fracture treatment
Possible permeability enhancement
Methods for natural fracture characterization and prediction needed

3. Outline Core and outcrop studies SW Indiana
Fracture types and properties
Fracture spatial organization
Well experiment, W Kentucky
Core and image log data, pilot hole
Prediction of hydraulic fracture treatment
Input data for modeling of interaction of hydraulic fractures with natural fractures

4. Core and Outcrop Locations
After Hasenmueller and Leininger, 1987
Kentucky data set and well experiment
Field work along the outcrop belt was combined with core studies in two areas; southwestern Indiana and western Kentucky.
2 southerly outcrops described in Schieber and Lazar (2004 Field Guide)

5. Diversified Operating
McAtee S26-IV, 2788 ft
Subsurface Fractures Steeply Dipping, Sealed Dominant trend NW-SE Secondary trend E-W Orientation data from 3 cored wells (Hamilton-Smith et al., 2000)
4 in diameter core

6. Weakly bonded fracture cement
Noble Solsman1-32H ft

7. Predicting fracture attributes
Intensity
Spatial distribution

8. Map views of fracture pattern models
Subcritical Crack Index & Network Geometry Geomechanical modeling by Jon Olson (FRAC)
Map views of fracture pattern models
n= n= n=80

9. Subcritical Crack Index Testing
High SCI
Mod SCI
Variable subcritical index 37 66
Mean subcritical indices
4 tests for upper unit
10 tests for lower unit
Apparent
concentration of
fractures
In the example shown with variable subcritical index, the lower layer has a higher index, with a more clustered fracture pattern, whereas the upper layer has a lower index and a more evenly spaced fracture pattern. Mechanical layer thickness also controls fracture spacing, with evenly spaced fractures spaced proportionally, but not necessarily equal to, mechanical layer thickness.

10. Subcritical crack index test results
Single sample – multiple test specimens
Variability between specimens
43 to 80
Compositional
variation?
2,280 ft

11. In Situ Stress (controls hydraulic fracture orientation)
Mid-Plate Compression Province, but local variation
Need to establish SHmax carefully

12. Hydraulic Fracture Treatments Pumping Phase
SW Indiana N
Secondary natural fractures E-W
Hydraulic fracture resumes in SHmax direction at natural fracture tip
Reactivation of natural fractures
Trace of part of horizontal wellbore with perforation
fracture clusters NW-SE
Dominant natural
Hydraulic fractures ENE-WSW
Natural fracture orientation data from 3 cored wells (Hamilton-Smith et al., 2000)
SHmax
~ 500 ft
Modified after Gale et al. (2007)

13. Well Experiment
McBride and Nelson (1999)
Slide: Doug Walser, Pinnacle

14. E-W trending en echelon fractures with carbonate cement at 2574-2575 ft
Image log with open fracture in the subsurface
Note offset at 2575 ft on image log.

15. Compacted Veins Blocher Member
July 2005
Compacted Veins Blocher Member
Present in subsurface
Mostly dolomite
Contain porosity
Looking west

16. In Situ Stress (controls hydraulic fracture orientation)
Mid-Plate Compression Province, but local variation
Need to establish SHmax carefully

17. Well Experiment Prediction
Natural Fractures parallel to SHmax
Horizontal wellbore
Dominant natural
fracture cluster E-W
SHmax
In the western Kentucky area the natural fractures and strike of normal faults are subparallel to SHmax and may cause the hydraulic fractures to propagate along them. Because the reactivation involves failure along one fracture wall, the connection to shale host rock is potentially half that of a fracture cutting the shale. That is, gas flow through a cement fill may be negligible. The result is that much less shale is broken in this scenario. A well experiment with microseismic monitoring is being conducted in this region.
~ 500 ft

18. Orientation of SHmax : E - W
Natural Fracture Input Data for Hydraulic Fracture Modeling Daugherty Petroleum Inc. (DPI) Christian Co.
Orientation of SHmax : E - W
Orientation of opening-mode fractures: E - W dip N or S
Orientation of faults: Surface faults E - W dip 60° N or S
Kinematic aperture: 0.05 to 2 mm
Hydraulic aperture: not known, likely 0 to 2 mm
Strength of fracture planes: Weak but no test data. Will use data from Barnett (fractures 1/2 as strong as host shale)
Height of opening mode fractures
1 m for fractures < 1 mm wide; 10 m for fractures > 1 mm wide
Spacing of opening mode fractures: 1 to 10 m as base cases

19. Impact of Natural Fractures: Assessment Workflow
Measure SHmax and natural fracture orientation in subsurface. Core, image logs, dipole sonic logs. Do not rely on surface data.
Determine composition of host rock and fracture fill
Measure subcritical crack index
Determine fracture timing and establish mechanical layering at the time of fracturing

20. Impact of Natural Fractures: Assessment Workflow
Use geomechanical models and fracture scaling theory to predict intensity and spatial organization
Verify with microseismic monitoring and core/image logs
Use to predict likely interaction of hydraulic fractures with natural fractures
Use production data to verify
Iterate to improve prediction capability

21. Conclusions Natural fractures common; diverse origins
Steep, partly or completely sealed likely most important for completions
Calcite-sealed fractures weak planes (reactivation)
Different origins yield different attributes
Subcritical index for spacing
Location critical
Sullivan/Pike Co. area near Wabash Valley Fault System (active)
Christian Co. area near Rough Creek Graben (E-W)
Present day SHmax E-W to ENE -WSW across region (need local data)

22. Acknowledgments
Fracture Research and Application Consortium (FRAC), University of Texas at Austin
RPSEA – funding for this project
GTI
Noble Energy Inc.
NGas
ResTech and Pinnacle
Indiana Geological Survey
Kentucky Geological Survey

23. Subcritical Crack Index Test Results
Osburn Trust 1-11H
(2528 ft)
Solsman 1-32H
(2557 and ft)

24. Active tectonics Earthquakes
Wabash Valley Fault System (normal faults)
18th April 2008 Magnitude 5.2
Strike slip, right lateral
Depth 18 km (~ 11 miles)
Active tectonics
McBride and Nelson (1999)

25. Natural Fracture Input Data Hydraulic Fracture Modeling Daugherty Petroleum Inc. (DPI) Christian Co.
Present day orientation of SHmax : E - W
Orientation of opening-mode fractures
E-W trend, steep dips to N and S
Orientation of faults
Surface faults trend E-W dipping 60° to N or S
Fault on image log at 2484 ft MD, dipping at approx 88° S

26. Natural Fracture Input Data Hydraulic Fracture Modeling Daugherty Petroleum Inc. (DPI) Christian Co.
Natural fracture apertures
Partly open in image log
Natural state; popped open by drilling; calcite cement abraded during air drilling
Calcite cement in core, breaking within cement
Euhedral calcite crystals on fracture surfaces ( 2 mm): pore space of > 2 mm needed to grow
Narrowest sealed fractures 0.05 mm wide
Kinematic aperture is of the order of 0.05 to 2 mm
Hydraulic aperture is not known. Likely 0 to 2 mm.

27. Natural Fracture Input Data Hydraulic Fracture Modeling Daugherty Petroleum Inc. (DPI) Christian Co.
Strength of fracture planes
Weak, but no tensile strength measurements
Will use data from Barnett tests (fractures half as strong as host shale)
Height of opening mode fractures
1 m for fractures < 1 mm wide
10 m for fractures > 1 mm wide
Spacing of opening mode fractures
Will be modeled using subcritical index and mechanical layer thickness
will use 1 to 10 m as base cases

28. Polyphase Cement Anschutz Corp. #16-19 Voelkel, Dubois Co., 2106 ft
July 2005
Polyphase Cement
Anschutz Corp. #16-19 Voelkel, Dubois Co., 2106 ft

29. Measuring Subcritical Propertiesd
P/2
P=Load P W m
Crack Guide
D=displacement L a
crack length
Crack
dual torsion test
test in air, water, brine, oil, …
multiple tests per sample
sample size 20 x 60 x 1.5 mm
Holder et al. (2001)

30. Outcrop Joints
July 2005
Appalachian Basin