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

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

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

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)

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

Weakly bonded fracture cement Noble Solsman1-32H 2568.8 ft

Predicting fracture attributes Intensity Spatial distribution

Map views of fracture pattern models Subcritical Crack Index & Network Geometry Geomechanical modeling by Jon Olson (FRAC) Map views of fracture pattern models n=5 n=20 n=80

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.

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

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

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)

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

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.

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

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

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

Orientation of SHmax : E - W Natural Fracture Input Data for Hydraulic Fracture Modeling Daugherty Petroleum Inc. (DPI) 2485-21 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

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

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

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)

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

Subcritical Crack Index Test Results Osburn Trust 1-11H (2528 ft) Solsman 1-32H (2557 and 2630.6 ft)

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)

Natural Fracture Input Data Hydraulic Fracture Modeling Daugherty Petroleum Inc. (DPI) 2485-21 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

Natural Fracture Input Data Hydraulic Fracture Modeling Daugherty Petroleum Inc. (DPI) 2485-21 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.

Natural Fracture Input Data Hydraulic Fracture Modeling Daugherty Petroleum Inc. (DPI) 2485-21 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

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

Measuring Subcritical Properties d 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)

Outcrop Joints July 2005 Appalachian Basin