Publication Date:
2007-04-01
Description:
Gas production from the Barnett Shale relies on hydraulic fracture stimulation. Natural opening-mode fractures reactivate during stimulation and enhance efficiency by widening the treatment zone. Knowledge of both the present-day maximum horizontal stress, which controls the direction of hydraulic fracture propagation, and the geometry of the natural fracture system, which we discuss here, is therefore necessary for effective hydraulic fracture treatment design. We characterized natural fractures in four Barnett Shale cores in terms of orientation, size, and sealing properties. We measured a mechanical rock property, the subcritical crack index, which governs fracture pattern development. Natural fractures are common, narrow (〈0.05 mm; 〈0.002 in.), sealed with calcite, and present in en echelon arrays. Individual fractures have high length/width aspect ratios (〉1000:1). They are steep (〉75°), and the dominant trend is west-northwest. Other sets trend north-south. The narrow fractures are sealed and cannot contribute to reservoir storage or enhance permeability, but the population may follow a power-law size distribution where the largest fractures are open. The subcritical crack index for the Barnett Shale is high, indicating fracture clustering, and we suggest that large open fractures exist in clusters spaced several hundred feet apart. These fracture clusters may enhance permeability locally, but they may be problematic for hydraulic fracture treatments. The smaller sealed fractures act as planes of weakness and reactivate during hydraulic fracture treatments. Because the maximum horizontal stress trends northeast-southwest and is nearly normal to the dominant natural fractures, reactivation widens the treatment zone along multiple strands. Julia Gale obtained a Ph.D. in structural geology from Exeter University in 1987. She taught structural geology and tectonics for 12 years at the University of Derby. She moved to the University of Texas at Austin in 1998, working as a research associate first in the Department of Geological Sciences and then the Bureau of Economic Geology. Her interests include fracture characterization in carbonate and shale hydrocarbon reservoirs. Rob Reed is a research scientist associate at the Bureau of Economic Geology. He received his B.S. degree and his Ph.D. in geological sciences from the University of Texas at Austin and his M.S. degree in geology from the University of Massachusetts. His current research focuses on various aspects of the microstructure of rocks. Jon Holder received a Ph.D. in physics from the University of Illinois at Urbana Champaign (UIUC) in 1968. He was a member of the Geology Department faculty at UIUC from 1969 until 1981, teaching and conducting research in areas of rock physics. He worked in geotechnical research in the private sector from 1981 to 1989 and then joined the research staff in the Petroleum and Geosystems Engineering at the University of Texas at Austin, where he continues to do research in mechanical behavior in porous media, with emphasis on fracture mechanics.
Print ISSN:
0149-1423
Electronic ISSN:
1943-2674
Topics:
Geosciences
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