Natural
Fractures in the Barnett Shale: Why They Are Important
Gale, Julia F. W.1, Jon Holder2,
Robert M. Reed1 (1) Jackson School of Geosciences, The University of
Texas at Austin, Austin, TX (2) The University of Texas at Austin, Austin, TX
Microseismic monitoring of
hydraulic-fracture treatments in the Barnett Shale shows that fractures
propagate in an array wider than that expected from knowledge of the
present-day stress field. It has been suggested that this is because hydraulic
fractures reactivate natural-fracture systems. To investigate this possibility,
we analyzed the orientation, size, intensity, and sealing properties of
opening-mode fractures in several Barnett Shale cores from the
Natural fractures in the Barnett Shale are
common. They are narrow (< 0.05 mm), sealed with calcite, and present in en
echelon arrays. They are typically steep (> 75°), and the dominant trend is
WNW. Other sets trend N-S. The narrow fractures are mostly sealed and cannot
contribute much to reservoir storage or permeability. The fracture population,
however, may follow a power-law size distribution, where the largest fractures
in the population are open. A mechanical rock property, the subcritical crack
index, was measured for different lithofacies. This index, along with
mechanical layer thickness, governs fracture clustering. The subcritical crack
index for all lithofacies tested is high, indicating strong fracture
clustering. These results suggest that large open fractures exist in clusters
spaced several hundred feet apart.
We also tested the effect of
calcite-sealed fractures on tensile strength of shale. Samples containing
natural fractures have half the tensile strength of those without. The junction
between the fracture-wall rock and cement is weak because the calcite cement
grows mostly over noncarbonate grains, thus providing a mechanism for hydraulic
reactivation of sealed natural fractures.
AAPG Search and Discover Article #90063©2007 AAPG Annual Convention, Long Beach, California