Natural Fractures in the Barnett Shale: Why They Are Important
Gale, Julia F. W., Jon Holder, and Robert M. Reed
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 openingmode
fractures in several Barnett Shale cores from the Fort Worth
Basin.
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 fracturewall
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 Discovery Article #90071 © 2007 AAPG Rocky Mountain Meeting, Snowbird, Utah