Hydraulic-fracture propagation in layered rock: experimental studies of fracture containment Page: 6 of 20
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layer when fluid pressure was applied to the sealed borehole
through the hollow steel packer. In each experiment the
open-borehole was filled with fracturing fluid (40 weight
oil) which was then quickly pressurized by an 80 MPa hand
pump (fracturing occurred within 60 seconds). The fluid
pressure was monitored by a transducer (with an accuracy
of 0.1 MPa) and recorded on a pressure/time recorder.
Experiments were conducted on monolithologic and dilitho-
logic (that is, outer layers A and C were different from
middle layer B) specimens by applying a stress up to 20
MPa normal to the layer interfaces. Rock types used in
the experiments include Arizona, Berea, Coconino, and Ten-
nessee sandstones and Lueders limestone. The porosity,
permeability, and a brief physical description of these
rocks are given in Table 1. The surface roughness of the
interfaces were varied by polishing them with 20, 40, 80,
and 240 grit abrasives. Average surface roughness was
measured with a surface profiler.
In order to relate fracture growth to the mechanical
properties of the different rocks, room temperature, uncon-
fined compression tests were conducted on right-circular
cylinders (4.76 cm in diameter and 10 cm long) with axial
and lateral strain gages to determine the uniaxial compres-
sive strength, Young's modulus, and Poisson's ratio. In
addition, indirect tensile strengths were determined from
Brasil tests. The mechanical properties of the rocks used
in this study are given in Table 2.
EXPERIMENTAL RESULTS
Monolithologic Experiments
In order to determine the influence of layer inter-
faces on hydraulic fracture growth, a series of experiments
were conducted on monolithologic layered specimens as a
function of normal stress. Since the layers on either side
of the interface have identical mechanical properties, the
telaLive importance of the shear strength of the interface
(resulting from the frictional effect of the applied normal
stress) and the material properties of the layers can be
assessed.
Compressive normal stress can be transmitted across
an interface, but the amount of shear stress transmitted
across the interface will depend on the inherent shear
strength and frictional properties of the interface. For
an unbonded interface the inherent shear strength is essen-
tially zero and the shear strength of the interface is
solely dependent on its frictional properties. Experi-
mental studies on the frictional properties of rock have
shown empirically that the shear strength of a sliding
surface fits a linear relation
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Teufel, L. W. & Clark, J. A. Hydraulic-fracture propagation in layered rock: experimental studies of fracture containment, article, January 1, 1981; Albuquerque, New Mexico. (https://digital.library.unt.edu/ark:/67531/metadc1184461/m1/6/: accessed July 16, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.