Oriented core samples from 23 Devonian gas shale wells in the Appalachian basin were used to determine microscopic and mesoscopic fracture patterns. The specific objectives were to note the preferred direction and nature of natural microcracks, to determine the preferred fracture propagation direction in laboratory mechanical testing, and to outline areas in the basin that are characterized by a high natural fracture density in the gas shales. This information provides a necessary background for the development of the in-situ stimulation technology which would most effectively connect natural fracture systems to a single well bore.

Mechanical tests under zero confining pressure conditions included point load, indirect tensile, laboratory hydrofracturing, and directional ultrasonic testing. Natural fractures were measured prior to testing. The preferred orientation of both induced and natural fractures throughout the basin was generally parallel to the trend of Paleozoic tectonic structure. This parallelism, as well as the details of the microfabric, suggests an "incipient cleavage" origin for the natural crack arrays. It thus appears that the residual effects of in-situ stresses do not influence the orientation of the induced fractures in laboratory tests. Tests under zero confining pressure are therefore not useful for determining the orientation of ^sgr_Hmax as other workers have previously suggested nor is the orientation of the fractures produced in these tests necessarily the same as that of an induced hydrofracture in the field.

The trajectory maps for in-situ stresses in the basin clearly illustrate the lack of parallelism with the mechanical fabric of the shale. However, analysis of the two patterns has been used to outline local areas in the basin where ^sgr_Hmax is parallel to the natural microcrack system. In these areas the natural crack array would

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be oriented so as to facilitate induced crack propagation.

Combined with an analysis of hydrocarbon potential and the location of detachment zones related to basement deformation, these relationships offer a useful rationale for targeting areas for future unconventional gas recovery programs. In addition they provide a framework for understanding the behavior of the rock mass in response to hydrofracture stimulation in less promising areas of the basin.

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