Hydraulic fracture characteristics are dramatically impacted by the measure of the modulus (ratio of stress to strain) of the rock. If the modulus is large, the rock is considered stiff. In stiff rock units, fractures grow long
and narrow away from the source of additional pressure, and in less stiff
materials, fractures are wider and vertically shorter because the pressures causing the fractures can penetrate and dissipate further laterally (into planes of weakness like bedding planes and existing fractures) within
the rock unit. Shales are usually very stiff, but highly fractured black shales, like the Marcellus, are generally much less stiff than unfractured grey shales. The stiffness of the Marcellus Shale is not uniform, so some areas are more prone to longer fractures than others. However, when a hydraulic fracture hits the less stiff, already fractured materials in the Marcellus Shale, the energy can move laterally within the shale and, as a result, cause shorter, wider fractures