Summary
Hydraulic fracture microseismic data often reveal fairly uniform mechanisms as inferred from composite patterns of P and S first motions and amplitude ratios. The alignment of nodal planes with hydraulic fracture orientation and principal stress direction can imply a preference for extension-shear fracturing. Extension-shear failure occurs in the tensile normal stress field of the Mohr diagram, the region transitional from tensile to shear fracturing. A preference for extension-shear fracturing requires conditions of low differential stress and high pore pressure, a condition produced immediately adjacent to a hydraulic fracture. Using the Cotton Valley microseismic data, we present a case where observations of prevalent extension-shear fracturing can be reasonably explained by rock driven to failure in the stress field altered by the hydraulic fracture.
