Extension-shear microseismic mechanisms during hydraulic fracturing
James Rutledge*, Robert Downie, Shawn Maxwell, Julian Drew, Schlumberger and Tom
á
Fischer, Charles
University in Prague.
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.
