The Signature of Shearing Driven By Hydraulic Opening

Hydraulic-fracture microseismicity in layered rock often exhibits fairly uniform source mechanisms characterized by one of the nodal planes aligned vertical and striking close (~10°) to the principal stress direction. Typically, displacements are dip slip and/or strike slip. When combined with precise source locations, the events sometimes form simple linear trends and distinct depth bands separated by aseismic intervals. The simple geometry and observed shearing on planes with little or no expected in-situ shear stress suggest that the signal generation is closely associated with the near-field stress and strain conditions of hydraulic-fracture opening. Further, the distinct depth bands suggest the seismic failure is controlled by the mechanical stratigraphy.

Based on the analog of natural tensile joint structures observed in layered rock, I interpret the aligned strike-slip and dip-slip mechanisms in terms of en echelon fringe cracks and bedding-plane slip on step-over features, respectively. Dip-slip mechanisms are common in shales, and may represent slip on bedding surfaces if the alternate, horizontal nodal plane is considered the fault plane. Modeling studies of bedding-plane slip and the formation of jogs or step overs of a tensile fracture along bedding suggest that the fracture growth is controlled by crack tip stresses and the mechanical properties of the layer interfaces. The observation of strike-slip events aligned in horizontal bands could be similarly associated with tensile joint behavior at layer interfaces where the parent tensile crack can break up into a set of en echelon cracks in response to local stress rotations. Breakdown into en echelon cracks is initiated by strike-slip shearing parallel to the tensile parent fracture, representing a mode III deformation. In this interpretation, of critical failure associated with bedding plane slip and fringe fracture break up, the microseismicity provides a direct picture of tensile fracture growth through the rock, highlighting the deformation at and near bedding boundaries as the hydraulic fracture rips through or along layer interfaces.