Relationship Between Microseismicity and Pre-Existing Fractures at the North Montney Tight Gas Field

The behavior of the fracture extension during a hydraulic fracturing operation in the North Montney tight gas formation is clearly revealed by comparing the microseismicity observed by a surface array with a geometric attribute of the 3D seismic (Ant-Track). Our microseismic data reprocessing provided not only the drastic improvements in the event detection and hypocenter location but also the source mechanism solutions for all the events. This was achieved by the careful refinement of velocity model based on the stack amplitude of moveout applied and polarity compensated waveforms.

Consequently, the strike-slip type microseismic events are consistently found to be densely induced in the vicinity of the minor sub-vertical faults with the most slip-able strike (ENE-WSW) in the regional strike-slip type stress regime. In the fracturing stages without such slip-able pre-existing faults, dip-slip type (or so-called half-moon type) events are dominantly observed nearby the wells, suggesting tensile vertical fractures were dominantly extended without interaction with pre-existing fractures. Despite the pre-existing faults along optimal orientation in the shallower layer, the exception includes the half-moon type dominant microseismicity. The depth dependent changes in stress regime and/or the higher viscosity of the fracturing fluid are the suspected cause of the exception, so far.