Online Journal for E&P Geoscientists

Williams-Stroud, Sherilyn ^*1; Smith, Kevin L.²; Barker, William ¹
(1) MicroSeismic, Inc., Houston, TX. (2) EnCana Oil & Gas (USA) Inc., Dallas, TX.

Microseismic data was acquired with a shallowly buried monitoring array during a hydraulic fracture stimulation treatment of a deep, over-pressured shale gas reservoir. The locations of the microseismic events define parallel trends that formed during all of the stages of the stimulation treatment. Microseismic location trends with this type of geometry are generally interpreted to indicate the formation of new fractures parallel to the direction of maximum horizontal stress in the reservoir. However, analysis of sonic velocity measurements from the reservoir indicates that the linear trends formed by the microseismic events developed at an angle nearly 30 degrees different from the average maximum horizontal stress direction of values reported to the World Stress Map database for the area around the well, and greater than 50 degrees different from the maximum stress direction measured in the reservoir. Source mechanisms derived from events induced during the treatment show that the shear failure mode was oblique dip-slip, supporting the interpretation that the failure plane orientation is not parallel to the maximum horizontal stress. Examination of core from a nearby well in the reservoir revealed calcite filled fractures; because the fractures were filled, velocity anisotropy from fractures was not detected. Predictive modeling of microseismicity was done using an interpretation of reactivation of healed natural fractures in mode II shear failure. Mode I tensile hydraulic fractures are modeled as having initiated near the wellbore, intersecting the existing natural fractures in the reservoir as fracture growth continued during the treatment. The lower cohesive strength of the cemented discontinuities in the rock relative to the unfractured rock strongly controlled the geometry of the fracturing during the treatment. The stresses and rock properties required to produce the modeled rock failure interactions combined with the locations of microseismic events, source mechanism analysis of the events, and information about the stress state in the reservoir provide feedback parameters that allow operators to better plan subsequent hydraulic fracture treatments.