Fracture Mechanics Interpreted from Stress Inversion Analysis on Microseismic Event Source Mechanisms in the Marcellus Shale

A multiple inverse stress analysis technique was applied to source mechanisms from microseismic events detected during a stimulation treatment of a horizontal well in the Marcellus Formation in order to determine the stress state(s) responsible for rock failure during the treatment. The microseismicity was mapped with a monitoring array of geophones located on the surface above the well and during the stimulation treatment, their locations formed two well-defined trends. The northeasterly trend is interpreted to be parallel to the J1 regional joint set. In most of the Applachian Basin, the J1 joints are often nearly parallel to the azimuth of the generally NE-SW direction of maximum horizontal stress, and 3 of the 7 stages of the hydraulic fracture stimulation treatment showed northeasterly oriented trends. Relatively strong microseismic activity was detected repeatedly along a trend nearly perpendicular to SHmax throughout stages 4 to 6. A large amount of the materials used to treat these stages was lost into the formation; well log information and subsurface mapping support the interpretation that the activity mapped along the northwest trend resulted from reactivation of a northwest-striking steeply-dipping fault during the stimulation treatment. The source mechanisms of selected events indicate that the failure mechanism is strike slip displacement in both microseismicity trends but with varying amounts of non-double couple component. Based on the source mechanism results, the events were lumped into two groups, one that has a minimum misfit for a right-lateral strike slip solution and the other for a left-lateral strike slip solution. The different sigma1 orientations indicated by the two solutions is interpreted to be the result of stress perturbations in the reservoir related to dilation of the existing fault zone and reactivation of fractures in the damage zone surrounding the fault.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California