Abstract
Hydraulic fracture stimulation treatments of 17 wells have been monitored with a shallow buried array for induced microseismicity in the Marcellus Shale in West Virginia, USA. The wide azimuth and large coverage area (18 square miles) of the shallow buried array allowed identification of the source mechanisms of all of the detected events, enabling a statistical analysis of failure mode and associated stress state at failure for all the events in space and time. Detailed analysis of source mechanisms of the largest events revealed heterogeneous failure plane orientations and slip directions, with a combination of dip-slip and strike-slip failure and varying amounts of volumetric failure. Stress inversion analysis of these source mechanisms allowed characterization of the local stress tensor, and how the stress tensor changed from the beginning to the end of the stimulation treatment. The failure mechanisms observed to occur more frequently at the beginnings of the fracture stages were dip-slip and strike-slip failure mechanisms were more common at the ends of the fracture stages. Utilizing the timing of the types of source mechanisms to define temporal groups, the stress inversion analysis showed that the stress state in the stimulated rock changed from being consistent with the regional NE-oriented maximum horizontal stress orientation with sigma1 vertical to a stress state where the maximum horizontal stress becomes sigma1 and is horizontal. The temporal stress state is quantitatively identified and it is demonstrated that the fracture growth directions responds to the temporal stress state. This information can be used by operators to respond to or exploit the expected fracture failure mode and direction, and also to design stimulation treatments that develop complex fracture networks.
