Geomechanics of hydraulic fracturing microseismicity: Part 1. Shear, hybrid, and tensile events

We investigate the geomechanical behavior of hydraulic-fracturing-induced microseismicity. Microseismic events are commonly used to discern stimulation patterns and hydraulic fracture evolution; however, techniques beyond fracture mapping are required to explain the mechanisms of microseismicity. In this series we present an approach to combine seismological and geomechanical techniques to investigate how microseismicity relates to propagating hydrofractures as well as existing natural fractures and faults. Part 1 describes the first analysis step, which is to characterize the microseismic events by their source parameters, focal mechanisms, and fault-plane orientations. These parameters are used to determine the mechanical conditions responsible for activation of discrete populations or subpopulations of microseismic events that then can be interpreted in their geological and operational context. First, we compare microseismic fault-plane populations from a Mississippian Barnett Shale, Texas data set that are determined using a traditional double-couple model (shear only) with a tensile source model (hybrid events), which may be more suitable for hydraulic fracturing conditions. Second, we employ a new method to distinguish fault planes from auxiliary planes using iterative stress inversion and critical stress (instability) selection criteria. The result is an enhanced microseismic characterization that includes geomechanical parameters such as slip tendency and local activation stress state during the operation. Using this approach on the Barnett Shale data, two microseismic fault sets are resolved: an inclined northeast southwest set with dominant shear, and a vertical north south set with more hybrid behavior. The results are used in part 2 to further investigate the heterogeneity of the stimulations and to compare models for microseismic activation.