Summary
We present results of application of stress field inversion on microseismic monitoring data. We inverted regional principal stress directions from the source mechanisms of microseismic events induced by hydraulic fracturing in a shale reservoir. We compare results of stress inversion from several groups of microseismic events and inversions. We compare inversions using source mechanisms inverted from manually picked amplitudes and automatically inverted source mechanisms. We show changes in the stress field orientation based on subset of dataset created according to observed type of source mechanisms and depth of microseismic events. The resulting stress fields are stable, highly similar and consistent with regional stress field. We get maximum regional stress in the vertical direction, which is typical for most of sedimentary basins, and maximum horizontal stress oriented approximately 75° from the direction of the drilled wells. The fact that we obtained the regional stress indicates that the regional stress determined the source mechanisms of induced microseismic events and we use Mohr diagrams to determine most likely fault planes of these events.
Introduction
Microseismic monitoring is considered to be a crucial tool for observing and mapping reservoir response to hydraulic fracture stimulation. Locations of microseismic events determine basic information about fracture geometry such as direction of fracture propagation, its length and height. Recently, an advanced fracture characterization using source mechanisms of induced microseismic events is also routinely provided (Baig and Urbancic, 2010). Locations and source mechanisms allow other advanced characterization such as discrete fracture network (Williams-Strout et al., 2010) and stress orientation.
Observed mechanisms are dominated by shearing (Stanek and Eisner, 2013; Rutledge et al. 2013). Each source mechanism of a shear event represents an (micro)earthquake with two potential fault planes and slip on these planes. According to a type of stress regime in the source area we observe different types of source mechanisms (Anderson, 1942) radiating specific directional dependent pattern of seismic waves (Aki & Richards, 1980). We use the waveforms of seismic waves to determine source mechanisms. The source mechanisms enable us to invert the most suitable stress field orientation which is consistent with the slippage on the fault planes determined from source mechanisms
