Characteristic ruptures of micro-seismic hydraulic fractures

Abstract:

Hydraulic fracturing is a process that involves the injection of fluids above lithostatic pressures to increase permeability of rocks at depth. During the fracturing process thousands of micro-seismic events are generated as the fracture front propagates outwards from the injection point. Sand or glass beads are frequently injected later in the stage to prop the fractures and maintain flow paths open. Because injection progresses in time and space changing the in situ characteristics the generated seismic events show a combination of seismic signatures between two end members: Coulomb stress transfer on favorably oriented fractures and fluid-induced tensile fractures. In this study we investigate the failure process of ~27,000 micro-seismic fractures induced during a hydraulic fracturing shale completion program. Our goal is to identify spatial and temporal distribution of families of events with similar characteristic rupture behaviors, for different injection phases and relative locations based on either rock formation, depth, source mechanism, fracture plane orientation, stress drop, etc., to classify distinct dynamic failure processes. In our analysis we estimate static and dynamic stress drop, radiated energy, seismic efficiency, moment tensor, fracture plane orientation, slip direction and rupture velocity. On average, the micro-seismic events have low radiated energy, low dynamic stress and low seismic efficiency failing in overshoot mode, with slow rupture velocities consistent with failure on fluid lubricated fractures with decreased friction resistance. Slip is accommodated on fracture planes with orientations dominated by the rock fabric and not always optimally oriented to the regional stress field. Subtle source characteristic differences can be identified: Events occurring in deeper formations tend to have faster rupture velocities and are more efficient in radiating energy. Variations in rupture velocity tend to correlate with variation in depth, fault azimuth and elapsed time, reflecting a dominance of the local stress field over other factors.