The risk of induced seismicity is often managed using methods based on the empirical scaling relationship between fluid injection volumes and cumulative seismic moment. Mitigation options involve reducing the rate of fluids injection when induced earthquake magnitudes reach a certain threshold. Field observations show that the magnitudes of some of the largest induced earthquakes exceed the values typically used to forecast the maximum magnitude of events. These larger events are hosted by pre-existing faults that are often considered to be hydraulically connected to the injected fluid source. To address the apparent limitations of these forecasted magnitudes, we investigate the correlation between spatial and temporal evolution of fracturing/faulting and measured physical properties of reservoir rocks and seismic parameters of laboratory earthquakes.
Intact core plugs from the Horn River Basin shale play (BC,Canada) were characterized in terms of their density, elastic moduli, ultrasonic waves velocity and seismic anisotropy, then loaded to failure at reservoir conditions (~2km depth) in a triaxial apparatus. Acoustic emissions were measured to monitor the evolution of seismic parameters during the progressive load-induced degradation leading to failure. To investigate fluid induced reactivation of a preexisting fault within the reservoir underburden, we injected pore fluids via a borehole in a composite sample consisting of a granite saw-cut and shale core.
During the prefailure stage, after yielding we observed a 3% decrease in P-wave velocity, an increase in acoustic emission rate to 0.35s-1 and a max relative magnitude of -4.35. During failure and sudden stress drop stage, emission rate increased to 24.7s-1, P-wave velocity dropped by 6% and relative max magnitude increased to -2.84. Acoustic events location shows that the evolution of seismic parameters at the transition between pre to co-failure corresponds to the development of a larger, throughgoing fault. During the composite sample test, injection of pore fluid at 40MPa reactivated the preexisting fault within the underburden. Then, at constant/decreasing pore pressure, location of acoustic events shows progressive fault reactivation within the underburden and fault growth within the reservoir. The development of throughgoing fault was associated with a decrease in P-wave velocity, and an increase in seismicity rate and magnitude, as the throughgoing propagated from the underburden into reservoir.
Our results show that a proactive risks mitigation strategy can be based on real time observation and monitoring of diagnostic systematic changes in physical properties and seismic parameters of the reservoir-underburden system.
