Effects of Seismicity Mitigation Practices Captured in Time-Lapse of Induced Earthquake Fault Activation

Abstract:

The evolving state of pore fluid pressure throughout induced earthquake sequences is of key importance in understanding and mitigating the development of subsequent seismicity. It is important yet challenging to assess the redistribution of pore pressure following mitigation efforts such as injection rate control and cementation of wastewater disposal wells. While it is well established that wastewater injection into the deep subsurface alters the effective normal stress and slip tendency on optimally oriented faults, the field-scale effects can be difficult to observe in practice. Quality time-lapse seismological observations can provide constraints on the state of pore pressure, stress, and structural geometries in the shallow crust near injection operations.

In this study, we present results from a multi-year seismic experiment conducted in close proximity to disposal wells associated with an induced earthquake sequence near Greeley, CO, between 2014 and 2019. High-precision relocations of the microearthquakes through time reveal a pronounced shift in the orientation of active structures, with events in the 2014-mid-2016 time period activating faults in both north-northeast and north-northwest orientations, and events in the mid-2016-present time period activating faults primarily striking in the the north-northwest direction. We evaluate the inferred structural orientations of active faults with the analysis of a limited number of arrival-based focal mechanisms. The changes in active structure orientations correspond to a number of injection rate decreases and bottom-well cementation implemented at the two wastewater wells nearest to the seismicity. Our results suggest that the combined mitigation at nearby wells was successful in restricting pore-pressure build up in basement faults beneath the injection sights. Mitigation at individual wells was seemingly ineffective until all wells in the area were cemented and restricted to a maximum operating pressure criteria. This work highlights the importance of subsurface structural knowledge and combined-well hydraulic contributions.