The spatial footprint of injection wells in a global compilation of induced earthquake sequences

Emily E. Brodsky, & Thomas H. Goebel

Submitted August 15, 2018, SCEC Contribution #8717, 2018 SCEC Annual Meeting Poster #183 (PDF)

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Fluid injection induced seismicity is commonly modeled by assuming purely fluid-pressure driven earthquakes. However, this assumption is challenged by induced sequences with observed far-field triggering. To unravel triggering mechanisms of injection-induced earthquakes, we examine the spatial decay and space-time migration of ~20 induced seismicity cases. We limit our analysis to point source, single well injectors in the context of scientific, geothermal and wastewater injection. We perform an initial quality assessment, excluding datasets that cannot be distinguished from random Gaussian location uncertainty and uniform background seismicity. The spatial decay of the remaining cases is determined from the areal density of surface distances between wells and earthquakes. All sequences show a spatial gap between well locations and the location of highest seismicity density. Moreover, we observe two different types of spatial decay: 1) sequences with a near-well density plateau and rapid spatial decay and 2) sequences with steady decay, that extend to distances of more than ~10 km in some cases. Induced sequences with abrupt decay are dominated by square-root space-time migration, a characteristic of pressure diffusion. Sequences with steady spatial decay are dominated by linear migration or the absence of migration. The steady decay can be described by a power-law with an exponent of 1.8, which is significantly smaller (i.e. more gradual) than the spatial decay of aftershocks. This power-law behavior may be indicative of poroelastically induced earthquakes which lack the commonly expected rapid spatial decay and gradual spatial-temporal expansion of fluid-pressure-driven induced seismicity sequences. Far-reaching poroelastic stresses can lead to inflated hydraulic diffusivity estimates and a strongly underestimated spatial reach of injection wells.

Citation
Brodsky, E. E., & Goebel, T. H. (2018, 08). The spatial footprint of injection wells in a global compilation of induced earthquake sequences. Poster Presentation at 2018 SCEC Annual Meeting.


Related Projects & Working Groups
Fault and Rupture Mechanics (FARM)