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On the mechanisms of stress-triggered seismic events during fluid injection

Yarushina, V.M. , Podladchikov, Y.Y. , Minakov, A. , Räss, L.
Poromechanics 2017 - Proceedings of the 6th Biot Conference on Poromechanics , pp. 795 - 800.
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Microseismicity is often assumed to be directly linked to the pore pressure increase by shifting stress conditions onto the Mohr-Coulomb yield surface. In addition, a large proportion of seismic events occurs mainly within the reservoir and have tensile opening component, characteristic of fluid-filled fractures. However, there are observations of injection-induced microseismicity at large offset distances from the operation site. Such seismic events often occur at some distance from the reservoir or pressure front and are characterized by mainly double couple source mechanism. We discuss mechanisms that might trigger such events and show that in deformable porous rocks stress changes and subsequent induced seismicity might happen ahead of the fluid pressure diffusion front. Localization of the fluid flow that might occur in rocks exhibiting time-dependent creep, e.g. shales, might lead to out-of-zone seismicity that continues even after injection has been stopped.
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