Abstract
In this contribution, we analyze in detail the spatio-temporal changes of the stress field caused by fluid injection and their effects on injection induced seismicity. We use linear superpositions to model a cylindrical injection source in a three dimensional poroelastic medium of infinite extent that mimics an open borehole section rather than a single point source. The influence on induced seismicity is monitored via changes in the Coulomb failure function. Previous theoretical and numerical studies indicate that injection induced stresses have a stabilizing or destabilizing effect depending on relative position with respect to injection point and principal stress directions prior to injection. Consequently the shape of seismic event clouds is affected by pore pressure diffusion and induced stresses.
Our results indicate that poroelastic induced stress has a direct impact on the triggering front of seismicity. In particular during shut-in we find a significantly different position of the triggering front compared to predictions from pore pressure diffusion models. The calculated three dimensional seismic event clouds are in good agreement with field observations.
Our findings emphasize the importance of poroelastic stress changes in addition to pore pressure diffusion effects in describing and analyzing injection induced seismicity.
1. OVERVIEW
Traditional analysis of injection-induced seismicity assumes pore pressure diffusion to be the main triggering mechanism [1,2]. Recent contributions however also emphasize the importance of induced poroelastic stress for the spatial evolution of the triggering front. Schoenball [3] investigated the influence of poroelastic stress compared to anisotropic diffusivity. The authors used a two-dimensional numerical model and found that either mechanism results in similar elongated microseismic clouds, rendering the two effects indistinguishable. Altmann [4] extended the analysis to three dimensions and concluded that fault reactivation is promoted along the maximum principal stress direction through poroelastic stress contribution. Field studies indeed reveal that seismic event clouds may be elongated along the maximum principal stress direction [5,6,7].
