To Bleed‐Off or Not to Bleed‐Off?

Abstract

Bleed‐off and shut‐in of injection have been implemented in different Enhanced Geothermal Systems (EGS) in an attempt to mitigate the induced seismicity at and after the injection stop. This article tackles the controversial discussion on the mitigation of the post‐injection induced seismicity through ending fluid injection by either a bleed‐off or a shut‐in. Both scenarios failed to mitigate the occurrence of large magnitude earthquake after stopping injection in numerous EGS. The two protocols are compared by using a seismological forecasting methodology coupled with a hydro‐mechanical model reproducing flow, poroelastic stress and fault failure in a domain including a discrete fault network based on Basel EGS (Switzerland, 2006). Unlike bleed‐off, shut‐in mitigates early post‐injection seismicity by smoothly reducing pressure, and consequently poroelastic stress changes in the modelled Basel EGS. Although post‐injection pore pressure diffusion eventually reaches distant faults and destabilizes them, seismicity is not initiated according to our simulation forecasting.