Pore pressure behavior at the shut‐in phase and causality of large induced seismicity at Basel, Switzerland

Abstract

Induced seismicity with unexpectedly large magnitude often occurs after shut‐in or end of stimulation, generating concerns at the end of stimulation. We investigated the physical mechanism of large‐magnitude induced seismicity during shut‐in following the hydraulic stimulation at Basel, Switzerland. Larger postinjection events occurred at the periphery of the seismic cloud. We estimated the pore pressure required to cause shear slip using Coulomb failure criteria from stress information, geometry of the fault planes of microseismic events, and a constant coefficient of friction. Time series analysis of pore pressure distribution indicated that pore pressure migrated to the far field even after shut‐in. Redistribution of pore pressure at shut‐in brought sufficient pore pressure increase to induce seismicity in the peripheral region. After shut‐in, the pore pressure gradient away from the well lessened and eventually pressure became uniform. These observations suggest that the higher pore pressure, which remained in the vicinity of the injection point, shifted to the farthest field. Shut‐in pressure migration caused uniform pore pressure distribution at the edge of the seismic zone. Shut‐in pressure destabilized a large part of the fault located at the edge of the seismic cloud, resulting in the shear slip of a large section of the fault. Meanwhile, during stimulation, only some parts of the fault entered the critical state because of the pressure gradient. The resulting shear slip on that specific part causes moderate magnitude events at most.

Triggering processes of microseismic events associated with water injection in Okuaizu Geothermal Field, Japan

“…a Location of the earthquake used for the example of scattering analysis; b observed (black line) and theoretical (red dashed line) waves; c sensitive area of initial P-waves for this example case projected on cross section including hypocenter (shown as circle) and receiver (shown as inverted triangle) the transfer of the stress fluctuation from the Test Well to Mqs2 is still unclear. Several studies (e.g., Häring et al 2007;Baisch et al 2010;Mukuhira et al 2016) have pointed out that seismic events likely occur at the edge of reservoirs after shut-in owing to redistribution of the pore pressure at/after the injection is stopped. If the stress fluctuation due to the injection rate changes has an effect similar to that in the shut-in phase, it might be a trigger for the microseismic events that occurred at the periphery of the fracture zone (see Fig.…”

“…Shapiro and Dinske (2009) developed a theory regarding a linear/nonlinear diffusion process. In addition, several studies attempted to clarify pore-pressure migration by including the selective water flow in permeable fractures and faults (hereafter referred to as the water flow model) (e.g., Evans et al 2005;McClure and Horne 2011;Mukuhira et al 2016).…”

Geophysics From Terrestrial Time‐Variable Gravity Measurements

“…Measuring gravity variations is important to constrain the mass balance within reservoirs and its lifetime, which is difficult to estimate (Ishido et al, ; Sugihara & Ishido, ). This is also required to mitigate the associated earthquake hazard, which depends on the amount and location of displaced fluid (Deichmann & Giardini, ; Holland, ; McClure, ; Mukuhira et al, ).…”

Postinjection Normal Closure of Fractures as a Mechanism for Induced Seismicity

“…The seismic recordings of the Basel and Paralana geothermal projects show that the locations of seismic events after the termination of injection are mainly at the outer rim of the previous seismic activity, i.e., the stimulation region (Albaric et al, ; Häring et al, ; Mukuhira et al, ). Here we analyze a novel hypothesis that pertains to the mechanism underlying elevated postinjection events occurring at the boundaries of the stimulation region.…”