Evolution of pore fluid pressures in a stimulated geothermal reservoir inferred from earthquake focal mechanisms

Abstract

AbstractThis paper proposes an inversion method to estimate the evolution of pore fluid pressure fields from earthquake focal mechanism solutions. Application of the method to induced seismicity in the Basel enhanced geothermal system in Switzerland shows the evolution of pore fluid pressure in response to fluid injection experiments. The pore fluid pressure in the reservoir was less than the minimum principal stress at each depth, indicating that hydraulic fracturing did not occur during the stimulation. This suggests that seismic events may play an important role in promoting the development of permeable channels, particularly southeast of the borehole where the largest seismic event (Mw 2.95) occurred. This is not directly related to a drastic decrease in fault strength at the hypocenter but rather to positive feedback between permeability enhancement and elastic/poroelastic stress loading from slipping interfaces.