Observation of Hydromechanical Fracture Movements During Stimulation of the EGS Basel Reservoir

Abstract

During the first 100 hours of the Basel (Switzerland) stimulation, numerous sudden and sharp pressure drops (up to -1 MPa) and pressure build-ups (up to 1.7 MPa) concur with the timestamp of seismic events that are located close to the injection interval and that have generally stronger magnitudes of up to 1.7. After 100 hours, the effects of induced seismicity on pressures vanishes also for stronger magnitudes of up to 3.4. A 1D radial flow model that incorporates spatio-temporal variations of flow dimension (n), hydraulic conductivity (K) and storativity (S), can adequately reproduce the observed pressure and flow rate behavior of approximately 9 days of the stimulation including injection, shut-in and back-flow periods. In particular, the temporal variation of the flow dimension reveals different hydraulic regimes during and after injection, i.e., pseudo-radial flow during stimulation and channelized preferential flow during shut-in and backflow periods. The modelled pressure profile is in good agreement with the propagation of the observed seismic cloud. The pressures necessary for the propagation of the stimulation front are estimated to be in the order of 2 to 3 MPa. We conclude that basic hydraulic modelling can be used as a good approximation for the radial pressure distribution during stimulation of an EGS reservoir.