SOURCE PROPERTY VARIATIONS OF INDUCED SEISMICITY IN GEOTHERMAL RESERVOIRS

As part of a geothermal project in Basel, Switzerland water was injected with high pressure for increasing the permeability of crystalline rock and creating an enhanced geothermal system. The hydraulic stimulation lasted about 6 days. More than 10,500 events were induced and several larger magnitude (up to M_L=3.4) earthquakes occurred shortly after well shut-in. The Basel case is a good example for the potential seismic risk associated with hydraulic fracking.

A better understanding of the physics of the induced events is needed to improve forecasting models that describe the induced seismicity. We analyze the seismicity with respect to local variations of the magnitude-frequency distribution (b-value), and event stress drop. Both properties are (i) seismological observables, and (ii) linked to the local stress field at the source. We can utilize these two observables as proxies for injection-induced changes in the stress field.

We estimate stress drops from the best-fitting corner frequency of the P-wave source spectra. The b-values are estimated for each event location using a fixed number of events in the vicinity. Both properties exhibit significant lateral and temporal variations: both change with radial distance from the injection point. The b-value increases over the first 100 m and then decreases with distance and time. Stress drop increases by about a factor of five between 10 m to 300 m. This suggests a correlation with injection-related medium perturbations.

We use an analytical solution of the linear diffusion equation to calculate the time-dependent pressure perturbation, assuming a homogeneous and isotropic medium. We find a good correlation of the pore pressure perturbation with stress drop and b-value. Stress drop is inversely proportional to the pore pressure perturbation. The largest events in the sequence are found at the edges of the seismicity cloud, in low b-value and high stress drop areas where the pore pressures changes are small compared to the stress changes caused by the seismicity itself. We attempt to describe the observations by forward-modeling the pressure-induced stress changes and the triggered seismicity based on Coulomb friction. We aim to establish a link between the mechanical properties of the source region and the probability of triggering an event of a certain magnitude.