The role of triggering by static stress transfer during geothermal reservoir stimulation

During creation of an Enhanced Geothermal System, massive fluid injections are conducted to induce fracture shear which generates reservoir permeability. In this study we analyze coseismic static stress transfer caused by induced seismic events during a stimulation at the European research project at Soultz-sous-Forets (Alsace, France). For this purpose we developed an efficient method to calculate coseismic static stress changes from an elliptical slip distribution on a circular fracture using superposition of rectangular sources, which enables us to apply an analytical solution for fast computation. This method is applied on a data set of 715 focal mechanisms derived from seismic recordings of the stimulation of the well GPK2 to calculate temporal evolution of static stress transfer. We find that the resulting structure of coseismic stress changes can be divided into three parts: a quiet zone where no spreading of seismicity occurs, an active zone within the created reservoir with ongoing fracturing and a process zone where the growth of the reservoir occurs. Static stress changes in the active zone are of the order of 1 MPa, both positive and negative, but may exceed this value considerably on a local scale. Analysis of stress changes from a cluster of events that occurred after shut-in lets us conclude that triggering by coseismic static stress changes is possible for some events. Our analysis shows that triggering by static stress transfer plays a minor role for injection induced seismicity in a volumetric reservoir, whereas it can be quite effective for rupture propagation along single large fault zones.