Interpretation of pore-pressure propagation and fluid flow in the Soultz geothermal reservoir based on induced microearthquake multiplets

In this paper, we discuss the pore-pressure propagation and fluid flow in the geothermal reservoir at Soultz geothermal field, France, by making use of the induced microearthquakes during hydraulic stimulation. The locations of possible seismic and hydraulically activated fractures can be estimated using the induced seismic events associated with the shear slip of preexisting fractures. We propose an evaluation method for the critical pore-pressure in fractures and the corresponding propagation in reservoir during hydraulic stimulation by using an induced multiplet-clustering analysis. The multiplet-clustering analysis helps to determine the precise source location of microearthquakes using cross-spectrum and clustering analyses. The accuracy of source location can be improved by this method, and the orientation of fracture planes which would induce shear slip and generate microearthquakes, can be estimated. The source locations of induced microearthquakes during the hydraulic stimulation at Soultz field, France, in 1993 were determined. Following the multiplet-clustering analysis, small planar clusters corresponding to each multiplet were identified. Since it can be interpreted that each cluster indicates the fracture plane for shear slip, we can calculate the best fitting plane using the source locations. We can also calculate the pore-pressure for fracture in the case of shear slip by using the orientations of fractures and the stress field based on Coulomb's criteria. The estimated critical pore-pressure required for shear slip suggest that the fluid pressure in fractures has a higher value near the fractured zones around 2900 m inside the well, and was found to decrease with increase of distance of these fractures from the zones. The results imply that the fluid pressure was transmitted into the fractures, and that the pore-pressure around the fracture zones increased up to near maximum of the fluid pressure in the fracturing well. It has been reported that the fractures near 2900 m have a low flow rate. The upper limit of the estimated critical pore-pressure started to decrease exponentially at distances around 120 m from the feed point. This result suggests that the injection pressure was almost the same near the borehole and decreased exponentially as a function of distance.