Energy analysis of microseismicity induced by fluid-injection in the Soultz-sous-Forˆets geothermal reservoir

Between 1993 and 2005, the Soultz-sous-Forˆets reservoir was stimulated through 4 different wells crossing the reservoir at two different levels R3 (about 3km deep) and R5 (about 5km deep). The figure below represents the N-S section of the reservoir with the geometry of the 4 wells. During these stimulation episodes, seismic and hydraulic data were recorded. Using hydraulic data (pressure and flow rate) and available seismic catalogs of the stimulation episodes in the Soultz-sous-Forˆets reservoir, an analysis of the evolution of the injected energy and seismic energy was made. The analysis revealed two seismic behaviors of the reservoir. First, the seismic energy grows linearly with the energy injected from a certain level of energy injected with a similar slope for wells stimulated a first time. The parts of the reservoir which are stimulated a second time (GPK1 in 1993 and 1996 and GPK4 in 2004 and 2005) show a more rapid growth of the seismic energy which can be explained by the Kaiser effect (a reservoir stimulated a first time will have to reach at least the maximum pressure level reached during the first stimulation to generate seismic activity again). Secondly, the seismic response of the deepest part of the reservoir (R5) is greater than the shallowest one (R3). Indeed, the injection efficiency, which is calculated by the ratio between the cumulated seismic energy and the cumulated injected energy shows a convergence towards 10−5 for R3 and 10−2 for R5.