| Abstract |
In contrast to geo-pressurized hydrothermal systems, where it is often possible to exploit the heat in a simple production mode, EGS (Enhanced Geothermal Systems) or HDR (Hot Dry Rock) systems in low permeability host rocks require the re-injection of fluid in order to sustain the pressure and a steady flow in the reservoir. In the ideal case, this is realized in a virtually closed loop system with a mass balanced circulation between the production and the injection wells. In turn, a balanced circulation places high demands on the connectivity of the man-made subsurface heat exchangers. To achieve the desired connections, a hydraulic stimulation has to be designed and performed in the context of the total reservoir geometry and not only for each well separately. Numerical computations show that even small gaps between the stimulated zones result in a significant drop in productivity, even though each well has been individually stimulated to a high degree. For example, such a situation can arise when a stimulation job is aborted too early. On the other hand, field examples also show cases where the efficiency of a hydraulic stimulation decreased during pumping without being notified in the hydraulic data. This creates superfluous costs if the pumping strategy is not adjusted accordingly. The risk of both, creating heat exchangers of poor connectivity and inefficient pumping can be significantly reduced when the spatio-temporal expansion of the stimulated zones is monitored and interpreted already during the operation. For this purpose, the analysis of microseismic events occurring during a massive hydraulic stimulation has established itself as the most promising method. However, the lack of computing power and reliable and fast algorithms, as well as the extraordinary size of the data sets, sometimes exceeding several ten-thousand events, has limited a real-time analysis of microseismic data in past experiments. In the field, mainly, and information on the spatial impact of the operation was often not available when needed. Recent developments have yielded a software package capable to process and interpret huge microseismic data sets automatically and in real-time speed. By an integrated seismo-hydraulic analysis this software provides a valuable tool for the decision making in the field. |