Seismicity Based Reservoir Characterization of Basel Geothermal Site

摘要

The hydraulic stimulation of a reservoir is used to enhance its permeability. It is a common technique for developing geothermal energy sites. In 2006, a stimulation experiment has been performed in the city of Basel. Thereby, more than 15,000 microearthquakes have been triggered since beginning of the operation, from which about 3,500 could be located. We analyse the spatio-temporal distribution of microseismic events induced during the period of continuous injection. Assuming that the triggering mechanism is a diffusion process of pore pressure perturbations initiated at the injection source, we define hydraulic characteristics of the stimulated reservoir using different SBRC (Seismicity Based Reservoir Characterization) techniques. They provide consistent estimates of scalar permeability as well as of permeability tensor by accounting for hydraulic anisotropy. We obtain a reservoir permeability in the range of 10-75 micro-Darcy. The results agree with permeability estimate from independent measurement. Furthermore, we analyse the frequency-magnitude distribution of induced events. We present a formulation for the cumulative probability of inducing a given magnitude event which accounts for time-dependent flow rates. It obeys the Gutenberg-Richter relation with an increased b-value compared to the distribution of natural seismicity in the Basel area.