Seismic Source Parameter Evaluation and Its Importance in the Development of an HDR/EGS System

Abstract

In 2000 and 2003, massive hydraulic stimulations
were carried out at the European Geothermal Hot Dry
Rock (or EGS) site at Soultz-sous-ForÍts, France.
These stimulations were performed in the 5000 m
deep wells in the crystalline basement. The objective
was to create a dense network of enhanced
permeability fractures, which would form the heat
exchanger. The injection of water in the fractured
rock generated a high level of microseismic activity.
Around 30,000 and 90,000 micro-earthquakes were
triggered during the injection of 2000 and 2003
respectively. From this around 14,000 and 9,000
events were then located and processed. Source
parameters for these events were calculated using
Brune's model on acceleration traces. An automatic
process based on a genetic algorithm was developed
to calculate source parameters such as seismic
moments, moment magnitudes, stress drops. Large
events were induced during the shut-in period and the
spatial distribution of stress drops suggests that a
redistribution of stresses may be occurring in the
reservoir. Some of these big events may be caused by
the release of locked up stress on specific fractures.
Events in the range of up to 2.9 Ml have been
generated during these periods and it is felt that the
detailed understanding of the source parameters may
be a way forward for reducing these larger events by
revaluating the stimulation strategy.