The ten days lasting stimulation phase was composed of three cycles: The first and last cycle were conducted pressure-controlled. They aimed to quantify the initial and final injectivity as well as the pressure required to activate the pre-existing faults. The actual stimulation test was performed during the second cycle, where the injection rate was step-wise increased from 10 to 35 l/min (each rate was held constant for about ten minutes).
Various types of pre-existing structures have been stimulated increasing the injectivity rate by a factor of up to 1’850 (initial rates from 0.0006 l/min/MPa to 0.8 l/min/MPa vs. final rates from 0.4 to 1.7 l/min/MPa). The seismic activity showed a very heterogeneous result with total numbers of microearthquakes for the individual tests ranging between 6 and 940. Figure 1 shows the packer system that was utilized for the stimulation tests (left) and parts of the monitoring set-up (right). Figure 2 shows the microearthquakes for one test that were recorded and localized automatically during the second cycle.
A manifold monitoring system was in place to observe and register even smallest changes in the rock. It consisted of 32 seismic sensors, three tiltmeters, 60 strains sensors, 150 m of strain sensing cable and 13 pressure monitoring intervals.