Microseismic Monitoring of Hydraulic Experiments Undertaken During Phase Iia of the Soultz HDR Project (Alsace, France)

Abstract

In the framework of the European HDR Project of SOULTZ, co-sponsored by European Community, two 50 hour-hydraulic injections were carried out in the main borehole GPK1. This borehole reached a depth of 2000 m (600 m of granite under a 1400-m-thick sedimentary cover) and a bottom hole temperature of 140?C. Both injection s tests were made between a packer sited at 1968 m and bottom hole, with injection flow-rates of 7 1/s and 15 l / s. 3-axis accelerometer probes designed to withstand high temperatures up to at least 135?C were installed at the bottom of three boreholes which reached the granite. This network was supplemented by high temperature hydrophone sensors deployed in a fourth borehole (EPS1) at depth of about 2000 m. Data were transmitted to an on-line computer to perform prel iminary interpretations. During the test s, 135 and 239 induced microseismic events respectively were recorded. Present results show that seismicity starts of f very close to the inject i o n point and spreads out to form a cloud with a North-West to South-East e l ongat i on.