Innovative Microseismic Monitoring Tools and Configurations for Geothermal Applications

Abstract

Stimulation and long term microseismic monitoring of Enhanced Geothermal System (EGS) developments must fulfil two roles; mitigation of seismic risk and imaging of the reservoir, for which the monitoring system must be both highly sensitive and accurate. In principle these aims can be achieved using a network of sensors in dedicated monitoring boreholes close to the reservoir. At the Utah FORGE (Frontier Observatory for Research in Geothermal Energy) such a network has been employed to monitor a stimulation at around 8500ft depth bGL. Innovative, high temperature 3C geophone, 8 level strings were deployed in three boreholes at ~1-2000ft offset from the stimulation zone where the tools were at up to ~205°C. In addition, a string of fibre optic 3C sensors, wireline DAS and behind casing DAS were also deployed on a trial basis at 3-4000ft depths. A more economic approach to the installation of sensors close to the reservoir has been trialled at the Bedretto Underground Laboratory in Switzerland. There a 4 level, 3C clamped geophone string was installed in the stimulation hole beneath the injection zone. These tools enabled microseismic events of ~-3Mw to be detected and located during an experimental stimulation and demonstrated how EGS monitoring may be achieved at significantly lower cost using sensors deployed in the injection hole rather than in dedicated, deep monitoring boreholes.