GEISER

Geothermal Engineering Integrating Mitigation of Induced Seismicity in Reservoirs

The aim of the project is to contribute to the improvement of the concept of Enhanced Geothermal Systems by addressing the need to investigate the role of induced seismicity.

The term induced seismicity (IS) refers to typically minor earthquakes and tremors that are caused by human activity altering the stress field of the Earth's crust. IS events are predominantly of very low magnitudes and, in most cases, human activity is merely the trigger for an earthquake that would have occurred naturally in any case but at a later time. Despite there being many similarities between IS and natural earthquakes, there are also notable differences. The most significant being the aspect of human intervention in natural processes.
Large Magnitude Events (LME). IS events range from moderately-sized (4<M<6), damaging earthquakes with rupture planes on the order of km² through rock bursts (2<M<4) down to microseismicity and acoustic emissions (M<2, rupture planes ≤ m²) for which humans are insensible and thus can be observed only by sensitive seismic instrumentation. The largest IS event recorded was a M=7.0 gas withdrawal event in Gazli (Uzbekistan) in 1984 (Simpson & Leith 1985). The largest mining induced seismic event was a M=5.6 rock burst that occurred in a potash mine near Völkerhausen, Germany in 1989 (Knoll, 1990). Significant IS events associated with long-term well injection include the 1966 M=5.2 event at the Rocky Mountain Arsenal, Denver (Evans 1966). EGS operations caused some notable events arising from short-term stimulation measures, for example the 2003 M=3.7 event at the Cooper basin EGS site, Australia (Baisch et al., 2006) and the 2006 M=3.4 event in Basel (Switzerland). Numerous sizable events have been associated with gas withdrawal including the 1996 M=3.8 event near Strachan (Alberta), events up to M 4.2 near Lacq, France (Feigner and Grasso, 1991), and up to M=3.5 in the Netherlands (e.g.,Van Eck et al., 2006). Enhanced oil and gas recovery events are usually below M=3, but possible IS associated with well operations have local magnitudes as high as 6.5 (Nicholson & Wesson 1992).
The mitigation of IS to an acceptable level is the major intent of this project. For this purpose, we set as our goals:
• to understand why seismicity is induced in some cases but not in others;
• to determine the potential hazards depending on geological setting and geographical location;
• to work out licensing and monitoring guidelines for local authorities, which should include a definition of what level of ground motion is acceptable; and
• to develop strategies to fulfil the task of the stimulation and improve the hydraulic properties of the geothermal reservoir without producing LMEs that pose a threat to buildings and disturb the public.