EGS: The Geysers: Why do the operators of EGS activities want to avoid large Earthquakes?
In addition to the obvious reason for wanting to avoid large earthquakes—not wanting to create seismic hazards, potential danger, and community alarm—there is also a very good engineering reason to avoid larger events. It should be pointed out that from an engineering standpoint, when one is trying to perform enhanced geothermal activities, there is a target size for the volume of the subsurface permeability enhancement. In general, geothermal operators do not want to exceed a certain radius of enhancement beyond the injection well (usually a few hundred meters) This is because to extract the maximum heat from a given volume of rock, operators want to avoid any “fast paths” that would “short-circuit” the water and not allow the injected water to heat to the desired temperature. Large, long fractures could cause such a short-circuiting of the desired paths and possibly make the EGS project less economically feasible. Ideally, a large matrix of many small fractures is desired, to give the maximum surface area for heating the injected water. Hence, from an operational standpoint, many small events of less than M = 2.0 (source radius of 100 m or less) are beneficial. It therefore follows that as EGS operations continue, an operator would want to “grow” the production volume with many small events rather than have large events. Seismic monitoring is one of the few methods with which this permeability enhancement can be monitored.