EGS: What is being done to understand and control the seismicity as well as interact with the local communities?
Induced seismicity is an important reservoir management tool, especially for EGS projects, but it is also perceived as a problem in some communities near geothermal fields. Events of magnitude 2 and above near certain projects have raised residential concern for both damage from single events and their cumulative effects. Some residents believe that the induced seismicity may cause structural damage similar to that caused by larger natural earthquakes. There is also fear that the small events may be an indication of larger events to follow. A related concern is that not enough resources have been invested in trying to answer some of the questions associated with larger induced events, and in providing for independent monitoring of the seismicity.
Recognizing the potential of the extremely large resource worldwide, and recognizing the possibility of misunderstanding about induced seismicity, the Geothermal Implementing Agreement under the International Energy Agency (IEA) initiated an international collaboration on induced seismicity in 2004. The purpose of this collaboration is stated in the “Environmental Impacts of Geothermal Development, Sub Task D, and Seismic Risk from Fluid Injection into Enhanced Geothermal Systems Geothermal Implementing Agreement (IEA/GIA)” as follows:
Participants will pursue a collaborative effort to address an issue of significant concern to the acceptance of geothermal energy in general, but EGS in particular. The issue is the occurrence of seismic events in conjunction with EGS reservoir development or subsequent extraction of heat from underground. These events have been large enough to be felt by populations living in the vicinity of current geothermal development sites. The objective is to investigate these events to obtain a better understanding of why they occur so that they can either be avoided or mitigated. Understanding requires considerable effort to assess and generate an appropriate source parameter model, testing of the model, and then calculating the source parameters in relation to the hydraulic injection history, stress field and the geological background. An interaction between stress modeling, rock mechanics, and source parameter calculation is essential. Once the mechanism of the events is understood, the injection process, the creation of an engineered geothermal reservoir, or the extraction of heat over a prolonged period may need to be modified to reduce or eliminate the occurrence of large events.
The U.S. Department of Energy was a principal player in this activity and helped organize and carry out activities as described in the agreement above. As an initial starting point for achieving a consensus, the Department of Energy Geothermal Technology office supported three international workshops that were organized in 2005 and 2006 with participants from various backgrounds, including geothermal companies and operators. After three international workshops and participation by 17 different countries, an initial induced seismicity protocol was developed for operators of geothermal fields to follow. This was intended to be a general document that set out general procedures. Specific protocols for different areas will supplement the general protocol, depending on potential for seismicity and proximity to local communities. These activities continue to address research needs and refine protocols for induced seismicity.