Induced seismicity associated with Enhanced Geothermal Systems

Abstract

Enhanced Geothermal Systems (EGS) have the potential to make a significant contribution to the world energy inventory. One controversial issue associated with EGS, however, is the impact of induced seismicity or microseismicity, which has been the cause of delays and threatened cancellation of at least two EGS projects worldwide. Although microseismicity has in fact had few (or no) adverse physical effects on operations or on surrounding communities, there remains public concern over the amount and magnitude of the seismicity associated with current and future EGS operations. The primary objectives of this paper are to present an up-to-date review of what is already known about the seismicity induced during the creation and operation of EGS, and of the gaps in our knowledge that, once addressed, should lead to an improved understanding of the mechanisms generating the events. Several case histories also illustrate a number of technical and public acceptance issues. We conclude that EGS-induced seismicity need not pose a threat to the development of geothermal energy resources if site selection is carried out properly, community issues are handled adequately and operators understand the underlying mechanisms causing the events. Induced seismicity could indeed prove beneficial, in that it can be used to monitor the effectiveness of EGS operations and shed light on geothermal reservoir processes.

Introduction

To produce geothermal energy economically on a commercial scale, sufficient fluid and permeability must be present in the targeted subsurface hot rock masses. In many cases, there is a need to increase permeability and/or fluid content, i.e. to enhance the natural geothermal systems. One of the issues associated with Enhanced Geothermal Systems (EGS) is the effect and role of the seismicity (or microseismicity) induced during the creation, or improvement in the properties, of an underground reservoir and subsequent extraction of geothermal energy (i.e. hot fluids) (Majer et al., 2005). Microseismicity has been successfully dealt with in a variety of environments. Cypser and Davis (1998) set out the legal responsibilities of reservoir impoundment projects, as well as oil and gas, mining and geothermal operations. In this paper, we review our current knowledge on the seismicity induced during the development and operation of enhanced geothermal systems, and highlight the gaps in knowledge that are an obstacle to a thorough understanding of the mechanisms generating the seismic events; we also present information that will hopefully prove useful when drafting and implementing protocols for monitoring and addressing community issues associated with induced seismicity.

Naturally fractured hydrothermal systems are the easiest sources from which to extract heat stored in the subsurface rocks, but the total resource and its availability tend to be restricted to certain areas. Their development proceeds where conditions are ideal for cost-efficient extraction. These hydrothermal systems are sometimes difficult to locate and also run a high risk of not being commercially feasible, if their geological, physical and chemical characteristics are not favourable.

The reasons for developing EGS technology are two-fold: (1) to bring uneconomic hydrothermal systems into production by improving their underground conditions (stimulation); and (2) to engineer an underground condition that creates a hydrothermal system, whereby injected fluids can be heated by circulation through a hot fractured region at depth and then brought to the surface to deliver the captured heat for power conversion or other uses. The second approach expands the available heat resource significantly and reduces the uncertainty of exploitation costs. However, the process of enhancing permeability and the subsequent extraction of energy may often generate microseismic events.

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 (e.g. the Soultz project in France; Baria et al., 2005) have raised residents’ concern related to both damage from single events and their cumulative effects (Majer et al., 2005). Some residents believe that the induced seismicity may result in structural damage similar to that caused by larger natural earthquakes. There is also fear that the small events may be the precursors of larger ones to come, that not enough resources have been invested in finding solutions to the problems associated with larger induced events, or in providing for independent monitoring of the seismicity prior to large-scale fluid injection and production operations. During the final phases of preparing this paper (December 2006–January 2007), a number of perceptible events (of magnitude¹ 3.4 or less) occurred in Basel, Switzerland, in the vicinity of the Deep Heat Mining project (http://www.dhm.ch/dhm-drillingInBasel.html). No structural damage was reported but the local authorities suspended operations until investigations were completed; it is not certain whether this project will be allowed to continue (see Section 9 below). This is an example of how a more comprehensive site selection study and understanding of the nature of the seismicity would have benefited the community at large, as well as the operators liaising with the public.

In recognition of the large potential of the geothermal resource worldwide, and in acknowledgement of the misunderstandings that might arise with regard to induced seismicity, the International Energy Agency (IEA) drafted a Geothermal Implementing Agreement (GIA), which took the form of an international collaboration (Majer et al., 2005). The mission of this collaboration, as stated in the “Environmental Impacts of Geothermal Development, Sub Task D, Seismic Risk from Fluid Injection into Enhanced Geothermal Systems Implementing Agreement (IEA/GIA)”, is 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.

As an initial starting point for achieving a consensus, three international workshops were organized with participants from a variety of backgrounds, including geothermal companies and operators. They were held during the Annual Meeting of the Geothermal Resources Council, Reno, NV, USA, in October 2005, and the annual Workshop on Geothermal Reservoir Engineering, Stanford University, Stanford, CA, USA, held in February 2005 and February 2006 (Majer et al., 2005, Baria et al., 2006). We present the results of these workshops, along with recent updates and recommendations for future studies and fieldwork.