A survey of the induced seismic responses to fluid injection in geothermal and CO2 reservoirs in Europe
Highlights
► We document the seismogenic response of crystalline and sedimentary rocks to fluid injection at 41 European sites. ► The objective is to identify factors that have a bearing on the magnitude of the largest seismic event. ► All cases of injection into crystalline rocks produce seismic events, albeit usually of non-damaging magnitudes. ► Injection at sites with low natural seismicity, defined by the expectation that the local peak ground acceleration has less than a 10% chance of exceeding 0.07 g in 50 years, has not produced felt events. ► The limited data suggest that low natural seismicity, corresponding to hazard levels at or below 0.07 g, may be a useful indicator of a low propensity for fluid injection to produce felt or damaging events. However, higher values do not necessarily imply a high propensity.
Introduction
Induced seismicity is recognised as a possible hazard in practically all engineering endeavours where stress or pore pressure in the subsurface is altered. This can be taken as a reflection of the realization that has dawned in the past 20 years that the Earth's crust generally supports high shear stress levels and is often close to failure. Historically, the most damaging events, which have sometimes caused fatalities, are associated with the impoundment of reservoirs (Gupta, 1992). However, earthquakes of a size sufficient to cause damage have also been associated with mining activity (Gibowicz, 1990), long-term fluid withdrawal (Segall, 1989) and fluid injection (Nicholson and Wesson, 1990).
Given that massive fluid injections to stimulate crystalline rocks have routinely been performed during Engineered/Enhanced Geothermal System (EGS) projects – formerly called Hot Dry Rock (HDR) projects – since the early 1970s, it is perhaps surprising that the issue of seismic hazard associated with these operations has only recently come to the fore. This is because the pioneering EGS developments at Fenton Hill (USA), Rosemanowes (UK), Hijiori (Japan) and Soultz (France) (3.5 km reservoir) did not produce events large enough to disturb the local population, whereas more recent attempts to develop systems at 4.5–5.0 km depth at Soultz, Cooper Basin (Australia) and Basel (Switzerland) produced events approaching or exceeding magnitude 3. There are also one or two instances where small but felt events have been associated with the operation of deep (∼3 km) hydrothermal systems.
The recent increase of interest in developing deep geothermal systems and of sequestering large quantities of CO2 underground makes it desirable to identify factors that influence the different seismogenic responses to fluid injection at various sites. This paper documents the first stage of an on-going study that seeks to determine such factors through examination of incidences where fluid injection has taken place without generating seismic events that were felt by the local population, as well as cases where it has. We make no distinction between induced and triggered seismicity.
The scope of this study is limited to reporting European case histories of injection-related seismicity, including the project to develop an EGS beneath the city of Basel in 2006. Most, if not all incidences of injection into crystalline rocks are included, regardless of whether events were felt. However, at this stage our coverage of injection into sedimentary rocks is not complete, especially for non-geothermal injections that did not produce felt earthquakes. Two sites where CO2 is injected into sedimentary rock are included. We exclude from consideration other types of induced seismicity associated with such activities as fluid withdrawal, excavation or reservoir impoundment since these may involve somewhat different mechanisms.
The case histories of injections into igneous and sedimentary target rocks are presented in separate sections and are ordered alphabetically according to country. This structuring has the disadvantage that case histories from the same geological province but different countries are not reported contiguously, although the incidences are few. The locations of the sites are shown in Fig. 1, and a summary listing of parameters for the sites is given in Table 1. A rock mass will be referred to as ‘critically stressed’ if the shear stress level it supports would produce failure of an optimally oriented fracture whose strength is described by Coulomb friction criterion with a coefficient 0.65. The maximum and minimum principal horizontal stresses will be denoted by SHmax and Shmin, respectively, and the vertical stress by SV.
For the purpose of comparing the natural seismic activity at a site, we used an index of estimated local seismic hazard which is based on an assessment for all of Europe that conforms to a single standard. This hazard is quantified in Table 1 and Fig. 1 by the peak ground acceleration (PGA) in units of ‘g’, the acceleration of gravity taken from the compilation of Giardini et al. (1999). The value of PGA for a site denotes the acceleration level on stiff soil that has a 10% probability of being exceeded in 50 years (equivalent to a recurrence period of 475 years). Regions of low hazard are characterised by values below 0.08 g, high hazard areas have PGAs above 0.24 g, and moderate hazard regions present intermediate values. It should be emphasised that this value is not necessarily a measure of the seismogenic response of the ground to injection, but rather, is a conveniently available index of natural seismic hazard at a site that is based on the peak earthquake-induced shaking that is likely to occur at the site. Consequently, PGA values will be high for localities with no natural seismicity if it neighbours a region where large events occur. Such situations are noted in the text.
Throughout this report, seismic magnitudes are given as either local magnitude, ML, duration magnitude, MD or moment-magnitude, MW. Whenever possible, macroseismic intensities are given in terms of the EMS-98 scale (Grüntal et al., 1998) and denoted ‘Io(EMS)’. Most macroseismic intensities given in this paper were derived before EMS-98 was defined. These values are denoted as ‘Io’ and should be broadly consistent with the EMS-98 classification. The most likely difference is that indistinct values such as Io = IV–V or Io = VI–VII would most probably be assessed as Io(EMS) = IV and Io(EMS) = VI respectively in the EMS-98 classification.
Section snippets
Le Mayet de Montagne, France
The site is located 25 km south-east of Vichy, France, at the northern fringe of the Massif Central where the granite outcrops. It was established in 1985 as an EGS test site with two boreholes drilled to 800 m depth. The local stress state is characterised as strike-slip with a Shmin/SV ratio of 0.55, and thus the rocks are critically stressed. Natural seismicity is low, although the historical record indicates that two events estimated to be of moment-magnitude MW 4.3 have occurred at distances
Upper Austria (South-German Molasse basin):
The region of Upper Austria within 25 km of the German border is host to a cluster of five doublets that utilise heat from the Malm carbonate aquifer. These are Simbach-Braunau, Altheim, Geinberg, Oberndorf and St. Martin (Goldbrunner et al., 2007). Here we describe the first three of these sites. The only available stress information in the area comes from a short (11 m) drilling-induced tension fracture imaged in a well at the Simbach-Braunau site which indicates an SHmax orientation that is
Discussion
The primary objective of this study is to document case histories of fluid injection with a view to identifying any systematic dependence of the seismic response on reservoir injection depth or the various parameters that constitute ‘geological setting’. It must be recognised from the outset that the available data are not ideal in this regard. Injections that involve a net fluid volume increase within the reservoir such as in hydraulic stimulation operations would, in principle, produce a
Acknowledgements
The work was supported by the CARMA (http://www.carma.ethz.ch) and GEOTHERM (http://www.geotherm.ethz.ch) projects funded by the Competence Centre for Environment and Sustainability and by the Competence Centre for Energy and Mobility of the Swiss Federal Institutes of Technology (ETH), and the Swiss Federal Office of Energy. We are grateful to the following people for providing unpublished data or information: Kristján Ágústsson (ISOR, Iceland), Gudni Axelsson (ISOR, Iceland), Julia Angerer
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