Accueil > EGW 2018 : 6th European Geothermal Workshop > Abstracts > Session 5 : Sustainability, Environment, Regulation Framework, and Societal (...) > Session 5 : Poster Presentations

Session 5 : Poster Presentations

S5.1 Earthquake size controlled by hydraulic injection parameters : case study of the 1993 Soultz-sous-Forêts injection

Léna Cauchie, Olivier Lengliné, Jean Schmittbuhl

PDF - 430.3 ko
Cauchie et al.

The injection of fluid in the upper crust, notably for the development or exploitation of geothermal reservoir, is often associated with the rise of induced seismicity. The occurrence of large seismic events during such operation needs to be reduced in order to preserve the infrastructure and population nearby the injection site. However it is not well clear how the injected fluid influences the characteristics of the induced events. Here we investigate earthquakes that occurred during an injection in Soultz-sous-Forêts, France in 1993 in order to study the link between the injected fluid and the source properties of the induced earthquakes. We take advantage that during this experiment, numerous seismicity was generated and borehole accelerometers were running at proximity from the injection site. We estimate the moment and radius of all recorded events based on a spectral analysis and classify them into 663 repeating sequences. We show that the events typically obey the typical scaling law between radius and moment but that at the scale of the asperity fluctuations of the moment are important while the radius of the events remain similar. We also evidence globally an increase of the events radius (and moments) over the course of the injection that follows the increase of the wellhead pressure. These observations suggest that the fluid pressure has a direct control on the rupture size of the induced events.

S5.2 State of knowledge about the risks and impacts associated with deep geothermal

Philippe Gombert, Franz Lahaie, Auxane Cherkaoui

PDF - 537.4 ko
Gombert et al.

Deep geothermal is a renewable and non-intermittent source of energy that can contribute to the global transition towards a lower emission of carbon and less greenhouse emitting energy mix. Only a small share of the world’s geothermal potential is being exploited today and many countries, including France, have included in their objectives an accelerated development in this renewable energy for the coming decades (Bertani, 2015).

Like any industrial activity, deep geothermal drilling is accompanied with potential inconveniences and possible risks for people and the environment, which must be clearly identified and controlled in order to make this industry fully compatible with the expectations and the needs of the citizens, especially those living near such facilities (UCS, 2016). In past years, some concerns have been expressed by local authorities regarding the development of deep geothermal projects, particularly in the field of high temperature, based on the risks associated with such underground operations.

We present the current knowledge about the risks and impacts associated with deep geothermal. In addition to the scientific literature, it is based on the feedback from incidents or accidents already recorded in this field . It also capitalizes on INERIS’ expertise in the field of risks related to other sectors, such as hydrogeology or oil and gas wells, to provide a larger perspective of deep geothermal technologies.

A census of accidents or incidents reported in the field of deep geothermal reported 35 events, of which 32 were deemed relevant to the safety conditions currently in the framework of this industry. Of these 32 events, one death and nine wounded were recorded in almost three decades of feedback and over about 1,700 geothermal installations operating in the world. Other types of consequences are property damage on the surface (buildings or infrastructures), local pollution or psychological impacts on the inhabitants. The overall impression that emerges from this assessment is that deep geothermal energy benefits from a rather weak accidentology. It should be noted that most of these types of accidents are not specific to deep geothermal energy and can appear in any well extraction of subsurface resources (hydrocarbons, drinking water supply, underground gas storage, etc.). On the other hand, the context of geothermal energy offers conditions that are more favorable to certain types of accidents and less favorable to others.

Main accidental risks can be divided into 3 main themes (Gombert et al., 2017) :
1) Accidental release of surface fluids (blowout), gaseous emissions or effluents of fluids (Kage et al., 1998) ;
2) Contamination of sensitive aquifers, due to longitudinal or lateral loss of integrity of a well (Vernoux et al., 2002) ;
3) Felt seismic events (Zang et al., 2014) and noticeable ground movements as upheaval or subsidence (Allis, 2000) ; seismic events are mainly due to the stimulation operations (particularly hydraulic), but also to the production phase ground movements are respectively due to overpressure induced in the geothermal reservoir or to significant reservoir depletion (incomplete reinjection of geothermal fluid).

All the risks and impacts have been summarized in Table 1 with a qualitative rating scale to compare them, in terms of their likelihood of occurrence and the severity of the consequences they might have. This assessment is carried out for each phase of the life cycle of a geothermal site (drilling, testing, production, post-abandonment) and is based on a scale of 4 values. It should be noted that this is a generic assessment, which is not intended to replace the specific analysis that has to be carried out in the context of each site.

S5.3 Similarity of natural swarms and induced seismicity earthquake catalogues

Martin Mazanec, Leo Eisner

PDF - 462.8 ko
Mazanec and Eisner

A growing number of confirmed cases of human induced seismicity (e.g. Ellsworth, 2013) and their socio-economic impact began receiving much more attention in recent years. This has created a strong demand to characterize this seismicity and to find suitable methods to distinguish induced from natural seismicity (Schoenball et al., 2015). Reliable differentiation of the earthquake origin is crucial for the ability to mitigate and control the induced seismicity and safely carry out various underground operations. However, no reliable and clear rules for the discrimination between natural and induced seismicity have been firmly accepted so far (Dahm et al., 2015).
Here we present analysis that examines statistical temporal-magnitude features of various natural and induced seismicity catalogues. We choose six criteria that are commonly used to characterize natural swarm or mainshock-aftershock earthquake sequence and apply them to ten different earthquake catalogues representing natural swarms, tectonic mainshock-aftershock (MS-AS) sequences as well as induced seismicity sequences to see if natural and induced earthquakes share the same behavior.

METHOD
For this analysis we choose ten different earthquake catalogues from all around the globe. These catalogues represent different types, swarms and MS-AS as well as natural and induced sequences. Catalogues source is either public or private.
a) Natural volcanic catalogues : Mt. Etna, Sicily, Italy ; Yellowstone, United States of America ; Arkansas, United States of America ; West Bohemia, Czech Republic
b) Tectonic catalogues : Hector Mine, California, United States of America ; Parkfield, California, United States of America ; Wheeler Ridge, California, United States of America
c) Induced seismicity catalogues : North America (private catalogue) ; Preese Hall, United Kingdom of Great Britain and Northern Ireland ; Azle, Texas, United States of America

DATASETS
We selected six criteria used for differentiation of tectonic and swarm seismicity as they are suitable for temporal-magnitude sequences without exact location. For each criterion, certain assumptions were discussed for which the dataset was interpreted either as a swarm or MS-AS.
a) Events density : comparing the total number of earthquakes to the maximum daily number of earthquakes (Mogi, 1963).
b) & c) Mainshock position within the sequence : (b) the ordinal number of the mainshock within the sequence and (c) the timing of the mainshock event relative to the rest of the seismicity in sequence.
d) Mainshock – aftershock magnitude comparison : based on method presented in Zobin et al. (2005) magnitude gap of two largest events in the swarm seismic sequence is usually smaller or equal to 0.5 magnitude units.
e) Comparison of the number of events and magnitude of the mainshock : based on e.g. Vidale and Shearer, 2006 comparison of the magnitude of the largest event (Mmax) in a sequence to the overall number of events above the level of magnitude completeness Mc may differentiate between swarms and tectonic earthquakes.
f) Inter-event time : the temporal distribution of events within the sequence using the time gap between two following events (inter-event time) above the completeness (Mc) (e.g. Shcherbakov et al., 2005).

CONCLUSIONS
Analysis shows that all three induced seismicity datasets are identified as swarms and can be distinguished from the MS-AS. This observation is in consistence with our assumption that human induced seismicity behaves similar to natural swarm. However, none of the used statistical criteria is able to clearly distinguish induce datasets from natural swarms. Perhaps additional information on human activity it is possible (e. g. correlation with fluid injection parameters).

Combination of the selected criteria may be a useful tool for distinguishing induced seismicity from natural tectonic MS-AS in region with lack of the natural swarm seismicity.

None of the used criteria is always reliable for distinguishing different types of earthquake sequences. Each of them failed for at least one dataset and gives good result for specific type of seismic sequences. The most successful criterion is a comparison of the number of foreshocks and aftershocks and criterion of IET distribution which are both observed to be less sensitive to the quality of the seismic catalogue. Combination of all six criteria reliably distinguished four out of five natural swarms and three out of four natural MS-AS.

S5.4 Microgravity monitoring of the Soultz-sous-Forêts and Rittershoffen geothermal reservoirs (Alsace, France)

Nolwenn Portier, Jacques Hinderer, Umberto Riccardi, Gilbert Ferhat, Marta Calvo, Yassine Abdelfettah, Jean-Daniel Bernard

PDF - 362.1 ko
Portier et al.

We studied the mass redistribution induced by the geothermal production of the Soultz-sous-Forêts and Rittershoffen geothermal plants in Alsace (France). To do so, we measured 13 gravity stations on each network in the period 2014-2017 including the most recent production episode. Indeed, the Soultz-sous-Forêts enhanced geothermal system has produced 1.7 MWe using 3 wells 5 km deep since the 24th of June 2016. The Rittershoffen geothermal plant has been designed to produce 24 MWth heat power with 2 wells 2.5 km deep since the 19th of May 2016. Time-lapse relative microgravity measurements had been performed since 2014 and 2015, on the Soultz-sous-Forêts and Rittershoffen networks, respectively. After tide and drift correction, gravity double differences were calculated to retrieve the gravity variation at each measuring point compared to a reference time and station. The vertical deformation was controlled by leveling measurements at gravity stations as well as by collecting continuous GNSS data near the production/injection wells. The leveling data are not correlated to the gravity double differences and they do not exceed 1 cm. Hence, we consider that the observed gravity variations are only due to the subsurface mass changes. Moreover, we monitored the stability of the reference stations by evaluating their gravity changes with respect to the gravity observatory of Strasbourg STJ9. Finally, no signals are detected on the Rittershoffen network. We do notice small changes at Soultz-sous-Forêts network in agreement with the position of the injection and the production wells. We also note that the maximum gravity value appears in the same area as the induced seismicity. However, a simplistic Newtonian attraction model of mass changes of geothermal origin does not explain the results when taking into account the known depth and rates of the injection/production flows.

S5.5 Conceptual model assessment for sustainable direct use of geothermal resources : The case of Los Andes – San Felipe basin (central Chile)

Matías Taucare, Linda Daniele, Gloria Arancibia, Benoît Viguier, Diego Morata

PDF - 480.7 ko
Taucare et al.

Around the world, heating is a major requirement. The growing demand of energy has led to significant pollutant emissions, which resulted in the development of unconventional heating technologies, such as geothermic. Low temperature geothermal resources are abundant, and its extraction is not so complicated due to the low reservoir depth and the recent technological advances. In many cases, groundwater is used as heat transfer medium (Self et al., 2013). For this purpose, the amount and the quality of the used groundwater resource need to be protected.

To ensure a successful operation over years, it is necessary to understand the hydrogeological functioning of the exploited aquifer, especially in arid and semiarid regions were the water resources are limited. In this context, the recharge processes have to be reliably assessed. In central Chile (30 – 38°S), the groundwater recharge occurs along the West Andean Front (mountain front). Due to the orographic gradient (from 2700 up to 500 m a.s.l at 33°30’S), this area receives more precipitation than the basin floor (ca. 500 m a.s.l at 33°30’S) and the localized infiltration in the fractured bedrock facilitates the recharge (Wilson and Guan, 2004).

At Los Andes-San Felipe area (32°30S), several low–temperature springs outflow at different elevations. In order to improve the understanding of the recharge processes supplying the regional groundwater flows, a hydrogeochemical study (major, minor and trace elements, combined with δD, δ18O) was realized on 29 groundwater points. Preliminary results show that major ion concentrations have a negative correlation with the altitude unlike the temperature, pH and Eh. Furthermore major ions suggest that water-rock interaction processes control the chemistry along a same groundwater flow path. Our results indicate that the West Andean Front area in central Chile is a high potential place for low temperature geothermic resources exploitation.

3 octobre 2018