Accueil > EGW 2018 : 6th European Geothermal Workshop > Abstracts > Session 6 : Exploration of Geothermal Reservoirs > Session 6 : Poster Presentations

Session 6 : Poster Presentations

S6.1 Comparison between land gravity and thermal gradient in geothermal area to discriminate potential zones

Yassine Abdelfettah, Jacques Hinderer , Marta Calvo, Eléonore Dalmais, Vincent Maurer, Albert Genter

PDF - 454.2 ko
Abdelfettah et al. 2

We present and discuss new results obtained from new accurate gravity data showing average data uncertainties around 0.02 mGal. The data were acquired in 2013 and 2016 in the Upper Rhine Graben within the “Electricité de Strasbourg” permits located in northern Alsace. We achieved a qualitative study as well as a quantitative interpretation based on existing 3D geological model built mainly from seismics and borehole data. The mismatch between the observed and the computed Bouguer anomalies is also interpreted in terms of negative and positive anomalies that express lack and or excess of density values. These studies including a stripping approach are conducted in the whole area but more attention has been given to the seven subareas where seven thermal gradient boreholes have been done. At the end, a very interesting correlation is observed between gravity and thermal gradient results, converging to two distinct boreholes F3 and F5 showing high geothermal potential compared to the other boreholes. The objective of this paper is to present this innovative approach using high accurate gravity data to discriminate high potential geothermal areas.

S6.2 Geochemical characterization of a low-enthalpy shallow hydrothermal system as a proxy for a non-conventional deep geothermal potential, an example from the southeastern Sierra Madre Occidental, Mexico

Andrea Billarent-Cedillo, Gilles Levresse

PDF - 453.2 ko
Billarent Cedillo et al.

In Mexico the productive geothermal fields are intimately related to recent volcanism (Flores-Armenta, 2014 ; Gutierrez-Negrín et al., 2015). However, some low enthalpy geothermal springs are distributed far from any volcanic evidences (Ordaz-Méndez et al., 2011). These non-conventional geothermal activities seem to be related to the crustal composition (U, Th, K rich magmatic event) or thickness (DiPippo, 2016). In central Mexico several Miocene-Eocene grabens, such as the Juchipila graben, were appointed to have shallow non-conventional geothermic activities. In this specific locality the hydrothermalism manifests as springs with temperatures of 35-60°C. The shoulders of the graben consist of a volcanic-sedimentary succession, comprised mainly of Oligocene ignimbrite, some rich in U, Th, K, and Miocene basaltic-andesitic lava flows. The valley is filled by late Miocene continental sedimentary rocks : sandstone, conglomerate, limestone, shale and ash-fall deposits (Lahier, 1982 ; Webber et al., 1994 ; Carranza-Castañeda et al., 2013). This research intends to characterize the shallow non-conventional geothermal system as an expression of a potential deeper reservoir related to the equivalent plutonic event.

This work presents the hydrogeochemical results of 40 samples from shallow wells (>50 m), superficial wells (<50 m) and springs collected throughout the Juchipila graben. Two major groups of water were distinguished from the physical and chemical properties : (1) Ca-Mg-Na- HCO3 and (2) Na-SO4-HCO3. The first group of water has temperatures of 25-32°C and is characterized by a high concentration in NO3. These samples were collected in wells located in the valley zone which corresponds to a shallow and granular aquifer. The second group of water has temperatures of 28-58°C and is characterized by a high concentration of Li, F and As. Most of these samples were collected in springs and wells located near the graben’s shoulders or close to faults within the valley, so they pertain to a deep fractured aquifer in the volcanic rocks. There are several water samples that consist of a mixture between these two groups.

In the classification diagram of thermal waters Cl-SO4-HCO3 (Giggenbach, 1991), the Ca-Mg-Na-HCO3 and most of the Na-SO4-HCO3 type of waters are classified as peripheric waters (rich in HCO3), three of the Na-SO4-HCO3 samples fall in the steam-heated field (rich in S). The relations between conservative elements such as Cl, Li and B show patterns that correspond to a mixture between the two sample groups. In the ternary diagram Cl-Li-B (Giggenbach, 1991) the Ca-Mg-Na-HCO3 and Na-SO4-HCO3 types of water have a clearly different distribution, which confirms the coexistence of two water families in a compartmented aquifer.

According to the ternary diagram Na-K-Mg (Giggenbach, 1991), all the Ca-Mg-Na-HCO3 samples correspond to immature water, while some of the Na-SO4-HCO3 samples fall in the partially equilibrated waters with temperatures below 120°C. The geothermometers of silica and Na-K-Ca (Fournier, 1981 ; Fournier and Potter, 1982) were applied to the latter, obtaining temperatures in of 40-90°C and 50-90°C respectively. The silica geothermometer temperatures agree with the temperatures obtained with geothermometric modeling of chalcedony saturation index (55-95°C). These chemical equilibrium temperatures are analogous with the temperatures measured in surface (28-58°C).

The δ2H and δ18O values distribute along one evaporation line which intercepts the GMWL in (-11,-80), evincing the meteoric origin of the thermal water. The Na-SO4-HCO3 waters are located closer to the GMWL, while the Ca-Mg-Na-HCO3 which correspond to the shallower aquifer fall further showing a higher extent of evaporation.

Two shallow aquifers were identified : (1) a cool (25-32°C), Ca-Mg-Na-HCO3 water-type, rich in NO3 and (2) a “hot” (28-58°C), Na-SO4-HCO3 water-type, rich in Li, F and As. The hot aquifer dynamic is controlled by a particular geologic succession that superimposes two hydrogeologic units : (1) igneous rock rich in U, Th, K and (2) the thermally insulating sedimentary sequence of a paleo-lake. This geologic configuration allows the generation of a non-conventional low-temperature geothermal system. The geochemical characterization of the fluids suggest the meteoric origin of both aquifer waters and more important no volcanic input to the system. On the other hand, the low chemical equilibrium temperatures (<100ºC) confirm the shallow position of the geothermal reservoir (< 1km). The thermal efficiency of such shallow reservoir exhorts the quest for an equivalent deeper plutonic thermal unit for water injection.

S6.3 Performance of Ground Source Heat Pump system and energy migration assessment around Deep Borehole Heat Exchanger

Chaofan Chen, Haibing Shao, Francesco Parisio, Olaf Kolditz

PDF - 436.9 ko
Chen et al.

To meet huge heat demand of buildings in densely inhabited district during winter, ground source heat pump systems (GSHPS) are often coupled with deep borehole heat exchangers (DBHE), in which the coaxial pipes are installed (Kong, et al. 2017). Recently, thermal power of GSHPS coupled DBHE is overstated and the decrease of outflow temperature of DBHE and coefficient of the whole system performance (CWP) is underestimated. In this work, a comprehensive numerical model was constructed in the OpenGeoSys (OGS) simulator (Shao, et al. 2016) including flow and heat transport inside the DBHE and surrounding ground, coupled with dynamic performance of heat pump. The numerical model was verified through comparing with analytical solution proposed by Beier (2014) and field data from Qingdao, China. Based on the geological parameters in Qingdao area, different scenarios were simulated to investigate the evolution of outflow temperature of DBHE system and performance of the whole system. It was found that the sustainable specific heat extraction rate is kept around 100-150 W/m. And the minimum outflow temperature of DBHE at every heating season reaches quasi-state after 15-year’s operation. Besides, heat flux distribution of DBHE and equivalent temperature drop in ground were calculated to quantify the amount of extractable deep geothermal energy via DBHE system. It was indicated that the geothermal energy in ground around DBHE is the main source of extracted energy, which accounts 99.83 % of the total energy during 115 days’ heating season considering the heat supply from deeper ground.

S6.4 Geophysical signature of the transition zone between the sedimentary cover and the basement : an analogue approach to help de-risking geothermal prospects.

Mathieu Darnet, Chrystel Dezayes, Jean-François Girard, Jean-Michel Baltassat, Catherine Lerouge, Thierry Reuschlé, Nicolas Coppo, François Bretaudeau, Julien Porté, Yann Lucas

PDF - 1.2 Mo
Darnet et al.

Within the frame of the ANR-funded CANTARE-Alsace project, we have undertaken a multi-scale and multi-disciplinary approach to increase our knowledge of the transition zone between the sedimentary cover and the basement and provide fundamental knowledge for the assessment of its geothermal potential. In this paper, we report out the results of a study performed on an exhumed transition zone in the Ringelbach area in the Vosges Mountain, on the flank of the Rhine graben. In this analogue of a deeply buried transition zone of the Rhine Graben, a thin layer of Permian sandstones is still present on the top of the fractured and altered granitic basement providing the unique opportunity to study in-situ the physical properties of this transition zone. In this paper, we focused on electrical and acoustic properties of the transition zone as they are the main physical parameters usually assessed with the help of geophysical methods during the exploration phase of a geothermal project.

To characterize the electrical resistivity distribution of the transition zone, we acquired both shallow electrical resistivity tomography (ERT) profiles and deep 3D controlled-source electromagnetic (CSEM) soundings (figure 1). To interpret such data, we looked into the resistivity logs acquired in two 150m-long exploratory boreholes drilled in this area cutting through the transition zone as well as resistivity measurements performed on representative core-plugs from the two boreholes. This study shows that the fractured and altered basement exhibits low resistivity signatures (from several tens to hundreds of Ohm.m) over several hundreds of meters, mainly caused by the presence of alteration clay minerals and fluid filling the fractures. On the contrary, unaltered and unfractured basement exhibits high resistivity signatures (several thousands of Ohm.m), mainly caused by the absence of alteration and fluid. This strong contrast of resistivity makes it possible to image deeply fractured and altered (and hence porous and permeable) transition zones with electromagnetic methods (e.g. Magneto-Tellurics, CSEM).

To characterize the acoustic properties of the transition zone, we acquired both shallow refraction and reflection seismic profiles. Similarly to the resistivity case, we interpreted such data with the help of sonic logs and laboratory measurements. Here also, the fractured and altered basement exhibits a low P-wave velocity signature (<4500 m/s) over several hundred of meters, mainly due to the presence of large fractured and altered zones. On the contrary, the unaltered and unfractured basement exhibits high velocities (>5000 m/s). This strong contrast of velocity makes it also possible to image deeply fractured and altered transition zones with seismic methods (e.g. reflection seismics, passive seismics).

This work provides fundamental knowledge on the physical properties (electrical and acoustic) of the transition zone between the sedimentary cover and the basement. It is not only useful for the interpretation of prospective electromagnetic and seismic surveys over geothermal prospects targeting the transition zone but also for the proper design of future exploration surveys (e.g. to ensure sufficient resolution is achieved at target level).

S6.5 Crustal fault zones : new targets for geothermal exploration ? Preliminary numerical simulations of the Pontigbaud fault (French Massif Central)

H. Duwiquet, M. Bellanger, L. Arbaret, L. Guillou-Frottier, M. Heap

PDF - 550.1 ko
Duwiquet et al.

Numerous recent studies indicate that crustal-scale fault zones represent efficient conduits for meteoric fluids to flow down to mid-crustal depths (Haines et al., 2016), in particular near the brittle-ductile transition where temperatures are around 350-450°C (Violay et al., 2017). However, the associated hydrothermal systems are poorly studied. The present study aims to understand the potential of a new and novel type of geothermal system for high temperature and electricity production : crustal fault zones, which are not well recognized and understood yet. One such example the Pontigbaud fault zone (French Massif Central) is a 30 km-long and 3 kmwide mineralized fault zone. Localized near a granitic body (the Gelles granite ; Negroni, 1981), the Pontgibaud fault zone is also characterized by number of CO2-rich-thermo-mineral springs. Moreover, this area is also defined by local and regional surface heat flow values of 110 mW/m2 (International Heat Flow Commission database) involving temperature gradients between 37 and 41 °C/km. The Pontgibaud fault zone has been well studied in the last few years (Bellanger et al., 2017) and according to primary models obtained from new geological and geophysical data acquired since 2015, two electrical conductivity anomalies have been identified (Ars et al., 2017). The deeper one below 12 km depth could be related to the presence of fluids (hydrothermal fluids or silicate melt), which seems to be connected to the other low resistivity zone at 3 km depth (probably due to presence of smectite). With the available geophysical database and additional geological field measurements, a numerical approach dedicated to the understanding of Pontgibaud hydrothermal system will be relevant, as demonstrated by 2D numerical models of Soultz-sous-Forets geothermal system (Alsace, France)
(Guillou-Frottier et al., 2013). There, the numerical models led to the prediction of two additional thermal anomalies, one of them (Rittershoffen, Alsace, France) being currently under exploitation since 2014 (Baujard et al., 2017). The present study aims at investigating the possibility for the Pontgibaud crustal-scale fault zone to host an active hydrothermal system. Structural observations, laboratory permeability and connected porosity measurements and X-Ray micro-tomography observations suggest that the hydrothermal system behaves like a double matrix-fracture permeability reservoir. This multi-disciplinary approach allowed us to formulate a numerical model based on the geological knowledge of the Pontgibaud area. Two main unknown variables play a key role on possible fluid circulation : fault zone geometry (dip) and permeability ratio “R” between the fault zone and its host rocks, which varies from 1 to 300. For a sufficiently permeable fault zone (greater than 4x10-15 m2), buoyancy-driven flow creates a positive thermal anomaly at a depth of 2-5 km. Benchmark simulations indicate that vertical structures promote largest thermal anomalies at shallow depths. Futhermore, for a vertical fault zone, the thermal anomaly is larger for high R values. Finally, numerical models of the geologically constrained Pontgibaud fault zone show that a temperature of 150 °C at a depth of 2500 m can be obtained for a fault network permeability of 9x10-15 m2. The resulting thermal regime down to 15 km depth appears consistent with previously obtained resistivity profiles. Using this multi-disciplinary approach, this work established a potential predictive tool for future high-temperature geothermal operations in a basement context.

S6.6 Passive seismic monitoring of the Los Humeros (Mexico) geothermal field

Emmanuel Gaucher, Tania Toledo, Marco Calo, Angel Figueroa Soto, Philippe Jousset

PDF - 1.7 Mo
Gaucher et al.

Extensive passive seismic monitoring was carried out between Sep. 17 and Sep. 18 over the Los Humeros (Mexico) geothermal field. This acquisition operation was conducted in the framework of the European H2020 GEMex project among other geophysical campaigns, but also geochemical and geological surveys. Seismic monitoring provided numerous data, whose processing is still on-going, to better characterize the underground structures and properties of the geothermal field. These results participate to the increase of our understanding of the local geothermal system. They can be utilized to propose new development areas, especially, in the northwestern part of the currently exploited zone, which shows temperature larger than 380°C at ca. 2 km depth.

For one year, a network consisting of 45 short- and long-period seismometers was deployed at the Los Humeros geothermal field. The network layout was chosen to comply with several types of passive seismic processing methods : induced and natural seismicity characterization, travel-time tomography, ambient noise tomography, etc. We present preliminary results associated with the recorded seismicity.

Besides several natural earthquakes detected in the region, local earthquakes were regularly detected at an average close to one event per day. The raw data screening was performed using a recursive STA-LTA algorithm, tuned for local seismicity detection and implemented in the Obspy library (Beyreuther et al., 2010). Manual review of the earthquake candidates was performed as well as the picking of the seismic phase arrivals using the Obspyck freeware (Megies, 2016). Finally, location of the identified local earthquakes was done using NonLinLoc (Lomax, 2018). The local seismic events appear clustered around specific well doublets (Figure 1), at depths between 1 and 3.5 km, consistent with previous results obtained in the area and consistent with the geothermal reservoir interval. However, these locations need to be confirmed after refinement of the velocity model.

S6.7 Neogene deformation in the Upper Rhine Graben area : Implications from GIS-based LiDAR-DEM5 lineament analysis and fault-slip data from the Kaiserstuhl volcanic edifice

Jens C. Grimmer

PDF - 476.1 ko
Grimmer

Cenozoic deformation in the Upper Rhine Graben is essentially derived from syndepositional faults in 2D-seismic geological sections constructed from drillhole and seismic data and fault offsets (see Grimmer et al. (2017) for a review and further references) : Paleogene WNW-extension was overprinted by Neogene to recent NE-transtension. In order to test this concept, the Miocene (19-14 Ma) Kaiserstuhl volcanic edifice in the southern part of the Upper Rhine Graben was studied for its deformation by structural field data and a GIS-based LiDAR-DEM5 lineament analysis.

The DEM reveals three preferred orientations of linear features : NE-SW, NW-SE, and NNE-SSW. These trends are similar to orientations of minor faults documented in outcrops of the Neogene Kaiserstuhl volcanic edifice : Older NW-striking sinistral extensional faults and NE-striking, dextrally sheared dykes were offset by NW-striking normal faults thus representing a younger fault population. This younger fault population is characterized by N-S-striking sinistral oblique strike-slip faults and NW-striking normal faults, which are the major structural trends that are also identified in geothermal reservoirs and by fault-plane solutions of natural earthquakes in the Upper Rhine Graben. These structures are thus compatible with an origin in the recent stress field. Stress inversion of fault-slip data refers to an older N-S-transtension along NW-striking sinistral oblique strike-slip faults and minor NE-striking dextral oblique strike-slip faults, followed by the recent stress field. Both deformation phases are compatible with W- and NW-striking joints indicating N-S-extension and NE-SW-extension, respectively. Dykes in the Kaiserstuhl volcanic edifice predominantly strike (N)NW, NNE, and NE and thus subparallel with lineaments and fault trends. Few joint and fault-slip data indicate a third, possibly the oldest, but poorly constrained phase characterized by (W)NW-(E)SE-transtension. These heterogeneous fault-slip data thus indicate a more complex deformation history in the Neogene than commonly considered for the Upper Rhine Graben area.

S6.8 Comprehensive fault zone characterization : Identification of Liquiñe-Ofqui fault zone next to Villarrica volcano using gravimetric techniques

Sebastian Held, Eva Schill, Thomas Kohl

PDF - 208.9 ko
Held et al.

Geothermal resources related to major fault zones have a high potential for sufficient mass flow and thus provide a great potential for geothermal energy production. Elevated geothermal potential related to fault zones is documented e.g. in Landau EGS (Bächler et al. 2003) or Habanero project (Baisch et al. 2015). The key for a successful exploitation of such resource is the identification of the fault zone characteristics especially porosity and clay mineral content effecting the permeability and thus productivity.

Within this study we investigated the N-S aligned Liquiñe-Ofqui Fault zone (LOFZ), which is a major intra-arc fault zone in the South-Chilean Andes (Lopez-Escobar et al. 1995). Volcanic activities and geothermal manifestations are related to this major fault zone. At the research area close to Villarrica volcano the LOFZ is crosscutting by WNW-ESE oriented Mocha-Villarrica-fault zone (MVFS). Both fault zones exhibit volcanic and geothermal activity differing in its, probably stress-field related, manifestation (Cembrano et al. 2009). Besides the occurrence of almost perpendicular fault zones, the research area was chosen, because of the availability of high-resolution magnetotelluric data (Held et al., 2016).

Given the relation between permeability and porosity, explorations techniques are needed revealing fault zone porosity especially in depth. EM methods allow an indirect determination of fault zone porosity via the enhanced conductivity of fluid filled pore space (e.g. Türkoglu et al., 2015 ; Held et al., 2016). However, the occurrence of secondary conductive phases (clay minerals, ore minerals, graphite) produces ambiguous results complicating electromagnetic estimation of fault porosity.

Negative gravity anomalies are often detected related to fault zones, e.g. shown for San Andreas fault zone (Wang et al., 1986). Guglielmetti et al. (2013) in their study of the Maritime Alps, document negative Bouguer anomalies coinciding with local faults, where geothermal fluids discharge. Negative density contrasts are also observed for faults inside the Soultz geothermal reservoir (Schill et al., 2010). In all these studies the negative anomalies are related to enhanced porosities caused by a combination of shearing and hydrothermal alteration modifying initial rock composition by re-crystallisation and dissolution.

Within this study 220 gravimetric measurements were conducted on two perfiles perpendicular to the major fault zones. Considering regional gravity field, classic corrections and using measured rock densities gravimetric processing results in the Bouguer anomaly. The analysis show negative Bouguer anomaly at the fault trace of LOFZ, that cannot be observed for the MVFZ. The negative anomaly has a sharp peak of high contrast to the surroundings. Using Butterworth filtering the signal can be related to a shallow origin. Forward modeling reveals a density contrast of -0.24 g/cm3 between the fault zone and the surrounding rock (Figure 1). By the combination with electromagnetic results an estimation of porosity and clay content is feasible by the incorporation of fault zone resistivity. Higher porosities >10% and low clay content can be determined.

S6.9 Template matching applied to the seismicity induced in the Rittershoffen deep geothermal reservoir

Rike Köpke, Olivier Lengliné, Emmanuel Gaucher, Jean Schmittbuhl, Thomas Kohl

PDF - 466.5 ko
Köpke et al.

The development of the deep geothermal site at Rittershoffen (Alsace, France) was monitored continuously by different seismic networks covering various operational periods from September 2012 to October 2014, including the drilling of the well doublet GRT1/GRT2, the thermal, hydraulic and chemical stimulations of GRT1 and circulation tests between the two wells. The seismicity induced by these operations has the potential to give valuable insight into the geomechanical behaviour of the reservoir and the geometry of the underground fracture network. However, to conduct such studies a robust seismic database is needed. In this work the parameter set up for a template matching code which will be used to obtain seismic catalogues from continuous waveforms recorded by the seismic networks at Rittershoffen is described. Furthermore, a first overview of the amount of seismic events detected by this algorithm during different operational periods and detection statistics are presented.

The basic concept of the template matching technique is to calculate the correlation coefficient between the waveform of a known event, the template, and the continuous waveform over the desired time period to detect additional seismic events with similar waveforms to the template. It outperforms conventional energy detectors in terms of sensitivity to events with low signal-to-noise ratio and picking consistency. As template database, we use a manually picked seismic catalogue covering the different stimulation periods of GRT1 and part of the drilling period of GRT2. This starting catalogue contains over 1300 events.

Test runs of the template matching code on a sample of the Rittershoffen data has shown that there are two parameters which can potentially influence the results significantly. One is the detection threshold, which gives the value the correlation coefficient has to exceed in order to consider a new event detected, the other is the range of the bandpass filter used to optimize the signal to noise ratio of the signals. The distribution of the correlation coefficient proved to depend strongly on which templates and stations are used for detection. To ensure an objective choice of the threshold optimized for each chosen setting we decided to implement an adaptive threshold using the approach of Slinkard et al., 2014. To determine the optimal band pass filter range a frequency analysis for the different seismic stations was conducted by comparing the frequency spectrum of the noise to the frequency spectrum of the template signals. After the parameters are set the code is applied to the periods that were already picked manually to compare the amount and distribution of the detected seismicity between the two catalogues. Additionally, an evaluation of the detection statistics is presented to see for example which templates detected most events and whether templates detect primarily events with arrival times close to their own or across all time periods.

S6.10 Towards monitoring deep geothermal reservoirs in Alsace with ambient seismic noise

Kula D., Vergne J., Schmittbuhl J., Zigone D.

PDF - 191.8 ko
Kula et al.

Ability to monitor deep geothermal reservoirs at various stages of their life cycle is crucial for a safe and efficient exploitation. Here we explore a promising monitoring approach based on the cross correlation of the pervasive ambient seismic noise and analyze its potentials and limitations in the context of deep geothermal reservoirs.

Our study focuses on the Rittershoffen geothermal project (northern Alsace, France) which consists of two wells, GRT1 and GRT2, reaching depths of 2526 and 2708m respectively. Our dataset consists of the continuous seismic records at two short period seismic stations located nearby the site and available since 2012, therefore encompassing the drilling of both wells , hydraulic stimulation of GRT1 and circulation phases. The ambient noise characteristics in the area were already studied in Lehujeur et al. 2015 due to what we have more constraints for our analysis.

The noise records are first pre-processed and cross-correlate for every day to produce ambient noise correlation functions (ANCF). We then use a Coda Wave Interferometry (Shapiro & Campillo 2004, Snieder 2004, 2006, Roux et al. 2005) method with a stretching approach and using multiple ANCF references (Sens-Schönfelder et al. 2014) to estimate velocity variations of the medium in the neighborhood of the two stations.

We analyze our results in three different frequency bands, 0.2-0.5Hz, 1-3Hz and 3-6Hz, corresponding to various sources of ambient noise and various sensitivities with depth, assuming that the coda part of our ANCF is mostly made of surface waves. We observe stable cross correlation functions and coherent velocity variations. But most of these velocity variations are probably not related to the reservoir itself as they can be explained either by temporal changes in the frequency content or the azimuthal directivity of the ambient noise sources or by sub-surface effects like variations of the water table elevation. However, a sudden decrease of coherence, not related to changes in the noise sources, is observed following the hydraulic stimulation in the GRT1 well.

S6.11 Predictive Mechanical model for fracture stimulation in an enhanced geothermal system (EGS) context

Baptiste Lepillier, David Bruhn, Alexandros Daniilidis, Pierre-Olivier Bruna, Richard Bakker

PDF - 455.8 ko
Lepillier et al.

Controlling fracture stimulation is fundamental for enhancement of geothermal production.

The development of an EGS is one of the goals of the GEMex project, an international collaboration of two consortia, one from Europe and one from Mexico. The research is based on exploration, characterization and assessment of two geothermal systems located in the Trans-Mexican volcanic belt, Los Humeros and Acoculco. Los Humeros has been a producing field for several years, but Acoculco is yet to be developed. Thanks to surface manifestations of hydrothermal activities (Canet, et al., 2015), the existence of a geothermal system is evident. However, two wells reached very high temperatures, but did not find any fluids. For that reason, the Acoculco Caldera is foreseen as EGS development site, hoping to connect existing wells to a productive zone (Pulido, et al., 2010).

In this study, we develop a workflow from fracture characterization at the outcrop all the way to a predictive mechanical model for fracture stimulation from the well borehole. For practical reasons, the Acoculco site has been used as a case study to illustrate the method. This approach includes the fracture identification and description using the scanline survey method (ISRM, 1979) and a method for processing the collected data to generate a geological discrete fracture network (DFN). With the DFN generated this way we used the finite element method to build a mechanical model, discretized and populated with properties determined experimentally in the rock physics laboratory. Finally, we calculated the fracture propagation using the non-local damage approach, combined with a cohesive-zone model.

This workflow is based on easily accessible data from the field, and gives an accurate mechanical model of the fracture propagation and the pressure distribution for well borehole stimulation. Thanks to its simplicity, this approach can be applied in most EGS case studies, as for example, in the ultra-deep geothermal systems planned to be accessed in the Netherlands.

S6.12 Improvement of Geothermometric Methods to Reduce Exploration Risk

Fabian Nitschke, Sebastian Held, Thomas Neumann, Thomas Kohl

PDF - 135.2 ko
Nitschke et al.

Being a key parameter for the assessment of the economic efficiency of future reservoirs, the determination of the reservoir temperature is of outstanding importance for geothermal exploration. Since decades, it is a common practice to use naturally outflowing geothermal springs as a window to the subsurface, deducing the deep reservoir temperature from the composition of discharged fluids. Though, it has been and it still is a very active field of research and great efforts have been made to thoroughly apply the methods, reservoir temperatures deduced from solute geothermometry are still subject to large uncertainties. The application of different geothermometer formulations to one sample, not uncommonly leads to deviations of results reaching 200 K (or even more)[1]. Based on these methods, it is not possible to estimate trustful temperatures, which would be needed to define drill targets, forecast economic efficiency and design a power plant process.

The presented study assesses the performance of classical solute geothermometers and the numerical approach of multi-component geothermometry. We identify the most sensitive factors and secondary processes interfering the calculation of equilibrium temperatures, which are : a) effects from different reservoir lithologies on fluid composition, b) dilution with young, low-mineralized meteoric waters, c) solvent loss as a consequence of boiling, d) re-equilibration processes (such as dissolution/precipitation) during ascent, e) degassing and associated change of pH values and f) analytical errors.

We propose a stepwise approach to correct from these interferences and to adjust the measured data to original in-situ fluid composition which enables the estimation of true reservoir temperatures. Therefore, an anthropogenic tracer (CFC) based mixing model is used to quantify dilution. Long-term laboratory experiments equilibrating reservoir rock analogues under in-situ conditions are conducted to account for different equilibrium compositions of fluids. Boiling and associated solvent loss processes are identified by measuring stable water isotopes[2]. Chemical dis-equilibrium, the in-situ pH and measurement errors of dissolved constituents are corrected numerically by conducting a sensitivity analysis to obtain the presumed initial multi-mineral equilibrium (multi-component method)[2].

To demonstrate the approach, we applied it to a data set of natural geothermal spring fluids sampled at the hydrothermal system of the Villarrica volcano Chile. The absolute in-situ temperatures and the uncertainties were quantified. It is shown that, for both methods, classical solute and multi-component geothermometry, the proposed procedure leads to strongly converging calculated temperature with significantly low uncertainties of ΔT < 20 K.

S6.13 High-conductivity zones linked with fluid pathways in the Villarrica geothermal system, S-Chile

Maximiliano Pavez, Eva Schill, Sebastian Held, Thomas Kohl

PDF - 516 ko
Pavez et al.

ABSTRACT
Villarrica geothermal system, located in a volcanic environment, is highly influenced by cortical fault systems. Proof of this are the several thermal spring’s manifestations of low-medium temperature. The main structures can be visualized by their electrical resistivity by magnetotelluric methods. In the Villarrica geothermal field, anomalies of low electrical resistivity ≤ 100 Ωm have been associated with existing tectonic structures, mainly fault zones, improving their electrical properties due to the presence of fluids characterizing areas of high conductivity associated with preferential fluid pathways.

1. INTRODUCTION
Chile is one of the most active volcanism regions of the world. Most occurrences of active volcanism are attribute to the Southern Volcanic Zone, which runs from 38°S to 43°S. High number of thermal sources of intermediate temperature (Sánchez et al. 2013), bordering volcanic chains and several fault zones and forming a high geothermal potential, as it is the case for the area around Villarrica volcano, which we call Villarrica geothermal system.

Field geophysical surveys are used to investigate and characterize the morphology of a geothermal system. One of the geophysical methods used to generate a visualization of the geothermally relevant depth is magnetotellurics (MT), exploration technique belonging to the electromagnetism field allowing obtaining information on the geoelectrical properties of the subsurface through the temporal fluctuations of the natural electric and magnetic fields.

In the Villarrica geothermal system, high-conductivity values were associated with fault systems due to the presence of hydrothermal fluid or hydrothermal alteration products such as clay minerals by Held et al. 2016. If fluids are responsible for high electrical conductivity of a subsurface, then the implication for rock strength and fault properties can be multifold. So that, the objective of this study is to investigate preferential fluid pathways along the LOFS by means of 3D inversion of MT data.

2. GEOLOGIC BACKGROUND
The Villarrica geothermal system is located in the volcanic arc of southern Chile next to the active Villarrica volcano (39.42°S, 71.93°W) with the last eruption occurring in 2015. The geothermal system is linked with by the Liquiñe-Ofqui Fault System (LOFS) that consists of two NNE straight dextral strike-slip, and curved features, which splay off the straight lineaments along 1200km (Cembrano and Lara, 2009). It is intersected by the WNW-ESE aligned pre-Andean Mocha Villarrica fault zone (MVFZ) and the Villarrica-Quetrupillán-Lanín volcanic chain that trends N50°W (Lara et al., 2006) and follows the run of the MVFZ. In the vicinity of the volcanic chain >20 geothermal springs discharge (Sanchez et al. 2013, Held et al. 2018) indicating the low-medium temperature field.

3. DATA ACQUISITION - PROCESSING
Magnetotellurics data were collected by 31 stations along two profiles that were orientated E-W and N-S across and parallel to the LOFS, respectively. The latter crosscuts MVFZ and volcanic chain. For MT acquisition, a period band of 10-3 - 512s was chosen. The data were processed using the routine by Egbert & Booker (1986). Details of data acquisition, processing and 2D inversion are given by Held et al. (2016). For the inversion, 3D grid Academic and ModEM (Kelbert et al., 2014) was used.

4. INTERPRETATION
3D inversion results demonstrate largest resistivity changes occur in the upper crust down to about 7 km depth, from anomalies of high resistivity (> 1000 Ωm) to rocks of low resistivity of about few Ωm.

A deeper rooting structure with significant high electric conductivity is situated at a branch of LOFS (Cembrano & Lara, 2009) and penetrates the zone of homogenous conductivity that may represent the ductile part of the curst. The less prominent conductor coincides with a minor NW-SE oriented fault zone (Sánchez et al., 2013). All of these have fault-related implications. For instance, the upper kilometers of the fault are characterized of high conductivity associated with fluids circulating in the damage zone of the fault (Unsworth et al. 2000 ; Becken et. El., 2008 ; Ritter et al. 2005 ; Bedrosian et al. 2004).

A conductive structure is identified below the NE slope of the Villarrica volcano at a depth of about 3 km. Due to the location beneath the Villarrica volcano, this may be associated to the magmatic system being part of a surface reservoir (Morgado et al., 2015). It has been stated earlier that eruptive centers result mainly from vertical fluid movements in the fault zones (Lara et al. 2006). Our observations indicate that low resistivity structure associated with LOFS, besides the development of preferential pathways for transport of fluids and hydrothermal alteration, may indicate channels of ascent for magmas, both, in monogenetic cones and the Villarrica-Quetrupillán-Lanín volcanic chain.

5. CONCLUSION
We have distinguish through magnetotellurics measurements different anomalies of high electric conductivity in the Villarrica Geothermal system. They may be associated to both, tectonic and volcanic features, i.e. the LOFS and the expected shallow magma reservoir.

S6.14 Resistivity imaging of an analogue of the transition zone between the sedimentary cover and the basement of deep sedimentary basin for geothermal exploitation in Alsace

J. Porté, M. Darnet, J-F Girard, N. Coppo, J-M Baltassat, F. Bretaudeau, P. Wawrzyniak, Samantha Neeb, Skah Bissavetsy and Pierre-Daniel Matthey

PDF - 432.5 ko
Porté et al.

The transition zone between the basement and the sedimentary cover is becoming an increasingly attractive target for the development of geothermal energy in deep sedimentary basin as encountered in the Upper Rhine Graben. Several geothermal power plants already exploit this target but the transition zone is however still poorly known, with presence of large permeability heterogeneities. Studies in the framework of the CANTARE-Alsace project are currently on going in order to develop conceptual models on how it is formed and how heat can be exploited. In this study, we evaluate the ability of resistivity imaging by Controlled-Source Electromagnetic (CSEM) method in frequency domain, to identify favorable areas for the development of Enhanced Geothermal System (EGS). We performed a land-CSEM survey on a shallow analogue of the transition zone in a well-known catchment basin at Ringelbach (Vosges), to assess the relevance of such data. The catchment basin of Ringelbach is a well-known hydrogeological site, analogous in surface to our target in depth. According to previous geophysical studies (Two well logs, ERT…), the site is composed of resistive sandstone (>1600 Ω.m) superposed over two granitic facies with a NE-SW fault separating the site in two blocks. The first one is identified as a weathered granite being conductive (250-1600 Ω.m), whereas the second one appears as a block of fresh granite with a strong resistive signature (>1600 Ω.m). We know from former laboratory measurements (Belghoul, 2007) that conductivity of Ringelbach granites is related to its alteration degree and porosity. Furthermore, weathered granites show signs of former hydrothermal alteration succeeding the granite burial (Wyns, 2012). Fresh granitic basement is not reached by wells (150 meters depth) or former geophysical studies (Limited around 50 meters depth).

The 3D Land-CSEM survey consists in a 3D-grid of 48 reception sites uniformly distributed over the whole basin and using a single transmitter located at 1.5 km north from the grid. We performed 2.5D inversions of a data subset with the 2.5D inversion code MARE2DEM (Key, 2016) to image resistivity structures through a profile of interest (Figure 1) and compared the result to a former Electrical Resistivity Tomography (ERT) inversion. 3D inversion of the entire dataset are currently on-going using the BRGM proprietary software POLYEM3D for 3D CSEM and MT data inversion. Preliminary 3D results are consistent with the 2.5D resistivity image.

CSEM inversion results extended shallow ERT images in depth and allowed to obtain a resistivity image of the transition zone analogue. Integration of these results with existing geological and geophysical knowledge allowed identifying and mapping the different resistivity blocks : the resistive sandstone ; the altered granite area more conductive, altered and potentially more permeable ; the resistive fresh granites with the “Bunker” block (Right on Figure 1) as well as the fresh granitic basement, less porous thus less favorable for a geothermal reservoir. Results of this analogue study demonstrate the importance of obtaining resistivity data at the target depth before drilling to maximize the success rate of a deep EGS project. Upscaling work will be part of the last work package of the CANTARE-Alsace project.

S6.15 Petrothermal Geothermal Energy in Bavaria : First Laboratory Results of an Analogue Study

Martin Potten, Heiko Käsling, Catharina Drexl and Kurosch Thuro

PDF - 441.2 ko
Potten et al.

In Bavaria, so far only the hydrothermal geothermal energy in the alpine foreland basin (Molasse Basin) has been used. The petrothermal potential in crystalline rock is focused in the northern part of Bavaria. The first signal for increased underground temperatures were found during a drilling campaign in the 1970s and 1980s (Gudden 1981). The exploration and use of a regional geothermal anomaly in the northern part of Bavaria has an unused petrothermal potential.

According to the zoning of the Variscian orogen of Kossmat (1927), the field of interest (Franconian Basin) is situated within the Saxothurian zone (Riemer 2011). In 2000 Bauer identified a regional geothermal anomaly. The local anomaly is located north of Bayreuth in the area of the Franconian Basin, in the spreading extension of the granite of the Fichtelgebirge. In this anomaly, the geothermal gradients reach up to 5 °C/100 m (Riemer 2011). In the Fichtelgebirge, the outcrops display properties of geothermal reservoirs.

The development and use of this future technology initially requires extensive research but has outstanding future potential. In this context, the determination of rock mechanical parameters is indispensable for the subsequent modelling of petrothermal reservoirs. For this purpose, it is to conduct laboratory tests using analogue samples from quarries. Based on the results of these experiments, a database has been created. This database improves the knowledge of the mechanical properties of representative rock types. In the framework of the 6th European Geothermal Workshop the first results of the laboratory tests will be presented.

Funding came from the Bavarian State Ministry of Education, Science and the Arts in the frame of the project Geothermal-Alliance Bavaria.

S6.16 Identification of the deep hydrothermal reservoir in the Acoculco Caldera, Mexico

Rosa Maria Prol-Ledesma, Eduardo Granados-Pastrana

PDF - 268 ko
Prol Ledesma et al.

The Acoculco volcanic complex is in the eastern of the Trans Mexican Volcanic Axis. The complex is considered an EGS ; however, shallow features indicate the occurrence of hydrothermal eruptions that would require the presence of hydrothermal fluids. The central part of the caldera hosts bubbling cold springs with strong discharge of CO2, as well as numerous areas with advanced argillic alteration and strong silicification. Two exploratory wells with depths close to 2000m have confirmed a geothermal gradient higher than 100°C/km but no permeability has been found. It is assumed that deep drilling may encounter a hot geothermal reservoir because the bottom hole temperatures measured in the exploration wells are 264 and 307°C .

A heat transfer study is an important tool to identify and locate conductive and convective transport mechanisms in a geothermal system. Determination of the geothermal gradient requires drilling and at this stage we used aeromagnetic data to calculate the Curie Point Depth (CPD) and define the main geological structures using image processing enhancement techniques applied to multispectral images and digital terrain model. Additionally, magnetic data were processed to define the intrusive bodies that may act as heat source.

Concurrent results were obtained with a pair of geothermal gradients registered in exploratory wells and with the surface manifestations observed inside the caldera, as well as a direct relationship in the change of the geothermal gradients and the presence of reported faults (García-Palomo et al., 2017), The integration of CPD, main structures and surface manifestations define the occurrence of the highest geothermal gradients in the northern zone, where future exploration drilling is proposed.

S6.17 Premium treatment system for granite and sandstone formations - Fluid development and field trial in a geothermal well

Nils Recalde Lummer, Omair Rauf

PDF - 411.1 ko
Recalde Lummer and Rauf

The primary objective of acidizing geothermal wells in fractured sandstones and granites is to remove scales blocking the pathway of water, thus increasing the productivity or injectivity of the formation. For this purpose, hydrochloric acid (HCl) - based fluids are commonly applied. At high temperature, however, the fierce reactivity of HCl often hinders deep penetration of the formation making stimulation of removed damaged zones difficult to attain [1], [2]. Selection of chemicals to reduce reactivity adds to the difficulties encountered in the acceptance by mining and water authorities.

Furthermore, the extreme corrosion tendency of hydrochloric acid requires high concentrations of corrosion inhibitors and intensifiers [3], [4]. Another problem often encountered during acidizing sandstones with HCl-based formulations is their incompatibility with clay minerals [5]. Core flood experiments and field results indicate that high temperature illitic sandstone is sensitive to conventional mud acid treatments. Here, the commonly employed HCl pre-flush degrades illite and chlorite leading to fines migration and formation damage.

This paper introduces an innovative fluid system, which is based upon a naturally retarded, biodegradable acid [6] and highlights its first field trial in a granite formation. Here, demanding borehole conditions (195°C BHT, vast openhole section, and a high concentration of illite) were encountered. Extensive laboratory tests regarding reactivity, acid corrosion tendency, and compatibility were conducted preparing the first field trial. In this context, core flood experiments with actual granite and sandstone samples, as well as solubility tests with cuttings from the target zones were performed. For core flooding results, please see figure attached.

In contrast to HCl-based fluids, this state of the art acid system has supreme dissolving capacity against carbonates and silicates. Furthermore, it shows a low corrosion tendency and an excellent compatibility profile with illite, even at high temperature.

For the first field trial, three target zones in the openhole section were selected for chemical injection via tubing. The superior chemical properties of this treatment system lead to a greatly enhanced injectivity of the well. Due to its naturally retarded reactivity, this fluid can be pumped at much lower rates, compared to most HCl-based formulations. Laboratory and field results impressively proved that this new treatment fluid is an excellent alternative to commonly used HCl-based systems.

S6.18 New insights on geothermal rock properties of the Los Humeros geothermal field, Mexico

Leandra Weydt, Kristian Bär, Ingo Sass

PDF - 346.1 ko
Weydt et al.

The Los Humeros geothermal system is steam dominated and so far explored by 65 wells of which 23 are used for production. With temperatures of more than 380 °C in the northern part of the field, the system is characterized as a super hot geothermal system (SHGS). The development of these areas is challenging due to the high temperatures and aggressive reservoir fluids which have led to corrosion and scaling problems in the wells.

Thick sequences of mainly Pleistocene hornblende and pyroxene-bearing andesites and rhyolites form the geothermal reservoir within the Los Humeros caldera complex, which are interrupted by thin tuff or basalt layers (Carrasco-Núñez et al., 2017). The geothermal system is capped by several pyroclastic sequences of ignimbrites, tuffs and andesitic lavas. Holocene to recent lava flows, pumice and scoria deposits overlay the pyroclastic sequences and represent the latest volcanic activity. The geothermal reservoir is settled on a Mesozoic basement composed of limestones and shales. Granitic and syenitic plutons of Cenozoic age intruded into the basement and led to local metamorphism of marble, hornfels and skarn (Ferriz and Mahood, 1984).

For better reservoir understanding and prospective modeling, extensive geological, geochemical, geophysical and technical investigations are performed within the scope of the GEMex project (EU-H2020, GA Nr. 727550, Jolie et al., 2018). We present the detailed results of investigations on the thermo- and petrophysical properties of each reservoir formation and their spatial distribution. Until now only few studies exist including detailed descriptions of cuttings or subsurface core samples in correlation with chemical analysis or rock property measurements (Contreras et al., 1990 ; Arrelano et al, 2003 ; García-Gutiérrez and Contreras, 2007 ; Carrasco-Núñez et al., 2017).

In order to complement and update previous studies, 64 samples were drilled from 35 core sections covering 14 wells of the Los Humeros geothermal field. The samples were analyzed for petrophysical (e.g. density, porosity, permeability) and thermophysical properties (thermal conductivity, thermal diffusivity, heat capacity) as well as ultra-sonic wave velocities and magnetic susceptibility. Detailed complementary thin section analysis combined with XRD and XRF measurements will provide information about the mineral assemblage, geochemistry, the intensity of hydrothermal alteration and fluid rock interactions.

The subsurface core samples show an irregular lateral distribution of the reservoir units. The average matrix permeability and porosity range between 10-15 and 10-18 m² and <5 % up to 25 %, respectively. The results indicate that large scale fluid flow within the reservoir is rather fault zone controlled. Furthermore, thermal conductivity (1.2 – 2.2 W m-1 K-1) and thermal diffusivity (0.7 – 1.2 m² s-1) are relatively low. Hydrothermal alteration of different intensities and leaching processes were observed. Ongoing investigations focus on determining the impact and extent of these processes on the rock properties.

S6.19 Sensitivity analysis of reservoir temperatures based on mineral phase saturations

Lars Yström, Fabian Nitschke, Sebastian Held, Thomas Kohl

PDF - 273 ko
Yström et al.

Reservoir temperature estimation is a key technique in successful geothermal reservoir exploration. Since often the errors in reservoir temperature estimations are high, statistical approaches are used (Giggenbach 1981, Reed und Spycher 1984, Giggenbach 1988, Cooper et al. 2013, Spycher et al. 2014, Nitschke et al 2017). The focus on this study lies in the application of multicomponent geothermometry to estimate the reservoir temperature in Krafla and Námafjall geothermal fields, Iceland.

In quantitative geothermometry the mineral saturation and element ratios of the geothermal fluid are used to obtain the reservoir temperature based on the geochemical equilibrium between mineral phases and the reservoir rock. Thereby, the saturation state of different minerals serves as a geothermometer and hence the coupling of several geothermometers allows for statistically robust estimates. Within this study the reservoir temperatures of the geothermal system are determined by in-situ measurements. The objective is to define a typical mineral set for basaltic reservoir rocks by calibration with the measured data.

Krafla and Námafjall are high-enthalpy geothermal reservoirs located in the neo-volcanic zone of Northeast Iceland. Magma heated geothermal brines are expected to reach 350°C at a depth of 2000 m. The geochemical data of the well discharges were collected by Guðmundsson and Arnórsson (2002) as well as Arnórsson et al. (1983).

Geochemical equilibrium calculations are done using PhreeqC, while the statistical evaluation in form of box-plots is conducted in Matlab. The evaluation of individual datasets from Krafla and Námafjall allows the calibration of a specific set of minerals allowing a most accurate temperature estimation in basaltic reservoirs. As in-situ measurements of pH, aluminuium concentration and redox potential do not reflect reservoir conditions (Nitschke et al 2017), a further sensitivity analysis is conducted to improve the estimated reservoir temperatures. Therefore, the variations of the value yield a minimization of estimations errors reflecting the most plausible valuation. It can be shown that in all cases the temperature estimations match with the measured temperature ranges of the reservoirs. While the variations of pH and aluminium concentration effect the estimated temperatures, the redox potential has only negligible effect and thus can be discarded. To test the developed and calibrated method the procedure is applied for natural spring water of the geyser Uxahver. The calculated reservoir temperature matches the measured reservoir temperature with an error of ±10%. In conclusion the developed method tends to be a promising tool for the estimation of reservoir temperatures. In addition, it is an economical exploration tool that allows a high precision temperature estimation. Since the multicomponent geothermometry uses secondary mineralization it can be adapted to different geothermal settings yet requiring further calibration.

3 octobre 2018