Vuataz, François

2010, Sonney, Romain, Vuataz, François, Cattin, Stéphane

Cl/Br ratios were studied in deep groundwaters to decipher the origin of dissolved mineral components from the Alps and neighbouring areas. Cl/Br molar ratio represents a good marker to define if the salinity comes from seawater or residual brines (655 and lower) or from dissolution of halite or halite-rich gypsum, often present in the Triassic formations (upper than the seawater ratio). It can be an interesting tool for projects dealing with exploration and production of geothermal fluids. Results of this study showed the presence of trapped seawater in formations of the large basins from the Quaternary to the basement, resulting from infiltration during different marine intrusion periods. This method also showed the presence of brines in crystalline aquifers. Some of these waters discharge along subvertical faults and are diluted to various degrees by different types of meteoric waters. In some cases, this method raises new questions about the true geological origin of deep circulations.

2010, Sonney, Romain, Vuataz, François, Cattin, Stéphane

The database BDFGeotherm, containing physical, chemical and hydrogeological information on more than 200 deep fluids from 84 sites in Switzerland and some neighbouring regions, was first compiled on ACCESS code and was later modified to improve its availability and attractiveness by using Google Earth free software and the CREGE website (www.crege.ch/BDFGeotherm/). BDFGeotherm is a functional tool for various phases of a geothermal project such as exploration, production or fluid re-injection. This database allows gathering existing geothermal data, generally widely dispersed and often difficult to reach, towards a user’s friendly tool. Downloading the file “BDFGeotherm.kmz” from the CREGE website makes possible to visualize the 84 geothermal sites from Switzerland and neighbouring areas. Each one is represented with a pinpoint of different colour, for diverse temperature ranges.

A large majority of sites is located in the northern part of the Jura Mountain and in the upper Rhone Valley. General information about water use, geology, flow rate, temperature and mineralization are given in a small window by clicking on the desired pinpoint. Moreover, two links to Internet addresses are available for each site in each window, allowing returning to the CREGE website or providing more details on each sampling point such as: geographical description, reservoir geology, hydraulics, hydrochemistry, isotopes and geothermal parameters. For a limited number of sites, photos and a geological log can be viewed and exported (Sonney et al., 2009).

2010, Portier, Sandrine, Vuataz, François

L’impact de la stimulation chimique sur le réservoir du Système Géothermal Stimulé (SGS) de Soultz est évalué par simulation numérique des processus couplés thermo-hydraulique-chimique. Les modélisations 2-D simplifiées du réservoir ont pour but d’interpréter et de prévoir les interactions acides-roche dans le granite fracturé du système géothermal stimulé de Soultz-sous-Forêts. Pour évaluer les interactions chimiques entre la roche hôte et un mélange de HCl et HF, ainsi que les effets potentiels du traitement chimique sur le réservoir SGS de Soultz, une nouvelle calibration du code FRACHEM a été intégrée. Les résultats de la simulation indiquent que la quantité prévue de minéraux dissous par injection d’un mélange d’acides Regular Mud Acid (RMA) est comparable au montant estimé lors du test de stimulation chimique du puits GPK4 sur le site SGS de Soultz. Par conséquent, la porosité et la perméabilité du réservoir peuvent être améliorées près du puits d’injection par la stimulation chimique., The Soultz Enhanced Geothermal System (EGS) reservoir's response to chemical stimulation is assessed by numerical simulation of coupled thermo-hydraulic-chemical processes. To assess chemical interactions between host rocks and a mixture of HCl and HF as well as its potential effects on the Soultz EGS reservoir, new modelling efforts using the FRACHEM code have been initiated. This article presents the model calibration and results. Simulations consider realistic conditions with available data sets from the EGS system at Soultz. Results indicate that the predicted amount of fracture sealing minerals dissolved by injection of a mixture of acids Regular Mud Acid (RMA) was consistent with the estimated amount from the test performed on GPK4 well at Soultz EGS site. Consequently reservoir porosity and permeability can be enhanced especially near the injection well by acidizing treatment.

2009, Sonney, Romain, Vuataz, François

Le site géothermal de Lavey-les-Bains (Suisse) est un système alpin d’écoulement profond dans des terrains cristallins fracturés. Les analyses acquises depuis 1973 sur les eaux souterraines montrent un processus de mélange entre une composante profonde et chaude (68°C et TDS 1.4 g/L) et une eau froide peu profonde. La production sur le nouveau puits profond P600, réalisé en 1997, a amplifié ce processus de mélange dans le puits P201, où une baisse de la température et des TDS a été observée. Des modèles numériques 2D et 3D de chaleur, d’écoulement et de transport de masse ont été développés pour reproduire le système géothermal et pour prédire le potentiel de ressource géothermique exploitable à long terme. La température calculée du réservoir profond induit (100–130°C) est en accord avec les géothermomètres alors que le flux d’eau thermale (5,400–9,000 m3/jour) est probablement sous-estimé. Plusieurs scénarios de production de fluide permettent de reproduire les phases de baisse et de stabilisation des températures dans le champ géothermal depuis 1997. Sur P201, le rapport de mélange calculé avant et pendant l’exploitation de P600 est comparable aux valeurs observées; la température modélisée sur P201 tend vers une stabilisation à 56 C après 10–15 ans de production sur P600. La proposition d’un nouveau puits est susceptible de réduire la production thermale des puits existants., The geothermal site of Lavey-les-Bains, Switzerland is an Alpine deep flow system in fractured crystalline rocks. Groundwater analyses since 1973 reveal a mixing process between a deep warm component (68°C and TDS 1.4 g/L) and cold shallow water. The production rate of the new deep well P600, installed in 1997, has amplified this mixing process in well P201, for which a decline in temperature and TDS has been observed. Numerical hydrogeological two-dimensional and three-dimensional models of heat, flow and mass transport have been developed to reproduce the geothermal system and to forecast the long-term exploitation potential of the geothermal resource. The computed temperature of the deep inferred reservoir (100–130°C) is in agreement with the geothermometers, whereas the simulated thermal water flux (5,400–9,000 m3/day) is probably underestimated. Different fluid production scenarios can reproduce the decline and stabilization phases of temperatures in the geothermal field since 1997. For P201, the mixing ratio calculated before and during the exploitation of P600 is comparable with observed data; the modelled temperature tends towards stabilization in P201 at 56°C after 10–15 years of production at P600. Another proposed new well is likely to reduce the thermal output of the existing wells.

2010, Sonney, Romain, Vuataz, François

During more than 30 years, chemical and isotopic geothermometers have been extensively used to calculate and estimate the temperature of geothermal reservoirs in various geological, petrographical and thermal conditions. In this evaluation, chemical analyses of deep boreholes from the Molasse Basin and the Tabular Jura in Northern Switzerland were used to estimate reservoir temperatures with geothermometers, and results were compared to measured temperatures at depth. The presence of thermal waters in subhorizontal formations with a temperature range of 12-112°C, is associated with various geological and petrographical settings (sedimentary, crystalline rocks).

Composition of geothermal fluids depends on various and sometimes competing processes, such as full or partial chemical equilibrium, mixing with shallow groundwater or trapped seawater, dissolution of evaporite, ionic exchange with clays, or residence time in the reservoir. These processes are constraining for the application of geothermometers. Moreover, some of them have a limited temperature range of application cannot be used in some petrologic environments. Thus, a range of possible reservoir temperatures is calculated depending on several assumptions and compared to measured temperature. Application of chemical and isotopic geothermometers, using several calibrations proposed in the literature, is discussed in this specific context.

2010, Portier, Sandrine, Vuataz, François

The Soultz EGS reservoir’s response to chemical stimulation is assessed by numerical simulation of coupled thermo-hydraulic-chemical processes. To assess chemical interactions between host rocks and a mixture of HCl and HF as well as its potential effects on the Soultz EGS reservoir, new modelling efforts using the FRACHEM code have been initiated. This paper presents the model calibration and results. Simulations consider realistic conditions with available data sets from the EGS system at Soultz. Results indicate that the predicted amount of fracture sealing minerals dissolved by injection of a mixture of acids (RMA) was consistent with the estimated amount from the test performed on GPK4 well at Soultz EGS site. Consequently reservoir porosity and permeability can be enhanced especially near the injection well by acidizing treatment.

2010, Sonney, Romain, Perrochet, Pierre, Vuataz, François

Groundwater flow, heat and mass transport in geothermal or hydrothermal systems locally occurs in the Alps Range where a series of precise conditions are met such as active downflow, permeability at depth, concentrated and fast upflow and favorable geomorphological surface patterns. Advanced studies of the regional and local geology, thermal water chemistry, mixing processes, infiltration area, water-rock interactions, circulation depth, uprising conditions, groundwater residence time, mineralchemical fingerprint, etc. allow the deep flow system to be understood. To carry out these studies, different methods can be employed and combined together such as geological, hydrogeological, geophysical, geochemical, isotopic and numerical investigations. Moreover, these investigations represent a scientific basis providing information required to better manage the short and long term exploitation of the discharged thermal water, qualitatively and quantitatively. They also provide crucial knowledge to prepare future geothermal projects with boreholes and to limit failures that may occur in subsurface and deep geothermal prospection. Due to the high costs for the implementation of a deep borehole, the risk of failure needs to be lowered as much as possible, i.e. it requires a good knowledge of the explored site. This is important for long term sustainable development of geothermal projects. The three selected hydrothermal sites are Lavey-les-Bains and Val d’Illiez in Switzerland and Saint- Gervais-les-Bains in France, where the uprising thermal waters are continuously exploited for various uses such as spa, heating buildings, medical care and cosmetics. Lavey-les-Bains and Saint-Gervaisles- Bains were selected due to several criteria: 1) deep flow systems in the basement, 2) similar chemical and isotopic properties, 3) presence of several end-members with mixing processes, 4) interesting properties of the geothermal reservoirs, 5) many data available and 6) on-going geothermal projects or planned. On the contrary, Val d’Illiez was selected for orther criteria: 1) deep flow system in the autochthonous sedimentary cover of the basement where the hydrothermal systems of Lavey-les-Bains and Saint-Gervais-les-Bains occur, 2) comparison of the chemical and isotopic properties of thermal waters from the cover with those of thermal waters flowing in the basement, 3) history of the site related to the Salanfe Lake and 4) many data available. These three studied sites are located around the Aiguilles Rouges Massif, one of the external crystalline massifs of the Western Alps. Lavey-les-Bains and Saint-Gervais-les-Bains represent the two low-elevation points of the Aiguilles Rouges basement, respectively on the north-eastern and southwestern sides. In contrast, Val d’Illiez is located out of the Aiguilles Rouges basement in a sedimentary domain belonging to the autochthonous cover which outcrops along the north-western edge of the basement. Firstly, the investigations the selected hydrothermal systems were used to study the geological setting and the fracturing conditions for each site. Then, a new sampling campaign was carried out with pumping tests aiming to define the chemical-mineral processes leading to the composition of groundwaters. Finally, two and threedimensional numerical models were established to validate the assumptions formulated from the geochemical investigations, and to represent the deep flow system, the geothermal anomalies and the mixing processes. Groundwater analyses since 1973 at Lavey-les-Bains have revealed a mixing process between a deep Na-SO4 and high-Cl thermal component circulating in the basement (68oC and TDS 1.4 g/L) and cold shallow water from the mountain slope and the Quaternary filling. The production rate of the new deep well P600, installed in 1997, has amplified this mixing process in the well P201, for which a decline in temperature and total dissolved solids has been observed. Numerical hydrogeological two and three-dimensional models of flow, heat and mass transport reproduced the deep flow system and forecasted the long-term exploitation potential of the geothermal resource. Computed temperature of the deep inferred reservoir (100-130oC) is in agreement with the geothermometers, whereas the simulated thermal water flux (5400-9000 m3/d) is probably underestimated. Different fluid production scenarios have been documenting the decline and stabilization phases of temperatures in the hydrothermal ield since 1997. For P201, the mixing ratio calculated before and during the exploitation of P600 is comparable with observed data; the computed temperature tends towards stabilization in P201 at 56oC after 10 to 15 years of production at P600. Another planned new well is likely to reduce the thermal output of the existing wells. The crystalline rocks are not directly outcropping at the Saint-Gervais-les-Bains spa but certainly exist beyond 300 m depth. Uprising waters are pumped from two different aquifers below the Quaternary deposits of the Bon Nant Valley. In the lower Trias-Permian aquifer crossed by De Mey boreholes (27-36oC), the ascending Na-SO4 and high-Cl thermal water from the basement (4.8 g/L) is mostly mixed by a Ca-SO4 and low-Cl cold water circulating in the autochthonous cover of the Aiguilles Rouges basement. The origin of the saline thermal water probably results from infiltration and circulation in the basement until it reaches deep thrust faults where there is leaching of residual brines or there are fluid inclusions at depth. The dissolution of Triassic halite is not possible at Saint-Gervais-les-Bains because the Triassic cold waters have a very low-Cl concentration (< 20 mg/L). For the De Mey Est borehole, gypsum dissolution is occurring with cationic exchanges involving Na, as well as low-temperature Mg dissolution from dolomite in the Triassic formations. The aquifer made of imbricated structures (upper-middle Trias) and crossed by the Lépinay well (39oC) contains thermal waters which are strongly mixed with a low-Cl water, where gypsum dissolution also occurs. The infiltration area for the thermal end-member is in the range of 1700-2100 meters, close to the Lavey-les-Bains hydrothermal system, and corresponds to the average elevation of the Aiguilles Rouges Massif. For the Ca-SO4 and low-Cl end-member, the infiltration area is lower (1100-1300 m), showing circulation from the Mont Joly Massif. The geothermometry method indicates a reservoir temperature of probably up to 65oC but not exceeding 100oC. The deep flow system leading to the thermal springs in the Val d’Illiez occurs at the bottom of the autochthonous cover of the Aiguilles Rouges basement, mainly inside the Triassic formations. The structure of the cover is a great recumbent anticline with an axial plane plunging towards the south-east which would be limited by a basal thrust fault related to the thrust system between the Aiguilles Rouges and Infa-Aiguilles Rouges basements. Mixing processes occur between a Ca-HCO3 cold groundwater and a Ca-SO4 and low-Cl thermal endmember, having a temperature and a total dissolved solids roughly of 30-31oC and 1.8 g/L respectively. The thermal component acquires its mineral composition from the dissolution of gypsum and dolomite occurring in the Triassic formations, in a different way compared to Lavey-les-Bains and Saint-Gervais-les-Bains. The thermal component has an infiltration area close to the elevation of the Salanfe Lake (1900 m) and the reservoir temperature of the deep flow system should probably not exceed 35-40oC at a depth of around 1 kilometre below the Val d’Illiez spring zone. Using the tritium data, the piston-flow model calculated an average residence time for the thermal end-member of around 5 years. A two-dimensional model of groundwater flow and heat transport was carried out to study the relation between the Salanfe Lake and the thermal springs. Four scenarios were tested with different values of water losses from the lake and a long-term cooling of the host rocks was simulated. The natural variations of parameters in the springs indicate that the lake probably does not act on the thermal regime as a single intake. It appears that temperature and chemistry of the thermal waters in the Val d’Illiez have a current steady evolution, while the flow rate seems to vary with the natural variation of the water level of the lake with a time lag of around 140 days. This should indicate that an upper aquifer is situated on the thermal aquifer, without mixing processes, adding a pressure which raises the flow rate of the thermal spring without changing its temperature and chemistry. The understanding gained through this study on groundwater flow, heat and mass transport in the Aiguilles Rouges Massif improves the knowledge for the other external crystalline massifs where hydrothermal sites are present at the front of them. The Mont Blanc Massif is also an area where a large amount of thermal water can discharge from its lowelevation points. The hydrothermal site of Saxon along the Rhone Valley in Switzerland is an interesting site to investigate for this reason. Generally, the density of hydrothermal sites is often higher in the Western Alps where the basement outcrops. Indeed, vertical faults in the basement facilitate the deep infiltration of water leading to deep flow systems, whereas in sedimentary domain the superposition of nappes tends to generate shallower flow systems. Consequently, it would be interesting to investigate other low-elevation zones bordering the external crystalline massifs where the Quaternary filling could mask areas of uprising thermal waters.

2010, Portier, Sandrine, Vuataz, François, Barrios Martinez, Antonina Luz, Siddiqi, Gunter

The exploitation of the Berlin geothermal field (El Salvador) started in February 1992 with two 5 MW electrical units. In December 1999, exploitation at a larger scale started with the operation of two turbo-generating condensation units of 27.5 MWe each. Since the beginning of exploitation until today, all the deep thermal fluids produced during the generation of electricity have been injected back to the reservoir. However, the well injectivities declined because of self-sealing processes of fracture thereby reducing permeability in the injection zones.

Our study focuses on the factors that may have caused the observed loss of injectivity of reinjection wells, using numerical simulation of fluid flow with coupled chemical reaction modelling. Such a model helps quantitatively understand the complex interplay of thermal, hydraulic and chemical processes (THC) and to predict the impact of reinjection on reservoir properties. The first preliminary simulation was carried out with FRACHEM code. This paper presents simulation results that reproduce in some way the observed decrease of permeability in the injection zone with time.

2010, Sonney, Romain, Vuataz, François

Le système hydrothermal de Saint-Gervais-les-Bains, France est localisé au niveau du point bas du massif cristallin des Aiguilles Rouges, côté sud-ouest. Le socle n’affleure pas directement sur la zone étudiée, mais est certainement présent au-delà de 300 m de profondeur. Les eaux thermales ascendantes sont pompées dans deux différents aquifères, sous les dépôts quaternaires de la vallée du Bon Nant. Dans l’aquifère du Permien-Trias inférieur traversé par les forages De Mey (27–36 °C), une eau thermale ascendante Na-SO4 riche en Cl issue du socle cristallin (4,8 g/L) est essentiellement diluée par une eau froide Ca-SO4, pauvre en chlorure, circulant dans la couverture autochtone du socle des Aiguilles Rouges. Cette eau thermale saline se serait infiltrée dans les Aiguilles Rouges et circulerait en profondeur jusqu’aux plans de chevauchement, avec le lessivage de saumures résiduelles d’inclusions fluides (rapport molaire Cl/Br inférieur à 655). La dissolution de la halite triasique (Cl/Br > 1000) n’est pas possible à Saint-Gervais-les-Bains, car les eaux froides ont des teneurs faibles en chlorure (< 20 mg/L). Des interactions eau–roche se produisent durant la remontée via des décrochements nord–sud affectant le socle et sa couverture sédimentaire. Pour le forage De Mey Est, la dissolution du gypse se produit avec des échanges cationiques produisant du Na et la dissolution de Mg à basse température, depuis la dolomie triasique se produit également. L’aquifère des écailles du Trias moyen et supérieur, traversé par le forage Lépinay (39 °C), contient des eaux thermales fortement mélangées avec des eaux pauvres en chlorure et la dissolution du gypse se produit également. La zone d’infiltration du pôle thermal est située vers 1700–2100 m d’altitude, altitude équivalente pour le système hydrothermal de Lavey-les-Bains correspondant au massif des Aiguilles Rouges. Pour le pôle Ca-SO4 pauvre en chlorure, la zone d’infiltration est plus basse (1100–1300 m), montrant des circulations depuis le massif du Mont Joly. L’application des géothermomètres indique une température du réservoir probablement supérieure à 65 °C, mais n’excédant pas 100 °C., The hydrothermal system of Saint-Gervais-les-Bains, France is located in a south western low-elevation point of the Aiguilles Rouges crystalline Massif. The crystalline rocks are not directly outcropping in the studied area but certainly exist beyond 300 m depth. Uprising waters are pumped from two different aquifers below the Quaternary deposits of the Bon Nant Valley. In the Lower Trias-Permian aquifer crossed by De Mey boreholes (27–36 °C), the ascending Na-SO4 and high-Cl thermal water from the basement (4.8 g/L) is mostly mixed by a Ca-SO4 and low-Cl cold water circulating in the autochthonous cover of the Aiguilles Rouges Basement. The origin of the saline thermal water probably results from infiltration and circulation in the basement until it reaches deep thrust faults with leaching of residual brines or fluid inclusions at depth (Cl/Br molar ratio lower than 655). The dissolution of Triassic halite (Cl/Br > 1000) is not possible at Saint-Gervais-les-Bains because the Triassic cold waters have a low-Cl concentration (< 20 mg/L). Water–rock interactions occur during the upflow via north–south strike-slip faults in the basement and later on in the autochthonous cover. For the De Mey Est borehole, gypsum dissolution is occurring with cationic exchanges involving Na, as well as low-temperature Mg dissolution from dolomite in the Triassic formations. The aquifer of imbricated structures (Upper-Middle Trias) crossed by the Lépinay well (39 °C) contains thermal waters, which are strongly mixed with a low-Cl water, where gypsum dissolution also occurs. The infiltration area for the thermal end-member is in the range 1700–2100 m, close to the Lavey-les-Bains hydrothermal system corresponding to the Aiguilles Rouges Massif. For the Ca-SO4 and low-Cl end-member, the infiltration area is lower (1100–1300 m) showing circulation from the Mont Joly Massif. The geothermometry method indicates a reservoir temperature of probably up to 65 °C but not exceeding 100 °C.

2009, Portier, Sandrine, Vuataz, François, Nami, Patrick, Sanjuan, Bernard, Gérard, André

Rock matrix stimulation is a method of enhancing well production or injection within a broad range of challenging environments, varying from naturally fractured limestones to sandstones with complex mineralogy. A common and often successful stimulation option, matrix acidizing, utilizes acids that react and remove mineral phases restricting fluid flow. Reviewed is the technology of chemical treatments available for oil, gas and geothermal wells and the key elements and results of the chemical reservoir stimulation program at the Soultz-sous-Forêts, France, Enhanced Geothermal System Project.