Toward an exploration method for sedimentary basins and basement

International audienceIn order to expand the geothermal energy exploitation, we need to explore various geological contexts, particularly the sedimentary basins, which concentrate customers. Nowadays, with the improvement of the binary power plant productivities and the need of industrial heat, the deep layers of sedimentary basins and the upper part of the basement are attractive to tap fluid with temperature between 120°C and 150°C. In this context, we propose to set up and validate an integrated exploration workflow that consists in building, step by step, models based on larger scale models and key situations identifiable by innovative and validated exploration methods. Therefore, three questions arise:-What are we looking for? What are the key parameters and the key situations that allow withdrawing the geothermal fluid?-Which methods are able to identify these key situations at the different scales?-How used these methods to find the best drilling place and reduce the financial risk of drilling? At the base, a geological and structural model has been done from public data and improves from geophysical and geological analysis. Thermal, Hydraulic and Mechanical models, independent or partially coupled, are built based on this geological model and from other data like temperature, fluid geochemical and stress data. Three scales have been considered in this pre-drill exploration workflow: continental scale (around 1000kmx1000km) considers temperature, heat flow and stress field that constrain regional scale; the regional scale (50 to 100km) covers a geological region considered as relevant; the local scale (10 to 25km) corresponds to the focus which is made at the last step. The way between the different scales is done by the analysis of the key situations defining the geothermal location. Finally, at the smaller scale, the best geothermal location is refined in terms of risk and cost for the first exploration well

Toward an exploration method for sedimentary basins and basement Chrystel Dezayes and the IMAGE-SP3 team

International audienceIn order to expand the geothermal energy exploitation, we need to explore various geological contexts, particularly the sedimentary basins, which concentrate customers. Nowadays, with the improvement of the binary power plant productivities and the need of industrial heat, the deep layers of sedimentary basins and the upper part of the basement are attractive to tap fluid with temperature between 120 • C and 150 • C. In a normal geothermal gradient (around 30 • C/km) context, this temperature range is reached between 3 and 5 km depth and corresponds to the deep layers of sedimentary basins (aquifers more or less permeable, namely HSA) and the upper part of the basement, more or less fractured and altered, having potentially used EGS technologies. In this context, we propose to set up and validate an integrated exploration workflow that consists in building, step by step, models based on larger scale models and key situations identifiable by innovative and validated exploration methods. Therefore, three questions arise:-What are we looking for? What are the key parameters and the key situations that allow withdrawing the geothermal fluid?-Which methods are able to identify these key situations at the different scales?-How used these methods to find the best drilling place and reduce the financial risk of drilling? At the base, a geological and structural model has been done from public data and improves from geophysical and geological analysis. Thermal, Hydraulic and Mechanical models, independent or partially coupled, are built based on this geological model and from other data like temperature, fluid geochemical and stress data. Three scales have been considered in this pre-drill exploration workflow: continental scale (around 1000kmx1000km) considers temperature, heat flow and stress field that constrain regional scale; the regional scale (50 to 100km) covers a geological region considered as relevant; the local scale (10 to 25km) corresponds to the focus which is made at the last step. The way between the different scales is done by the analysis of the key situations defining the geothermal location. Finally, at the smaller scale, the best geothermal location is refined in terms of risk and cost for the first exploration well

Fluid paleocirculations at the cover/granite interface in the Rhine graben

International audienceThe Rhine Graben is a major site of development for the geothermal heating production in France. Targeted geothermal reservoirs are in deep Hercynian granitic basement which is fractured dominated system, and more recently at the cover/basement interface.In this framework of geothermal exploration, a better understanding of the hydraulic behaviour of the fracture network and fluid/rock interactions is needed. For that fracture fillings inHercynian granitic basement and in the formations of the cover (Permian rhyolites, Permian and Triassic sediments) were studied for mineralogy, fluid inclusion microthermometry and (C, O, Sr) isotopes in order to trace paleocirculations at the cover/granite interface in the Rhine Graben. Data were acquired on fracture fillings in samples of the basement/cover interfaces from the EPS1 borehole at Soultz-sous-Forêtin the Rhine grabenat ~ 1417 meters depth, and from outcrops in quarries on the flanks of the graben (Waldhambach, Saint Pierre Bois, Windstein, Heidelberg...). Mineral sequences of polyphasedfillings were interpreted in relation with the geological contextincludinglate evolution of the Hercynian basement and major extensive tectonic events. Quartz, carbonates, sulfates and illiteare major minerals identified in fractures crosscutting Hercynian granites, Permian rhyolite (Waldhambach) and Permian and Triassic sedimentary cover. Although quartz being considered as a major mineral filling fractures, petrological observations showed that carbonates are also an important and probably underestimated phase of filling, and of interest for two reasons. Firstly, from a geothermal point of view, they contribute to the clogging of fractures. Secondly, from a scientific point of view, they are informative on the variations of fluid chemistry through geological times. Among carbonates, dominant dolomite with minor ankerite, Mn-bearing carbonates and siderite was identified by CL, SEM and EPMA in fractures. A same generation of dolomite was identified in fractures crosscutting Hercynian granite and in Permian and Triassic sandstones of the Rhine Graben (EPS1 borehole) but also on the flank of the Rhine Graben in Hercynian granite of Waldhambach, Heidelberg and Windstein quarries, and in Permian rhyolite at Waldhambach. This dolomite is Fe and Mn poor, formed at ~120-130°C, and has a 87Sr/86Sr of ~0.708-0.709. Barite is the major sulphate observed in fracture filling, already formed later than dolomite. Rare microthermometric data combined with strontium isotopes provide evidence of several generations of barite with one generation formed at ~130°C. Preliminary data on dolomite provide evidence of large fluid circulations at the cover/granite interface.This dolomite is observed at least at depth down to 1650 m of the granite in the EPS1 borehole, i.e. with a minimum penetration of 200 m into the granite. Alternating deposition of ankerite and dolomite in fracture corridors strongly suggest pulses of fluids. Such fracture fillings at 1641 meters depth were attributed to present-day fluid circulations. However similar generation of dolomite also observed in fracture corridorcrosscutting the Hercyniangranite and Permian rhyolite at Waldhambachon the flank of the Rhine graben demonstrate that the fluid circulation

Underground thermal energy storage in subarctic climates: A feasibility study conducted in Kuujjuaq (QC, Canada)

Underground thermal energy storage can provide space and water heating and has been used in temperate climates so far. A step forward is to evaluate the efficiency and viability in arctic to subarctic environments, where rather low ground and air temperatures can make the design of such systems difficult. The present contribution describes the design of an underground storage system in Kuujjuaq (Québec, Canada) to heat the drinking water distributed in the town. The system was designed and modeled with TRNSYS and a parametric study was carried out to improve its efficiency based on 5-year simulations. The 20% of the 425 MWh annual demand can be satisfied by a solar collector area of 500 m2 coupled to a 10,000 m3 underground storage through two short term tanks. Further improvements could be adopted to reach the target of 50% energy from the underground store

Caractérisation et interprétation d'un volume rocheux fracturé à partir de données de forages : les forages géothermiques de Soultz-sous-Forêts

La caractérisation des réseaux de fractures est essentielle pour comprendre la circulation des fluides à l'intérieur d'un volume rocheux. Situé en profondeur, un volume ne peut généralement être exploré que par forage. Notre intérêt s'est donc porté sur l'échantillonnage des fractures en forage, qui constitue cependant une vision limitée du plan de fractures. De plus, les forages généralement verticaux ont une faible probabilité de recouper les fractures qui lui sont parallèles. L'étude présentée a été menée sur deux sites différents. A Soultz-sous-Forêts (Alsace, France), trois forages profonds traversent le socle cristallin du fossé du Rhin. Les fractures ont pu être observées sur des carottes et sur des images dia graphiques des parois. Sur le site de Ravenscar (Yorkshire, G.B.), 18 forages peu profonds ont été réalisés dans une série sédimentaire deltaïque, dont la fracturation a été reconstituée à partir des photographies de carottes. A plus grande échelle, la fracturation est constituée par des traînées d'inclusions fluides observées en lame mince dans les cristaux de quartz. Cette fracturation a été reconstituée à partir de l'observation réalisée dans trois plans de coupe perpendiculaires. Il apparaît ainsi deux familles N90°E semblant correspondre à une extension N-S permo-triasique. Quatre familles subméridiennes apparaissent liées aux régimes de tension E-W paléogène, et sont comparables aux familles de mésofractures observées sur les carottes. Enfin, deux familles subverticales semblent provenir de la compression NW-SE récente. Les familles de mésofractures correspondantes sont donc sous-échantillonnées par les forages verticaux. En conclusion, nous suggérons pour l'étude des fractures en forage: - de réaliser les observations sur des carottes orientées - d'avoir à notre disposition différents forages dans plusieurs directions afin de permettre un échantillonnage complet des fractures.pas de résum

A new geothermal exploration workflow for deep sedimentary basins and basement

International audienceIn order to expand the geothermal energy exploitation, we need to explore various geological contexts, particularly the sedimentary basins, which concentrate many customers. Nowadays, with the improvement of the binary power plant productivities and the need of industrial heat, the deep layers of sedimentary basins and the upper part of the basement are attractive to tap fluid with temperature between 120°C and 150°C. In a normal geothermal gradient (around 30°C/km) context, this temperature range is reached between 3 and 5 km depth and corresponds to the deep layers of sedimentary basins (aquifers more or less permeable, namely HSA) and the upper part of the basement, more or less fractured and altered, having potentially used EGS technologies. In such complex and diverse geological setting, we propose to set up and validate an integrated and scale-dependent exploration workflow that consists in building, step by step, more and more refined subsurface models based on larger scale models To achieve this, three questions arise:-What are we looking for? What are the key parameters and the key situations favorable for geothermal heat/fluid extraction? In order to optimize the exploitation, the three essential parameters to know are obviously temperature (the highest at the shallowest depth), the natural permeability (both from porosity and fractures), and the presence of brines and recharge waters. These key parameters are related to the geological patterns and subsurface phenomena, which are called key situations and constrain spatially the favorable areas.-Which methods are able to identify these key situations at the different scales? Standard exploration methods (e.g. active/passive seismics, electromagnetics, geothermometry and geology) have been tested on the specific case of basement and deep sedimentary layers to validate which techniques are relevant to identify key situations. Whereas some of these can be very capital-intensive (e.g. active seismics), the economic benefit to the project through its geological risk reduction can easily compensate for the upfront investment