Opalinus Clay is currently being assessed as the host rock for a deep geological repository for high-level and low- and intermediate-level radioactive wastes in Switzerland. Within this framework, the 'Full-Scale Emplacement' (FE) experiment was initiated at the Mont Terri rock laboratory close to the small town of St-Ursanne in Switzerland. The FE experiment simulates, as realistically as possible, the construction, waste emplacement, backfilling and early post-closure evolution of a spent fuel/vitrified high-level waste disposal tunnel according to the Swiss repository concept. The main aim of this multiple heater test is the investigation of repository-induced thermo-hydro-mechanical (THM) coupled effects on the host rock at this scale and the validation of existing coupled THM models. For this, several hundred sensors were installed in the rock, the tunnel lining, the bentonite buffer, the heaters and the plug. This paper is structured according to the implementation timeline of the FE experiment. It documents relevant details about the instrumentation, the tunnel construction, the production of the bentonite blocks and the highly compacted 'granulated bentonite mixture' (GBM), the development and construction of the prototype 'backfilling machine' (BFM) and its testing for horizontal GBM emplacement. Finally, the plug construction and the start of all 3 heaters (with a thermal output of 1350 Watt each) in February 2015 are briefly described. In this paper, measurement results representative of the different experimental steps are also presented. Tunnel construction aspects are discussed on the basis of tunnel wall displacements, permeability testing and relative humidity measurements around the tunnel. GBM densities achieved with the BFM in the different off-site mock-up tests and, finally, in the FE tunnel are presented. Finally, in situ thermal conductivity and temperature measurements recorded during the first heating months are presented.
The concentrations of CO2 and H2S in undisturbed liquiddominated high-temperature geothermal reservoir waters are generally controlled by temperature dependent equilibria with various mineral buffers. These equilibria cause the concentrations of these gases to increase with temperature. The presence of equilibrium steam in the reservoir (two phase reservoir) will cause the gaseous concentrations in the fluid to be higher than the aqueous equilibrium concentrations at any particular temperature. In the range of about 230-300°C, the CO2 buffer is considered to be clinozoisite + prehnite + quartz + calcite. In hightemperature waters of low salt content, which are strongly reducing, the H2S buffer is considered to be pyrite + pyrrhotite + epidote + prehnite. In waters of higher salinity, the respective H2S mineral buffer may consist of pyrite + magnetite + hematite. The concentrations of CO2 and H2S in steam of wet-steam wells producing from liquiddominated reservoirs are higher than those of the parent fluid, frequently in the range 50-300 and 2-20 mmoles/kg of steam, respectively. However, values as high as 1000 mmoles/kg for CO2 and 50 mmoles/kg for H2S are not uncommon. The concentrations of these gases in steam from wet-steam wells depend on 1) their concentration in the parent geothermal water, 2) the steam fraction, which has formed by depressurization boiling, 3) the reservoir steam fraction, if present, 4) the steam separation pressure, and 5) the boiling processes, which lead to the steam formation. Long-term utilisation of geothermal reservoirs may lead to decline in the concentrations of CO2 and H2S in the steam. The decline can be caused by recharge of cooler water into producing aquifers and/or progressive boiling of water retained in the aquifer rock by capillary forces. Further, enhanced boiling, which is a consequence of reservoir pressure draw down, and steam separation during lateral flow into production wells may cause the well discharge to become depleted in gas. The separated steam may form a steam cap over the liquid reservoir and/or enhance fumarolic activity. Although gas emissions from geothermal power plants may be enhanced much during the early years of production relative to natural discharge, in the long run, the integrated gas emission may not exceed that of the natural gas flux. A steady state may be reached between the flux of gases from the magma heat source into the geothermal system and from the geothermal system into producing wells and fumaroles. The source of noble gases, apart from He, in geothermal fluids is air saturated meteoric water. The relative abundance of noble gases in geothermal steam may aid assessment of which processes are responsible for changes in the concentrations of the environmentally important CO2 and H2S.
Abstract Ophiolites occur at several places in the Lower Penninic of the W and Central Alps. They are generally ascribed to oceanic crust of a so-called “Valais ocean” of Cretaceous age which plays a fundamental role in many models of Alpine paleogeography and geodynamics. The type locality and only observational base for the definition of a “Valais ocean” in the W Alps is the Versoyen ophiolitic complex, on the French-Italian boundary W of the Petit St-Bernard col. The idea of a “Valais ocean” is based on two propositions that are since 40 years the basis for most reconstructions of the Lower Penninic: (1) The Versoyen forms the (overturned) stratigraphic base of the Cretaceous-Tertiary Valais-Tarentaise series; and (2) it has a Cretaceous age. We present new field and isotopic data that severely challenge both propositions. (1) The base of the Versoyen ophiolite is a thrust. It overlies a wildflysch with blocks of Versoyen rocks, named the Méchandeur Formation. This “supra-Tarentaise” wildflysch has been confused with an (overturned) stratigraphic transition from the Versoyen to the Valais-Tarentaise series. Thus the contact Versoyen/Tarentaise is not stratigraphic but tectonic, and the Versoyen ophiolite has no link with the Valais basin. This thrust corresponds to an inverse metamorphic discontinuity and to an abrupt change in tectonic style. (2) The contact of the Versoyen complex with the overlying Triassic-Jurassic Petit St-Bernard (PSB) series is stratigraphic (and not tectonic as admitted by all authors since 50 years). Several types of sedimentary structures polarize it and show that the PSB series is younger than the Versoyen. Consequently the Versoyen ophiolitic complex is Paleozoic and forms the basement of the PSB Mesozoic sediments. They both belong to a single tectonic unit, named the Versoyen-Petit St-Bernard nappe. (3) Ion microprobe U-Pb isotopic data on zircons from the main gabbroic intrusion in the Versoyen complex give a crystallization age of 337.0 ± 4.1 Ma (Visean, Early Carboniferous). These zircons show typical oscillatory zoning and no overgrowth or corrosion, and are interpreted to date the Versoyen magmatism. These U-Pb data are in excellent agreement with our field observations and confirm the Paleozoic age of the Versoyen ophiolite. The existence of a “Valais ocean” of Cretaceous age in the W Alps becomes very improbable. The eclogite facies metamorphism of the Versoyen-Petit St-Bernard nappe results from an Alpine intra-continental subduction, guided by a Paleozoic oceanic suture. This is an example of the long term influence of inherited deep-seated structures on a much younger orogeny. This might well be a major cause of the inherent complexity of the Alps.
This paper presents the study which has allowed determining the best adapted Na/Li thermometric relationship for the High-Temperature (HT) dilute waters collected from wells located in the Krafla geothermal area, North-east Iceland. This work was carried out in the framework of the European HITI project (HIgh Temperature Instruments for supercritical geothermal reservoir characterization and exploitation). This relationship, which can give estimations of temperature ranging from 200 to 325°C with an uncertainty of ± 20°C, was also successfully used on HT dilute fluids collected from wells located in other Icelandic geothermal areas (Namafjall, Nesjavellir and Hveragerdi). So, a more general Na/Li geothermometric relationship including these other geothermal areas was obtained. These new relationships determined for Icelandic HT dilute geothermal fluids, which are very different from that commonly used for the dilute world geothermal waters, will be tested on the fluids collected during the international Iceland Deep Drilling Project (IDDP) for which the main objective is to drill deep wells (> 5 km) in supercritical reservoir conditions (T > 375°C), located in the Krafla and Reykjanes geothermal areas. Another Na/Li relationship determined for HT saline geothermal fluids derived from sea water and basalt interaction processes will be tested on the Reykjanes supercritical reservoirs. The existence of several different Na/Li geothermometric relationships seems to show that the Na/Li ratios not only depend on the temperature but also on other parameters such as the fluid salinity and origin, or the nature of the geothermal reservoir rocks in contact with the deep hot fluids. Thermodynamic considerations and some case studies in the literature suggest that the Na/Li and Na/K ratios for the HT dilute geothermal waters from Iceland could be controlled by a full equilibrium reaction involving a mineral association constituted, at least, of Albite, K-Feldspar, Quartz and Clay minerals such as Kaolinite, Illite and Na-, Li-Micas.
