3-D Magnetotelluric Investigations for geothermal exploration in Martinique (Lesser Antilles). Characteristic Deep Resistivity Structures, and Shallow Resistivity Distribution Matching Heliborne TEM Results

Within the framework of a global French program oriented towards the development of renewable energies, Martinique Island (Lesser Antilles, France) has been extensively investigated (from 2012 to 2013) through an integrated multi-methods approach, with the aim to define precisely the potential geothermal ressources, previously highlighted (Sanjuan et al., 2003). Amongst the common investigation methods deployed, we carried out three magnetotelluric (MT) surveys located above three of the most promising geothermal fields of Martinique, namely the Anses d'Arlet, the Montagne Pel{\'e}e and the Pitons du Carbet prospects. A total of about 100 MT stations were acquired showing single or multi-dimensional behaviors and static shift effects. After processing data with remote reference, 3-D MT inversions of the four complex elements of MT impedance tensor without pre-static-shift correction, have been performed for each sector, providing three 3-D resistivity models down to about 12 to 30 km depth. The sea coast effect has been taken into account in the 3-D inversion through generation of a 3-D resistivity model including the bathymetry around Martinique from the coast up to a distance of 200 km. The forward response of the model is used to calculate coast effect coefficients that are applied to the calculated MT response during the 3-D inversion process for comparison with the observed data. 3-D resistivity models of each sector, which are inherited from different geological history, show 3-D resistivity distribution and specificities related to its volcanological history. In particular, the geothermal field related to the Montagne Pel{\'e}e strato-volcano, is characterized by a quasi ubiquitous conductive layer and quite monotonic typical resistivity distribution making interpretation difficult in terms of geothermal targets. At the opposite, the resistivity distribution of Anse d'Arlet area is radically different and geothermal target is thought to be connected to a not so deep resistive intrusion elongated along a main structural axis. Beside these interesting deep structures, we demonstrate, after analyzing the results of the recent heliborne TEM survey covering the whole Martinique, that surface resistivity distribution obtained from 3-D inversion reproduce faithfully the resistivity distribution observed by TEM. In spite of a very different sampling scale, this comparison illustrates the ability of 3-D MT inversion to take into account and reproduce static shift effects in the sub-surface resistivity distribution.Comment: Wordl Geothermal Congress 2015, Apr 2015, Melbourne, Australi

Effects of fro and HzO on andesite phase relations between 2 and 4 kbar

International audienceExperimentalp hasee quilibriah ave been investigatedo n three medium-K silicic andesite (60-61 wt % SiO2) samples from Mount Pelhe at 2-4 kbar, 850-1040øC, under both vapor-saturatedC O2-freea nd vapor-saturatedC O2-bearingc onditions.M ost experiments were crystallizatione xperimentsu singd ry glassesp reparedf rom the naturalr ocks.B oth normal-a nd rapidq uenche xperimentsw ere performed.T wo rangeso f oxygenf ugacity( fo2) were investigatedN: NO (Ni-NiO buffer) to NNO + 1 and NNO + 2 to NNO + 3. At 2 kbar for moderatelyo xidizing conditionsp, lagioclase( pl) and magnetite( mt) are the liquidus phases, followed by low-Ca pyroxene (opx); these three phases coexist over a large temperature( T)-H20 range( 875-950øC and 5-7 wt % H20 in melt). Amphibole (am) is stable undern ear vapor-saturatedC O2-freec onditionsa t 876øC. At 900øC, ilmenite (ilm) is found only in experimentsle sst hano r equalt o NNO. Upon increasingp ressure( P) underv aporsaturatedC O2-freec onditionsp, l + mt is replacedb y am + mt on the liquidusa bove3 .5 kbar. For highly oxidizingc onditionsm, t is the solel iquidusp hasea t 2 kbar, followed by pl and opx, except in the most H20-rich part of the diagram at 930øC, where opx is replaced by Carich pyroxene( cpx) and am. Compositionso f ferromagnesianp hasess ystematicallyc orrelate with changingfo2E xperimentalg lassesr angef rom andesitict hroughd acitict o rhyolitic, showings ystematicc ompositionavl ariationsw ith pl + opx + mt fractionation( increaseo f SiO2a ndK 20,d ecreasoef A1203C, aO,F eOt, andM gO).F eO*/MgOm oderateilnyc reases with increasingS iO2.F orfo2 conditionst ypical of calk-alkalinem agmatism( approximately NNO + 1), magnetite is either a liquidus or a near-liquidus phase in hydrous silicic andesite magmas,a nd this shoulds timulater eexaminationfo r the mechanismso f generationo f andesitesb y fractionationf rom basalticp arents

Géothermie haute température : le défi énergétique

Alors qu'elle représente un potentiel important pour la production d'électricité dans la plupart des îles volcaniques de l'Outre-mer, qui voient grimper leur facture énergétique et leurs rejets en CO2, la géothermie haute température a du mal à s'y développer. Les objectifs ambitieux du Grenelle de l'environnement qui visent à produire, à l'horizon 2020, 50 % de l'électricité de l'outre-mer grâce aux énergies renouvelables, sont une opportunité, sans précédent, pour mettre en place une stratégie cohérente de développement de la géothermie dans ces îles

Analysis of fractures in boreholes of the Hot Dry Rock project at Soultz-sous-Forêts (Rhine graben, France)

The Hot Dry Rock (HDR) geothermal drill site is located in the Upper Rhine Graben at Soults-sous-Forêts (Alsace, France). Two boreholes reach a highly fractured basement of porphyritic granite, potential site for a geothermal reservoir. Data from the striated faults observed on cores enabled us to compute the palestress tensors correspondng to the tectonic regimes that affected the granite. Four stress states have thus been determined: a N20°E compression , an N100°E extension, a N80°E compression ans a N120°E compression. Both naturaland induced fractures were recognised in cores. The mean strike of natural fractures changes progressively with depth from N170°E to N200°E. Theses fractures from conjugate systems and their dip directio

OVER 10 YEARS OF GEOLOGICAL INVESTIGATIONS WITHIN THE HDR SOULTZ PROJECT, FRANCE

International audienceSeveral deep wells were drilled in the Rhine graben (Soultz, France) to evaluate the geothermal Hot Dry Rock potential of a deep fractured granite reservoir. Three main boreholes, which reached 2200, 3600 and 5100 m depth, intersected a crystalline basement overlain by 1400m of Cenozoic and Mesozoic sediments. Based on extensive geological database collected from 1987 to 1999, deep geology of the Soultz reservoir including hydrothermal alteration studies as well as an extensive fracture evaluation were characterised from core analysis, well-logging and borehole imagery interpretations. Conceptual models of hydrothermally altered and fractured zones are proposed. In order to build small-scale fracture distribution observed at Soultz from core data, simulation of 3D fracture network was done stochastically. At higher scale, several deterministic geometrical models were built by assuming fracture size in 3D. They showed that some potential connecting paths are outlined between the deep wells

Comparative analysis of direct (core) and indirect (borehole imaging tools) collection of fracture data in the Hot Dry Rock Soultz reservoir (France)

International audienceAttributes of several thousand fractures were collected in three boreholes of 2.2, 3.6, and 3.8 km depth, penetrating the Soultz Hot Dry Rock reservoir (France). The fractures were sampled from cores and from several high-resolution imaging techniques such as borehole televiewer (BHTV), ultrasonic borehole imager (UBI), formation microscanner (FMS), formation microimager (FMI), and azimuthal resistivity imaging (ARI). A comparison was made between the data collected on cores and those provided by different imaging techniques. The comparison clearly establishes that the different wall-images are not as exhaustive as the core data and cannot provide a complete characterization of the fracture network. Discrete fractures thinner than 1 mm are not properly detected. This is also the case for discrete fractures closer than 5 mm, which appear only as single traces. The imaging techniques are, nevertheless, very powerful for characterizing altered fracture clusters. Whatever the technique used, the fracture strikes were correctly sampled with the different systems. This comparison allowed us to calibrate the fracture population data obtained from the imaging system in order to correct for the filtering effect introduced by the technique itself and by the alteration of the rock mass

Magma storage conditions and control of eruption regime in silicic volcanoes: experimental evidence from Mt. Pelée

International audienceDifferences of eruption regimes in silicic volcanoes, e.g. effusive versus explosive, have commonly been ascribed either to stratification of volatiles in the magma storage region or to gas loss through permeable conduit walls. Recent Plinian and Pelean eruptions of silicic andesite magmas from Mt. Pelée (P1: 650 yr B.P., 1902, 1929) show no systematic variations in bulk rock and phenocryst and glass compositions. Rare coexisting Fesingle bondTi oxide pairs in Pelean products yieldT between 840 and 902°C, and ΔNNO between +0.4 and +0.8. Pre-eruptive melt H2O contents, calculated from plagioclase-melt equilibria, span values from 1.9 to 5.5 wt%. Glass inclusions from the P1 Plinian fallout have H2O contents between 4.2 and 7.1 wt%. In contrast, the Pelean inclusions have H2O contents commonly <3 wt%, due to post-entrapment modifications upon eruption. Phase equilibrium studies allow pre-eruptive conditions to be precisely determined and demonstrate that recent eruptions, either Plinian or Pelean, tapped magmas with melt H2O contents of 5.3-6.3 wt%, stored at 2 ± 0.5 kbar, 875-900°C and ΔNNO = +0.4-0.8. Differences in eruptive style at Mt. Pelée are unrelated to systematic variations in pre-eruptive magmatic H2O concentrations, but may be caused by contrasting modes of degassing in the conduit

Life Cycle Assessment of High Temperature Geothermal Energy Systems

International audienceEuropean and French regulations state that 50% of the energy mix in the French Caribbean should be sourced from renewable energies by 2020. Because of the volcanic conditions of the French Caribbean islands, geothermal energy would seem to be a very favorable solution to reach this ambitious objective, as, unlike other renewable sources, it is continuous and weather independent. According to the Intergovernmental Panel on Climate Change (IPCC), geothermal energy source is recognized as a competitive energy source (with a carbon footprint around 50 gCO 2eq /kWh over its lifetime) compared to conventional energies such as coal or oil (with a carbon footprint around 800 g CO 2 eq /kWh). The IPCC make their overall environmental assessments of energy pathways using Life Cycle Assessment (LCA). LCA assesses the environmental and human health impacts throughout the life cycle stages of a product by providing a " cradle-to-grave " environmental profile. A LCA of an existing high temperature geothermal system is reported here with two objectives: quantifying the environmental impacts of a geothermal plant installed in the French Caribbean islands, and comparing and identifying technological alternatives which potentially reduce its environmental impacts. The geothermal power plant assessed in this study is Bouillante geothermal power plant located in the Guadeloupe island. Built in the 80s, Bouillante is a high temperature geothermal system (the reservoir temperature is around 250°C) which is representative, in terms of size, spatial and technological constraints, of future power plants to be developed in French overseas territories. Its medium size (15.75 MW) enables it to supply 6 to 7% of Guadeloupe's annual electricity needs. It has two production units: UB1, a double flash technology (4.75 MW), and UB2, a simple flash technology (11 MW). The data inventory is mainly based on site-specific data, extracted from drilling reports: annual environmental and exploitation reports, and technical sheets completed with personal communication with experts. This power plant however presents some unusual design configurations related to the age of its construction: use of a sea water cooling system and absence of geothermal fluid reinjection. To model a configuration that fits better with current practices, two new scenarios based on alternative technologies are considered: a cooling tower or air dry cooling condensers. Three scenarios are assessed via a multicriteria approach using a selection of life cycle environmental indicators: climate change, water consumption, eutrophication, land use, ecotoxicity, primary energy demand, abiotic depletion, acidification and human toxicity. These environmental indicators are assessed at all phases of the plant life cycle: drilling, construction and installation of the surface equipment, operation, and end of life (decommissioning). First results show that greenhouse gases (GHG) are mostly generated at the operation step (around 90% of total GHG) and are mainly due to leakage of CO 2 and CH 4 emissions (a geothermal stream is composed of non-condensable gases fraction such as CO 2 , and CH 4 which are emitted due to the decrease in pressure). Results range from 38 to 47 gCO 2eq /kWh over the 3 scenarios. Primary energy demand is mainly due to the construction and installation phase (around 70% of total energy consumption) from background processes such as steel or copper production processes. The primary energy demand and GHG for the reference scenario and the cooling tower system alternatives are found to be lower than those for the aerocondenser cooling system scenario (for the same energy production). As an outcome of the study, we establish the development of a general parameterized LCA model developed for conventional geothermal systems with a temperature reservoir ranging from 230°C to 300°C. Results obtained from this model enable high temperature geothermal systems to be positioned from an environmental perspective in comparison with other energy systems, and also highlight the main drivers leading to the reduction of environmental impacts of future geothermal systems

Fracture analysis and reservoir characterization of the granitic basement in HDR Soultz project (France)

International audienceWithin the framework of the European Hot Rock (HDR) project, the natural fracture geometry of the deep granitic basement in the Upper Rine Graben has been studied at Soultz-sous-Forêts (France). The porphyritic granite is characterized by hydrothermally altered and fractured sections. Detailed structural analysis of continuous core samples and borehole wall imagery indicates that fracture zones constitute a complex network of fault segments dominated by N-S trends in accordance with the geometry of the local Soultz horst structure.Some of these fault segments still carry hydrothermal brines. The attitude of the faults is closely related to rifting of the graben during the Tertiary. The fractures strike in a direction nearly parallel to the maximum horizontal stress. In this favorable situation, hydraulic injection will tend to reactivate natural fractures at low pressures

Geothermal exploration of the Lamentin area using Controlled-Source EM method (Martinique, Lesser Antilles, France)

Within the framework of renewable energy development in France and the specific will of energetic independence of Martinique, a large exploration program founded by the FEDER, ADEME, Regional Council and SMEM has been designed to explore the geothermal potential of Martinique. During 2012 and 2013, multi-method investigations have been carried out on the four main regions already recognized for their high enthalpy geothermal potential. The Lamentin area is partly urbanized (including housing and commercial centers close to Fort-de-France) and is equipped with an international airport. The remaining zone is whether densely cultivated whether in the mangrove. Considering the high anthropic noise (powerlines and various installations) it was then proposed to investigate at depth using a Controlled-Source Electro-Magnetism approach (CSEM). The conditions for applying standard processing of CSAMT being not fulfilled, it was necessary to compute a specific normalization to correct the source effect. Indeed, when it is possible to measure the receiver stations far enough from the source, the distortion due to the controlled-source disappear. But the geography of the area of interest and the pre-existing exploration boreholes used to inject current at depth make it impossible in this case. This presentation presents the campaign realization and the results obtained. The results were confronted to previous geophysical campaign results performed in the 1980's and included in the interpretation of the multidisciplinary approach recently performed