Surface gas geochemistry above the natural CO2 reservoir of Montmiral (Drôme, France), source tracking and gas exchange between the soil, biosphere and atmosphere

International audienceOne of the options considered to mitigate greenhouse gas concentrations in the atmosphere is underground storage of CO2. There is a strong need for enhancing and developing methods that would help throughout the duration life of such underground storage, to ensure the safety and able to monitor the evolution of the injected CO2 plume. Among these, geochemical methods can play an important role. Here, we describe results acquired under the research programme “Géocarbone-Monitoring”, partially funded by the French National Research Agency, on the Montmiral natural analogue in South-Eastern France. Other results obtained under the same research programme in the French Massif Central are reported elsewhere in this volume.Spot sampling methods allowing a great geographical coverage and continuous measurements on selected points were undertaken in 2006 and 2007, in order to determine soil gas concentrations and fluxes as well as carbon isotope ratio determinations. One important result is that without any evidence of deep CO2 leakage, both CO2 concentrations and fluxes appear to be higher than can be explained only by biological activities. Further investigations are thus needed to understand the gas evolution better throughout the year

Geochemical Study of Natural CO2 Emissions in the French Massif Central: How to Predict Origin, Processes and Evolution of CO2 Leakage

International audienceThis study presents an overview of some results obtained within the French ANR (National Agency of Research) supported Géocarbone-Monitoring research program. The measurements were performed in Sainte-Marguerite, located in the French Massif Central. This site represents a natural laboratory for CO2/fluid/rock interactions studies, as well as CO2 migration mechanisms towards the surface. The CO2 leaking character of the studied area also allows to test and validate measurements methods and verifications for the future CO2 geological storage sites. During these surveys, we analyzed soil CO2 fluxes and concentrations. We sampled and analyzed soil gases, and gas from carbo-gaseous bubbling springs. A one-month continuous monitoring was also tested, to record the concentration of CO2 both in atmosphere and in the soil at a single point. We also developed a new methodology to collect soil gas samples for noble gas abundances and isotopic analyses, as well as carbon isotopic ratios. Our geochemical results, combined with structural geology, show that the leaking CO2 has a very deep origin, partially mantle derived. The gas rises rapidly along normal and strike-slip active faults. CO2 soil concentrations (also showing a mantle derived component) and CO2 fluxes are spatially variable, and reach high values. The recorded atmospheric CO2 is not very high, despite the important CO2 degassing throughout the whole area

Geochemical monitoring of the thermal manifestations located near the Bouillante geothermal power plant, in Guadeloupe (FWI)

International audienceThe Bouillante high-temperature geothermal field, 250-260°C, is developed around the Bouillante Bay along the western coast of Basse-Terre Island, in Guadeloupe (French West Indies, FWI). Since 2005, the new geothermal power plant has an installed capacity of 15.5 MWe gross, which represents about 6-7% of the annual electricity needs in the Island. Among the research efforts carried out by the BRGM group and partially funded by ADEME (French Environment and Energy Management Agency), different geophysical and geochemical monitoring methods of the geothermal exploitation have been started, tested and developed 1 in order to optimize and secure electricity production and to control its impact on the immediate environment of the power plant. This is especially important because the power plant is located inside the Bouillante town. Among these methods, this paper presents the main results and conclusions obtained for the geochemical monitoring of the terrestrial thermal manifestations located near the geothermal power plant. This study shows that, except for the " Tuyau " hot water, located inside the main production area and for which the steam contribution increases and salinity decreases when the power plant is in production (decreasing reservoir pressure), no significant geochemical change depending on the geothermal exploitation is observed in the fluids of the other terrestrial thermal manifestations. Consequently, the quality of the thermal waters of the Bouillante area can be considered as not damaged by the geothermal exploitation

Impact of carbo-gaseous saline waters registered by soils

International audienceThe impact of carbo-gazeous saline spring waters, rich in Ca, Fe, As and P and chemically stable through time, on the chemistry (major and trace elements) and mineralogy of soils developed from anatexite is presented. The soils developed beyond the influence of the spring are typical of a granite pedogenesis on a granitic bedrock with Ca loss, Si, Al and K conservation. The soils influenced by the springs are enriched in Ca and Fe, respectively, precipitated as carbonates and oxides. In such soils, the presence of two Ca-enrichment peaks may be explained by the occurrence of two distinct precipitation mechanisms for the carbonates: (1) related to degassing of the carbo-gaseous waters upon emergence at the surface, and (2) in the water-unsaturated zone, related to capillary rise and evaporation processes. The precipitation of iron oxides is related to a change in the redox potential of the mineral waters, following their emergence at the surface.The simultaneous association of As+P with Fe, as evidenced by principal components analysis and in the patterns in concentrations vs. depth observed in soils, can be explained by adsorption and/or coprecipitation of As and P during iron-oxide formation, while The As and P enrichments and the carbonate formation are independent.The extent of the spring influence was studied: the soil enrichments in Ca and, particularly, Fe sharply decrease with increasing distance from the spring on a metric scale

An assessment of anisotropic phase-field models of brittle fracture

International audienceIn several classes of ductile and brittle materials consisting of different cleavage planes, an orientation dependency of the fracture process is observed. It leads for instance to complex failure behaviours and crack paths in polycrystalline or architected materials. This paper focuses on modeling anisotropy of brittle fracture by means of a variational phase-field approach. More precisely, we study different models including several phase (or damage) variables corresponding to different damage mechanisms. First, we recall a multi-mechanism gradient damage model based on an anisotropic non-local fracture energy. We then consider a model accounting for an anisotropic degradation of the elasticity stiffness tensor. Both types of anisotropies are compared in terms of their influence on analytical homogeneous solutions under uniaxial and biaxial tensile loadings. Weak and strong anisotropies are captured via the chosen multi-mechanism damage framework. The models are implemented numerically by using a finite element discretization. In order to improve numerical performance, we implement an algorithm based on a hybrid direct-iterative resolution of the displacement sub-problem. Accuracy of model prediction is assessed by comparing numerical results to theoretical solutions under uniaxial loading. Benchmark numerical tests on notched and perforated plates highlight the role of material parameters on the fracture anisotropy. Furthermore, both models are able to retrieve zigzag crack patterns observed in prior numerical and experimental studies. Finally, we discuss the predictions of a model combining both types of anisotropies

Natural flow and vertical heterogeneities in a sedimentary geothermal reservoir (Paris Basin, France): Geochemical investigations

Three geothermal wells tapping the Dogger aquifer were studied in detail for their variations in chemical composition with time or conditions of exploitation. Analytical improvements for the determination of Cl, SO{sub 4}, Ca, Mg, Na and K make it possible to detect variations respectively of 0.15, 0.8, 0.6, 1.8, 1.8 and 1.4 %. Despite the fact that the natural flow may be important in some parts of the basin aquifer, we conclude that this factor is not responsible for the small variations noticed in mineralization within the one year survey period. The results concerning reactive and nonreactive species are best explained if a vertical heterogeneity of the chemistry of the fluid is assumed. A number of calcareous sub-layers, already demonstrated by geological studies, contribute to varying degrees to the production of the hot water. The changes in pumping rates, which are fixed according to external requirements, play a major role in the hydrodynamic and chemical disequilibrium of the wells. The consequences for the geothermal exploitations are emphasized

Chemistry of a low temperature geothermal reservoir: The Triassic sandstone aquifer at Melleray, FR

The Triassic sandstone aquifer offers on a regional scale, a large potential for low-temperature geothermal exploitation in the Paris Basin. The Na-Cl water n the aquifer has highly variable mineralization (TDS = 4 to 110 g/l) and a wide range of temperature (50º to >100ºC). Chemical studies have been carried out on the Melleray site near Orléans, where a single wel was producing a Na-Cl geothermal water (TDS = 35 g/l) at a wellhead temperature of 72ºC to provide heat for greenhouses. The purpose of these studies is to understand the chemical phenomena occurring in the geothermal loop and to determine the treatment of the fluid and the exploitation procedures necessary for proper reinjection conditions to be achieved. During the tests performed after the drilling operations, chemical variations in the fluid were noticed between several producing zones in the aquifer. Daily geochemical monitoring of the fluid was carried out during two periods of differing exploitation conditions, respectively pumping at 148 m{sup 3}/h and artesian flow at 36 m{sup 3}/h. Vertical heterogeneities of the aquifer can explain the variations observed for the high flowrate. Filtration experiments revealed that the particle load varies with the discharge rate and that over 95 weight % of the particles are smaller than 1 micrometer. The chemistry of the particles varies greatly, according to their origin as corrosion products from the well casing, particles drawn out of the rock or minerals newly formed through water-rock reactions. Finally, small-scale oxidation experiments were carried out on the geothermal fluid to observe the behavior of Fe and SiO{sub 2} and to favour particle aggregates for easier filtration or decantation processes