Water sampling in low productive boreholes: how to ensure of the representativeness of sampling?

International audienceGood practices of wellbore purging advice to draw three to five times the volume of the water column prior to sample. An extensive literature in the past 20 years has established the biases that may be linked to this procedure with emphasis on contaminant sampling and the benefit low-flow sampling may have to avoid redistribution of contaminants in boreholes because of the flow-weighted average character of pumping (Einarson, 2006, Handbook Environmental Site Characterization and GroundWater Monitoring). Low-flow sampling may produce flow-biased samples if operated in long-screened boreholes where vertical gradients exist (McMillan et al., 2014, J. Contam. Hydrol. 1699, 50-61) and is better suited for high permeability boreholes (ISO Standards). Water sampling in low permeability aquifers remains challenging especially for boreholes drilled for water table level monitoring (long-screened boreholes; pumps may not be lowered down in the screened interval). Fluid Electrical Conductivity logging prior to pumping and sampling may help in locating the productive levels albeit the information obtained under ambient flow conditions may be of less relevance than the data collected using salt injection (e.g. Lasher and Nel, 2013, Groundwater Division Conference, Durban). Fiber optic Distributed Temperature Sensing may also resolve hydraulic (McMillan, 2015, PhD dissertation) but is not of common use. We refer to investigations performed in two boreholes (Labruguière and Valdurenque) of low permeabilities (10 −6 to 10 −7 m.s −1), located in detritic formations in SW France. These two boreholes, of 170 m and 123 m depth respectively, have long screened sections (128-170 m and 75-123 m respectively). With such geometries, the sub-mersible pump cannot be placed in the screened interval to perform volume purge. This raises the question of how long the pumping has to be done to get water representative of the downhole chemistry. The purge of three times the volume of the water column is unrealistic. For Labruguière borehole, it would take 12 hours cumulated at ≤1 m 3 .h −1 pumping rate, each session cannot last more than 2 hours (dewatering) and 12 hours are needed to recover the water table level. We thus refer to deep sampling to assess the usefulness of such a method. Several levels were determined on the basis of ambient logging (temperature, conductivity, pH, dissolved oxygen, redox potential). In parallel we use pumping to assess the purge process inside the borehole and determine the minimum amount of drawn water needed to get water from the screens. This also highlights that alternative method to judge of representativeness, such as stabilization of physico-chemical parameters, may lead to false positives, i.e. the parameters were stabilized but the water chemistry was not that of the screened section. Three cycles of logging – deep sampling – pumping were done in each borehole. Based on field data and laboratory analyses, it appears that a protocol for deep boreholes characterization may refer to 1) borehole logging (information on ambient structure of the water column), 2) slight solicitation of the borehole by pumping (renewal of water at productive levels), and 3) deep sampling at the depth(s) suggested by borehole logs

Towards a Better Knowledge of Natural Methane Releases in the French Alps: A Field Approach

International audienceWe report investigations performed at some hydrocarbon gas seeps located in the French Subalpine Chains in zones of outcropping Jurassic black shales, increasing the reported number of such occurrences in this part of the Alps. We present the characteristics of each of the seeps, based on soil flux measurements and soil gas measurements. Gases emitted are CH4-rich (87-94%) with the exception of one site (78.5% CH4 + 8.2% CO2) where an active landslide may induce dilution by atmospheric air. CO2 is generally measured at low levels (<1.6%). Concentrations in C2H6 are more variable, from less than 1% to more than 2.3%. Gas is emitted over areas of various sizes. The smallest gas emission area measures only 60x20cm, characterized by a strong hydrocarbon flux (release of about 100kg of CH4 per year). At a second site, hydrocarbon emissions are measured over a surface of 12m(2). For this site, methane emission is evaluated at 235kg per year and CO2 emission is 600kg per year, 210kg being related to gas seepage. At the third site, hydrocarbons are released over a 60m(2) area but strong gas venting is restricted to localized seeps. Methane emission is evaluated at 5.1 tons per year and CO2 emission at 1.58 tons per year, out of which 0.53 tons are attributed to gas seepage. Several historical locations remain uninvestigated at present, and numerous others may still be unknown. We outline strategies to search for such unrecorded sites. Considering the topography of the potential alpine and perialpine emission areas, the possibilities to detect gas emissions appear of the size recorded so far seem to be restricted to ground-based methods or to methods offering the possibility to point orthogonally to the soil towards the seep maximum. If such sites are to be investigated in the future in the frame of Environmental Baseline Assessment (EBA), even establishing appropriate monitoring protocols will be challenging

Surface and near surface geochemical surveying of a CO2 injection pilot: application study to the French Pyrenean foreland (Rousse CCS pilot)

The geochemical monitoring of the Rousse injection pilot, operated by TOTAL Exploration Pproduction France, is presented over a 5- years long time period. The monitoring consisted in the acquisition at regular frequency of soil gas concentrations and fluxes at selected sampling points, coupled with the study of the geochemical parameters evolution of a perched aquifer overlying the storage reservoir through a dedicated 85 m depth borehole. Baseline data were acquired between September 2008 and December 2009 then the monitoring shifted to the surveying of the pilot during the operating phase. This second phase ended in March 2013. Data acquired during these two phases are presented and discussed

Main geochemical characteristics of the deep geothermal brine at Vendenheim (Alsace, France) with constraints on temperature and fluid circulation

International audienceThe first analytical results relative to the native geothermal brine discharged from the two deep wells drilled at Vendenheim, in the Rhine Graben, in Alsace (France), obtained within the framework of the Vendenheim FONROCHE geothermal project and the H2020 European DEEP-EGS project, suggest the geochemical composition of this brine is very similar to that of the fluid which was discharged from the neighboring Cronenbourg deep well, in the past. It is also close to that of the brines discharged from the other deep wells located in more northern areas of the Rhine Graben, such as Soultz-sous-Forêts, Rittershoffen, in France, and Landau, Insheim, in Germany. This Na-Cl brine has a TDS value around 100 g/l and a pH value close to 5, before cooling and degassing. Except for the calcium and strontium concentration values, which are much lower than those in the Soultz-sous-Forêts, Rittershoffen, Landau and Insheim brines, the other concentration values of major and trace species are comparable. Given the similarity of the geochemical composition of all these waters, their origin is probably similar but also multiple, because it results from processes of mixing between primary brines formed by advanced evaporation of seawater (probably until the stage of halite precipitation) and meteoric freshwaters, plus contributions from halite dissolution following successive marine transgression-regression cycles from the Triassic to Oligocene. As for the other deep brines, the main solute cation geothermometers give estimations of reservoir temperature close to 225 ± 25°C for the Vendenheim native brine. This estimation probably corresponds to the temperature of equilibrium at which this brine acquires its chemical composition by interaction with the reservoir rocks. The mineralogical assemblage in equilibrium with the brine at this temperature was described in previous studies. The concordant estimations of reservoir temperature, using thermometric relationships such Na-Li and Mg/Li, especially developed for oilfield and sedimentary basin brines, and existing thermal gradients from 40 to 60°C/km, suggest that the deep brines discharged from the granite basement could probably originate from Triassic sedimentary formations (Buntsandstein, for example) located at great depth (≥ 4 km), in the centre of the Rhine Graben, in which they acquire their high salinity and chemical composition at temperatures close to 225 ± 25°C. This assumption seems to be also supported by their Li, B and Sr isotopic signatures. These hot brines would then migrate through a complex, but still poorly defined system of deep faults (probably NE-SW but also NW-SE faults), from the sedimentary centre of the Rhine Graben to the granite-fractured basement and the Graben's NW borders

CO2 leakage in a shallow aquifer - Observed changes in case of small release

International audienceGeological storage of CO2 in deep saline aquifers is one of the options considered for the mitigation of CO2 emissions into the atmosphere. A deep geological CO2 storage is not expected to leak but potential leakage monitoring is required by legislation, as e.g. the EU Directive relative to Geological Storage of CO2. To ensure that the storage will be permanent and safe for the environment and human health, the legislation require that the CCS operators monitor the injection, the storage complex and if needed the environment to detect any CO2 leakage and its hazardous effects on the environment. Various monitoring methods are available for the monitoring of CO2 storage sites and the environment as listed by the IEA-GHG and the monitoring selection tool. Geophysical based methods have a greater area of investigation but may suffer from insufficient sensitivities to detect small leakages. At the opposite, geochemical monitoring methods may have insufficient investigation area but may be able to detect more subtle changes even if monitoring in deep environments is not straightforward. Leakage detection is not yet well constrained and research efforts and tests are required to gain confidence into monitoring strategies. In the framework of the CIPRES project, funded by the French Research Agency, a shallow CO2 release experiment has been performed in October 2013 in a chalk aquifer from the Paris basin. The Catenoy site has been characterised since March 2013 through several wells set on a straight line oriented along the local flow (see Gombert et al., this conference). Such an experiment is designed to gain confidence in leakage detection in subsurface environments by understanding processes and principles governing seepage occurrence. Contrary to other experiments such as ZERT or CO2FieldLab ones, where gaseous CO2 was injected directly in the water, the injection was done with water saturated with CO2 at atmospheric pressure. 10 m3 of water were pumped from the aquifer, then saturated with 20 kg of food-grade CO2 and injected during 40 hours between 12 and 25 m depth. Daily monitoring of soil gases and water was performed during injection and post-injection phases (2 weeks duration) in the area previously delimited by a tracer test. The aim is to determine if geochemical methods are accurate enough to allow detecting small release in shallow environments. If successful, such an experiment can help to gain confidence in leakage detection. As expected, no change was noticed in the unsaturated zone. The shape of gas concentrations distribution at the surface (CO2, O2, N2, 4He, 222Rn) observed during the injection is strictly similar to the repartition of gas species observed since March 2013. The main process observed is respiration and no change linked to the injection was highlighted, only seasonal effects. Slight changes were observed in the saturated zone. The water was collected at 15 m deep excepted for one stratified borehole where water was sampled at 15 and 18 m. The pH of the injected water was lower (mean value: 5.3±0.1) than the initial pH of the aquifer (7.1-7.2) due to CO2 dissolution. Only two monitoring boreholes set 10 m and 20 m downstream from the injection well may be considered as influenced by the experiment. A probable enrichment in HCO3 linked to interaction of the CO2 saturated water with chalk was noticed, with an enrichment close to +8 to +10% of the initial value. For one borehole the pH value remained nearly stable in relation with pH buffering and in the other borehole a slight decrease was observed (-0.1 to -0.15 pH unit). However this decrease is significant as it is above the instrumental uncertainty of the electrodes. In addition, a slight increase of the electrical conductivity was noticed but it did not exceed +6% compared to baseline data. Such slight changes in the physico-chemical parameters are related to small variations in dissolved elements. Apart from HCO3, the other major ion affected by CO2-water rock-interaction is Ca as the aquifer is mainly composed by calcite. Concentrations increases by +8 to +9% whose amplitude is in agreement with the increase of HCO3. Trace elements were also little affected, the main change concerned Sr (+8 to +10% increase). Modifications occurring during this CO2 release experiment have small amplitude as expected but these results highlight that geochemical methods are able to detect small leakages. Consequently, effects were noticed only during a short period of time. It is not possible to determine if all the injected CO2 has migrated downwards in the direction of flow or if partial lateral migration has occurred, but post-injection monitoring and boreholes logging 12 days after the stop of injection did not reveal any discrepancy in the water columns. On the other hand, the magnitude of the pH change is consistent with the behaviour of the co-injected tracer (dilution ratio ~30). In the perspective of getting more information on the remobilisation of trace metal elements, a push-pull test will be performed in 2014 on the basis of the learning of this first experiment

Hydrochemical impatcs of CO2 leakage on fresh groundwater; A field scale experiment

One of the questions related to the emerging technology for Carbon Geological Storage concerns the risk of CO2 migration beyond the geological storage formation. In the event of leakage toward the surface, the CO2 might affect resources in neighbouring formations (geothermal or mineral resources, groundwater) or even represent a hazard for human activities at the surface or in the subsurface. In view of the preservation of the groundwater resources mainly for human consumption, this project studies the potential hydrogeochemical impacts of CO2 leakage on fresh groundwater quality. One of the objectives is to characterize the bio-geochemical mechanisms that may impair the quality of fresh groundwater resources in case of CO2 leakage. To reach the above mentioned objectives, this project proposes a field experiment to characterize in situ the mechanisms having an impact on water quality and the CO2-water-rock interactions and also to improve the monitoring methodology by controlled CO2 leakage in shallow aquifer. The tests ran on an experimental site in the chalk formation of the Paris Basin. The site is equipped with an appropriate instrumentation and previously characterized (8 piezometers, 25 m deep and 4 piezairs 11 m deep). The injection test was preceded by 6 months of monitoring in order to characterize hydrodynamics and geochemical baselines of the site (groundwater, vadose and soil). Leakage into groundwater is simulated via the injection of a small quantity of food quality CO2 (~20 kg dissolved in 10 m3 of water) in the injection well at a depth of about 20 m. A plume of dissolved CO2 is formed and moves downward according to the direction of groundwater flow and probably by degassing in part to the surface. During the injection test, hydrochemical monitoring of the aquifer is done in situ and by sampling. The parameters monitored in the groundwater are the piezometric head, temperature, pH and electrical conductivity. Analysis on water samples provide chemical elements (major, minor and trace metals), dissolved gases, microbiological diversity and isotopes (13C). The evolution of the composition of the groundwater in terms of major elements, trace elements and isotope signatures is interpreted in terms of geochemical mechanisms, and the water-rock-CO2 interactions are characterised. Modification of the chemical composition of the water in the aquifer due to CO2 injection is assessed in term of groundwater quality i.e. metal element release and the possibility of exceeding references and quality of water for human consumption. One outcome of the CIPRES project will be to highlight mechanisms that can impact groundwater quality when a CO2 leakage occurs and to propose recommendations to prevent or/and eliminate negative effects and any risks to the environment and human health. This project is partially funded by the French Research Agency (ANR)

CO2 leakage in a shallow aquifer – Observed changes in case of small release

AbstractGeological storage of CO2 in deep saline aquifers is one of the options considered for the mitigation of CO2 emissions into the atmosphere. A deep geological CO2 storage is not expected to leak but potential leakage monitoring is required by legislation, as e.g. the EU Directive relative to Geological Storage of CO2. To ensure that the storage will be permanent and safe for the environment and human health, the legislation require that the CCS operators monitor the injection, the storage complex and if needed the environment to detect any CO2 leakage and its hazardous effects on the environment. Various monitoring methods are available for the monitoring of CO2 storage sites and the environment as listed by the IEA-GHG and the monitoring selection tool. Geophysical based methods have a greater area of investigation but may suffer from insufficient sensitivities to detect small leakages. At the opposite, geochemical monitoring methods may have insufficient investigation area but may be able to detect more subtle changes even if monitoring in deep environments is not straightforward. Leakage detection is not yet well constrained and research efforts and tests are required to gain confidence into monitoring strategies.In the framework of the CIPRES project, funded by the French Research Agency, a shallow CO2 release experiment has been performed in October 2013 in a chalk aquifer from the Paris basin. The Catenoy site has been characterised since March 2013 through several wells set on a straight line oriented along the local flow (see Gombert et al., this conference). Such an experiment is designed to gain confidence in leakage detection in subsurface environments by understanding processes and principles governing seepage occurrence. Contrary to other experiments such as ZER

CO 2 Migration Monitoring Methodology in the Shallow Subsurface: Lessons Learned From the CO 2 FIELDLAB Project

International audienceA CO 2 migration field laboratory for testing of monitoring methods has been established in the glaciofluvial-glaciomarine Holocene deposits of the Svelvik ridge, near Oslo. A shallow CO 2 injection experiment was conducted in September 2011 in which approximately 1700 kg of CO 2 was injected at 18 m depth below surface. The objectives of this experiment were to (i) detect and, where possible, quantify migrated CO 2 concentrations, (ii) evaluate the sensitivity of the monitoring tools and (iii) study the impact of the vadose zone on measurements. This paper describes the injection, discusses the joint interpretation of the results and suggests some recommendations for further work