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International audience; This paper investigates the construction of a metamodel for coastal flooding early warning at the peninsula of Gâvres, France. The code under study is an hydrodynamic model which receives time-varying maritime conditions as inputs. We concentrate on Gaussian pocess metamodels to emulate the behavior of the code. To model the inputs we make a projection of them onto a space of lower dimension. This setting gives rise to a model selection methodology which we use to calibrate four characteristics of our functional-input metamodel: (i) the family of basis functions to project the inputs; (ii) the projection dimension; (iii) the distance to measure similarity between functional input points; and (iv) the set of functional predictors to keep active. The proposed methodology seeks to optimize these parameters for metamodel predictability, at an affordable computational cost. A comparison to a dimensionality reduction approach based on the projection error of the input functions only showed that the latter may lead to unnecessarily large projection dimensions. We also assessed the adaptability of our methodology to changes in the number of training and validation points. The methodology proved its robustness by finding the optimal solution for most of the instances, while being computationally efficient.

International audience; In the domain of reservoir engineering, fault stability assessment is traditionally performed by assimilating fault systems to surfaces. However, faults are complex and heterogeneous geological systems, whose compartmentalized architecture generally corresponds to an inner core surrounded by outer, often fractured, damage zones (DZ). To date, few studies have assessed the potential role of the DZ characteristics (anisotropy, complex spatial distribution, etc.) on the shear behavior of the whole system, the main challenge being the proper (numerical) integration of fracture networks in reservoir simulators. The present study was motivated by the stochastically generated fracture networks representative of DZ investigated on outcrops (Cirques de Navacelles, South of France). We propose here a methodology relying on homogenization techniques for deriving the spatial distribution of effective anisotropic hydro-poro-elastic DZ properties from these stochastic fracture networks. Using them as inputs of 2D finite-element coupled hydro-mechanical models of a reservoir-scale fault zone, the key role of the spatially-varying DZ stiffness and Biot's coefficients are highlighted: neglecting them might lead to a high over-estimation of the maximum sustainable injection pressure (by nearly 50% in the considered cases).

International audience; The performance of Permanent Scatterers PS Interferometry (PSI) analysis is highly limited where the presence of large vegetated cover (agricultural terrains / forests) reduces signal coherence. A possible solution relies on the installation on ground of artificial devices (Corner Reflectors CR) to complement the existing PS network. Yet, the number of such CRs (spatial density typically ~ 1 / km2) can be limited when the deformation pattern affects a large area (tens of km2) especially for CO2 geological storage in open aquifers. In order to support the surveillance of such sites, we address here the question of how to estimate the spatio-temporal distribution of the ground displacements over the whole area using only the sparse CR + PS network. We propose to test the feasibility of the Geostatistics Output Perturbation (GOP) method, enabling to combine the results of the reservoir model calibrated on the limited number of ground displacements’ time series and the spatial correlation between such calibrated results and the observations. A test case was constructed using the signal measured during CO2 injection (from 2004–2009) in the KB501 well of the In-Salah site, Algeria at only a few tens of spatial measurement points either corresponding to: 1. a “realistic” PS network selected in a highly vegetated region in western France and 2. a series of CR selected through a space-filling criterion. The comparison with the observations over the whole area confirmed that 80% of the temporal observations were fitted by the GOP method over the region defined by the CR network with a density of ~ 1 CR / km2. Comparisons with the calibrated model results and with the direct application of a spatio-temporal kriging confirmed the better performance of GOP as well.

Fault systems can play a significant role regarding several risk issues related to CO2-injection-induced mechanical perturbations: fluid flow enhancement or compartmentalization within the reservoir, loss of integrity of the reservoir-caprock systems, potential triggering of seismicity and generation of new leakage pathways or permeability barriers. Several methods exist to model hydro-mechanical behaviour of fault zones. A first "conventional" approach aims at evaluating the fault response by directly post-processing the results of the large-scale coupled hydro-mechanical simulations. This consists of: estimating the changes of the effective stress field in the reservoir-caprock system during CO2 injection; computing changes of shear and normal stresses acting on the fault plane; comparing them to a fault reactivation criterion. However, this approach does not account for the effects of the presence of the fault on the stress and pressure field in the surrounding rock matrix. More sophisticated (and physically more realistic) modeling strategies have been proposed, which explicitly integrate the fault zone as a distinct element in the large-scale simulation, i.e. by representing it as a linear discontinuity with various hydraulic and mechanical properties. From a CO2 storage perspective, such a model still remains limited regarding two issues: 1. Faults are complex and heterogeneous geological systems, which do not correspond to discrete surfaces as already postulated by many authors. A fault zone is composed of an inner core made of fine material, often impermeable, and where slip is concentrated. It is surrounded by an outer damage zone that often acts as a hydraulic pathway, because of the presence of a fracture network, whose characteristics (fractures' orientation, connectivity, lengths, density, number of fractures' families, etc.) depend on the distance to the core; 2. Chemical interactions (dissolution and precipitation processes, chemically-induced weakening, etc.) between CO2-enriched brine and the minerals constituting the fault zone. can affect the mechanical and transport properties of the faulted/fractured system. In particular, chemo-mechanical processes can either stabilize the system if the compaction rate is increased or destabilize it if new micro-fractures are created. The FISIC project intends to overcome those limitations by accurately modelling the hydro-chemo-mechanical (HCM) complexity of a fault zone. The main goal is to provide appropriate theoretical and numerical models for accurate evaluation of fault stability in the context of CO2 geological storage, i.e. improving the stability analysis of a fault both undertaking pressure increase and alteration due to the presence of an acidic fluid. Four research questions are addressed and the progress regarding each of them is presented: 1. How to represent a fault zone in a tectonic setting, which has a priori been selected far from major potentially seismically-active faults, i.e. a moderate-to-low-deformed setting; 2. What are the fractures' organization within the damage zones of faults, i.e. their spatial distribution? 3. What are the dominant chemo-mechanical processes resulting from aqueous CO2 in fractured/faulted systems? 4. How to integrate (numerically) the hydro-chemo-mechanical (HCM) behaviour of fault zones in large-scale (reservoir or basin-scale) simulations? Regarding the first and second question, a geostatistical approach is adopted based on observations of fault structures at the Cirque de Navacelles. This site, located in the late Jurassic platform carbonates of Languedoc (southern France), can be considered a good analogue for CO2 storage reservoir. The third question is addressed at laboratory scale focusing on the CO2(aqueous)-induced effect on: i. the slow growth of cracks (subcritical fracturing) and their healing; ii. the degradation of mechanical properties such as fracture toughness or shear strength. Finally, the fourth question is numerically addressed by relying on: i. advanced meshing tools developed for converting complex geometries to finite element models; ii. analysis of fracture propagation condition within multiphase fractured porous media; iii. the rheological behavior derived from the experimental study.

Persistent scatterers synthetic aperture radar interferometry (PSI) analysis is viewed as a promising monitoring technique for multiyear detection of ground surface displacements during long term CO2 injection into deep aquifers (of the order of 25 to 50 years). This technique was successfully implemented at the CO2 storage site of In-Salah (Algeria). Yet, this success may not be generalized to future onshore industrial-scale CO2 storage sites, which are planned in far more complex contexts mostly presenting natural terrains variationeither agricultural or vegetal areas. These areas arecharacterized by poor Persistent Scatterer (PS)density, which might locally drop to zero, hence hampering the deployment of the PSI technique. For illustration purpose, a sector in the Paris basin (of 3,455 km²), where the potential implementation of CO2 storage sites has been investigated, is characterized by a land use composed of less than 5 % of artificial or urban areas and more than 95 % of natural (mainly forests) and agricultural areas. In such natural terrains, the absence of bright man-made structures might lead to sparse PS network (with density ~10 PS/km² to less than 1PS/km²). This constitutes a major obstacle to the phase unwrapping stage, for which the solutions are directly dependent on the PS density and makes reliable estimation of deformation using PS techniques a challenging task. The present study aims at exploring the applicability of two approaches to enhance the performance of the PSI technique in such constraining conditions for long term monitoring of CO2 storage sites. The first approach relies on the implementation of network of corner reflectors (CR) corresponding to artificial devices installed on ground to complement the existing "natural" PS network. A strategy for optimization (number and position) the CR network is proposed based on the position of the existing PS and on hydro-mechanical large-scale simulations constrained by the observations (through history-matching). The second approach relies on the use of two advanced PSI processing techniques: 1. Diffuse scatterers analysis (based on Ferretti et al., 2011), which can improve considerably the density of usable "natural" scatterers on certain un-urbanised area; 2. MulSAR technique (Pinel-Puyssegur et al., 2012), which is a processing method, that produces denoised wrapped phase time series, i.e. which facilitates the retrieval of the deformation signal obscured by decorrelation noise. To test the efficiency of these approaches, a synthetic case was constructed as follows: 1. we use as a "reference case" the deformation field observed at In-Salah during CO2 injection into the KB-501 well between 2004 and 2009; 2. the surface characteristics of this reference case are then modified by "artificially" applying the surface conditions (urbanized areas, natural terrains, either agricultural or vegetal areas) of an on-shore site in Western France, which can be considered representative of future industrial-scale storage sites in Europe. 3. by comparing the deformation field retrieved by the PSI technique combined with the afore-described approaches with the one of the reference case, the level of improvement can be estimated. Results show that the direct application of advanced processing techniques may be limited for zones with very high density of natural terrains (>90 % of the surface area), but can still be efficient for zones with moderate-to-high density of natural terrains. On this basis, we propose to improve PSI analysis in highly vegetal / agricultural areas for long term CO2 storage monitoring based on a combination of advanced processing PSI methods and an optimized CR network. References Ferretti, A., Fumagalli, A., Novali, F., Prati, C., Rocca, F., Rucci, A., 2011. A New Algorithm for Processing Interferometric Data-Stacks: SqueeSAR . IEEE Transactions on Geoscience and Remote Sensing 49 (9), 3460-3470. Pinel-Puyssegur, B., Michel, R., Avouac, J-P., 2012. Multi-link InSAR time series: enhancement of a wrapped interferometric database. IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing 5(3), 784-794.

Industrial-scale CO2 injection into deep aquifers might induce far-field pressure perturbations on the basin scale, potentially impacting other underground uses present within the same aquifer formation. In the present paper, we investigate an approach to trace back the area of elevated pressure induced by CO2 storage operations through an injection strategy in a cyclic manner over time. In this manner, the CO2 injection-induced pressure field is characterized by a low magnitude harmonic signature very specific to the considered CO2 storage, the frequency being chosen as different to naturally fluctuating phenomena or other anthropogenic underground activities. We rely on the combination of de-trending and FFT technique to detect the harmonic pattern associated the CO2 storage injections. We apply the methodology on synthetic pressure signal numerically generated using a typical injection scenario in the Paris basin case. On this basis, we discuss how trace back the CO2 storage site causing the pressure impact, in particular regarding the existing sources of noise.

This study aims at investigating the role played by the overlying aquifer formation as a safety barrier in case of CO 2 leakage accumulation (i.e. CO 2 accumulated in an overlying aquifer after its leakage from the CO 2 storage reservoir) by focusing on its natural capacity to prevent any further upward migration. Based on numerical simulations performed using TOUGH2/ECO2N incl. hysteretic module, we assess the processes influencing the quantity of mobile CO 2 within the leaking plume and perform a sensitivity analysis to point out the key-parameters and conditions for an efficient natural trapping by dissolution and residual trapping. Additional simulations of a leak - active remediation scenario on a complete system (storing reservoir connected to an overlying aquifer) show the importance of the natural trapping capacity of the overlying aquifer. This capacity could be integrated in the mitigation strategy, associating natural and engineered safety barriers. Gaining more knowledge on these formations can support first estimations of this natural capacity, and hence can help building the corrective measure plan and designing potential interventions during operations.

12 pages; International audience; Geological storage of the greenhouse gas CO2 has the potential to be a widespread and effective option to mitigate climate change. As any industrial activity, CO2 storage may lead to adverse impact on human health and the environment in the case of unexpected leakage from the reservoir. These potential impacts should be considered in a risk assessment process. We present an approach to assess the impacts on human health in case of CO2 leakage emerging in the unsaturated zone under a building. We first focus on the migration of the CO2 in the unsaturated zone and the foundation through numerical simulation with sensitivity analysis. Our results show that the intrusion of CO2 into a building is substantially attenuated by the unsaturated zone and the foundation and may lead only under very specific conditions (very low ventilated parts of buildings, high flow rate and/or building situated very close to a leaking pathway) to hazardous CO2 indoor concentrations. We have then integrated the former results in a global toolbox that provides an efficient and easy-touse tool for decision support, which enables to assess the impacts on human health of CO2 leakage from the reservoir to a building.

International audience; Assessing the collapse susceptibility of abandoned cavities at a regional scale is associated with large uncertainties that are mainly related to the very nature of the phenomena, but also to the difficulty in collecting exhaustive information at such a scale on often "forgotten" structures. In this context, the expert's role is essential, because he is able to synthesize the information resulting from the inventory and from the commonly imprecise, if not vague, criteria on the basis of his experience and his knowledge of the regional geological and historical and economic context. In this article, we propose mathematical tools for representing and processing this information in order to give flexibility to this step and manage the uncertainty inherent in the expert's information. The first tool, based on the weight of evidence theory, is for managing the uncertainty due to the heterogeneous spatial distribution of the data, whereas the second tool, based on the fuzzy set theory, is for managing the imprecision and incompleteness of available data, which hinder the definition of the class boundaries of the quantitative decision criteria. Based on an appropriate representation of the uncertainty sources (related to the input data and to the expert diagnostic), we then propose a methodology that integrates the uncertainty in the final output of the collapse susceptibility assessment and provides a confidence indicator useful within the decision-making process. The proposed methodology is applied to the Arras territory in the North of France, where abandoned chalk pits (dating back to the Roman ages) and war saps (i.e. covered trenches) located in the vicinity of the First World War front lines both raise difficulties for urban planning.

For carbon dioxide capture and storage to make an effective contribution in reducing greenhouse gas emissions, a large number of storage sites will be required. Safety will have to be demonstrated for each of these sites. We present in this paper a simple and flexible framework to assess safety of a CO 2 storage site in a time-efficient manner. It includes the identification by an expert panel of the relevant risk scenarios. Their evaluation privileges analytical or semi-analytical modelling with conservative assumptions. Uncertainty is managed in order to thoroughly represent and propagate the nature of information relative to the site properties. We illustrate the application of this framework with a case study in the context of the Paris Basin. The workflow described is not a complete risk assessment framework, but it constitutes a useful tool for an operator at an early stage of a project or to iteratively screen safety issues, or for a regulating authority to review the safety conditions of a project.

This paper deals with the electrical resistivity monitoring of the Ketzin CO 2 injection pilot (CO2ReMoVe EC project) through time-lapse CSEM measurements. There, 3 boreholes about 800 m deep have been especially designed for current injection at reservoir (sandstone) depth. CO 2 is directly injected in a saline (∼240 g/l) aquifer. Prior modelling results indicated that the increase of electrical resistivity generated by the CO 2 plume (gaseous and liquid CO 2 phases) supposed to be highly resisitive, would generate measurable changes in the EM fields on the surface, when injecting current directly inside the reservoir. In order to highlight and follow these expected resistivity changes, 3 CSEM surveys were performed in August 2008 (baseline prior to injection), June 2009 and August 2010. Each time, 13 EM stations have been recorded during current injection of a square wave at 3 frequencies (0.125 Hz, 0.5 Hz and 4 Hz) in two configurations (“double mise a la masse” (2×MAM) and “mise a la masse–surface” (MAM-Surface)). This paper only presents results of the 2×MAM configuration at 0.5 Hz. In spite of a very noisy area (gas pipes, high voltage power lines), we measured signal amplitude 10 times higher than noise amplitude. We show that EM fields vectors (both inphase and quadrature components) measured on the surface are very similar to the forward modelling EM responses computed with COMSOL Multiphysics ® . Models also show that electric field spatial distribution is strongly affected by a thin and resistive layer (35 m–200 Ωm) of anhydrite above the reservoir, making E field diverging from the boreholes whereas a dipolar pattern was expected for the dipole current injection used here. Moreover, while June 2009 survey highlighted the expected strong increase of electric field (increase of resistivity), August 2010 survey showed electric field amplitudes similar to the 2008 baseline survey, revealing therefore major changes of the reservoir properties. Finally, the directional sensitivity of the 2×MAM array is tested through modelling residuals computed for five CO 2 plume spatial distributions. Results show that a north-eastward migration of the CO 2 plume is expected to fit field data.

A leakage of CO2 from a geological storage site into an aquifer exploited for human use could raise concerns about potential changes of groundwater quality. We use the reactive transport code TOUGH2-ECO2N/TOUGHREACT in order to apprehend the possible geochemical mechanisms that could affect a freshwater aquifer. Our aquifer model is based on the mineralogical and geochemical features of an existing glauconitic-sandstone aquifer, to which trace elements bearing minerals are added in average proportions found in similar mineralogies. The code computes kinetic precipitation/dissolution processes. 2D sensitivity tests on: i) intrusion position (at the ceiling and at the floor) of pure CO2- ii) intrusion rates - iii) natural groundwater flow, were performed, exhibiting several dynamics. Dissolution of CO2 decreases the pH to 5.0 and leads to dissolution of trace elements bearing minerals, enriching the waters in trace elements including Cd, Pb, Zn. Their concentrations may locally exceed quality parameters. Gaseous CO2 migrates upwards and water enriched in dissolved CO2 moves downwards due to the density gradient. The geometric impact extends and the total amounts of mobilized health-significant-elements increase with increasing intrusion rates. Groundwater flow may have an impact on these parameters too. An interruption of the CO2 stream is immediately followed with decreasing tendencies of these parameters. A 3D test at an intrusion rate of 0.1 g/s/m2 during 1 year shows that, at this rate, the initial shape of the plume is much more extended horizontally than vertically. A strong limitation of the model is currently that sorption processes on clay minerals as well as on oxy-hydroxydes have not been taken into account, which most certainly means that our computations are conservative, as these mechanisms could strongly and quickly inhibit the mobilization of traces that would follow acidification and dissolution processes.

Geophysical methods are adequate for imaging the CO2 plume in order to follow its migration within the reservoir and possible leakages through the caprock formation. In particular, changes in density, seismic velocity or electrical resistivity are associated with changes in the gas saturation and make methods such as gravity, 4D seismic and electrical resistivity tomography (ERT) or CSEM powerful tools to follow up the fate of injected CO2. Respect to the minimum amount of CO2 stored that can be quantified for verification purpose, there is an order of magnitude between 4D seismic (few 100s of Kt) and the other two methods (few Mt). Respect to the detection of leakage at the reservoir level, only 4D seismic could be considered as useful. Using downhole measurements, such as crosshole electric or downhole gravity will increase the resolution of these methods and therefore its ability to detect leakage. In case of CO2 leakage upwards and accumulation within a secondary reservoir located at a few hundreds of meters depth, the resolution of the three methods is increased by several order of magnitude and small amounts of CO2 could be detected, depending whether it is in gaseous phase or dissolved. It is expected that controlled experiments of leaking CO2 at shallow depth will help to define more precisely the conditions of use of the three methods.

The development of a methodology for defining safety criteria for CO 2 geological storage, realised under the ongoing CRISCO2 project, is depicted in this paper. Generic lists of eleven risk events and nine types of targets are proposed to serve an expert panel for identifying risk scenarios. Assessing the effects of risk events and comparing targets exposure to critical thresholds should lead to setting safety requirements. We focus on simple modelling, such as abacuses, analytical or semi-analytical models. An uncertainty management framework completes this methodology. Further work will concentrate on effectively deducing safety criteria.

The present study provides a set of numerical tools for modeling the geomechanical aspects related to the safety of geological CO2 storage, namely, caprock damage and fault reactivitation due to reservoir pressure rise. Large scale finite element models are used to describe the injection process. The change of the effective stress field is investigated. Different cracking mechanisms are considered to examine the caprock integrity for high injection rates and different initial in situ stress conditions. Last, a special hydromechanical joint element is presented to model the response of the faults / fractures affected by the pressure build-up in the reservoir.