Induced-seismicity geomechanics for controlled CO2 storage in the North Sea (IGCCS)

The aim of the current study, IGCCS (2017–2020), is to evaluate the feasibility of micro-seismic (MS) monitoring of CO2 injection into representative storage candidates in the North Sea, based on broad and quantitative characterization of relevant subsurface behavior with respect to geology, geomechanics and seismicity. For this purpose, we first group potential CO2 storage sites in the North Sea into three different depths. Then, advanced triaxial rock mechanical tests are performed together with acoustic emission (AE) acquisition under representative loading for CO2 storage sites in the North Sea and for formations of each depth group, covering shale, mudstone and sandstone cores. Our work focuses particularly on quantifying the effects of injected fluid type and temperature on mechanical behavior and associated MS response of subsurface sediments. The experiment results show that each depth group may behave differently in responses to CO2 injection. Particularly, the occurrence of detectable MS events is expected to increase with depth, as the combined effects of rock stiffness and temperature contrast between the host rock and injected CO2 are increasing. In addition, lithology plays an important role in terms of the MS response, i.e. high AE event rate is observed in sandstones, while aseismicity in shale and mudstone. The test results are then scaled up and applied to advanced coupled flow-geomechanics simulations and a synthetic field-scale MS data study to understand micro-seismicity at fracture, reservoir and regional scales. The numerical simulation of scCO2 injection scenario shows quite different stress-strain changes compared to brine injection, resulting mainly from the thermally-induced behavior. Furthermore, the numerical simulation study via so-called Cohesion Zone Modeling (CZM) approach shows strong potential to improve our understanding of the multiphase-flow-driven fracture propagation. Our synthetic MS data study, focused on slow-earthquake scenario, also suggests that sensors with high sensitivity at low frequency might be necessary for better signal detection and characterization during CO2 injection. This manuscript covers the main findings and insights obtained during the whole study of IGCCS, and refers to relevant publications for more details

COMPORTEMENT HYDRO-MECANIQUE DE ROCHES RESERVOIR SOUS CONTRAINTES :<br />Relations entre évolution de perméabilité et échelles des mécanismes d'endommagement

The Understanding and the prevention of damage mechanisms, which have an impact on the kinetics of production and the rate of recovery, remain an outstanding issue in reservoir engineering. The aim of this study, based on the characterization of the hydromechanical behaviour of sandstone and carbonate rocks in 'reservoir conditions', is the identification of the local mechanisms responsible for changes in permeability measured macroscopically. The experimental work was performed with an original triaxial set-up, which allows measurements of the stress-induced permeability evolutions in the principal directions of the stress tensor. A first experimental campaign, consisting in hydrostatic and uniaxial compression tests, has been performed on Fontainebleau and Bentheim sandstones. In one hand, we showed that experimental end-effects might affect significantly the ‘classical' axial permeability measurements, and in the other hand, we have determined the impact of brittle failure on directional permeabilities. Compression experiments, following different stress-paths, were also carried out on a carbonate, the Estaillades limestone. In elastic deformation regime, the reduction of permeability was modelized by pore network simulations, based on 3D reconstructions of μ-tomography RX and a simplified representation of the pore space. While brittle fracture of carbonate samples induced slight permeability evolutions, at higher effective pressure, permeability drops can reach 90% to the initial values and are associated with mechanisms of pore collapse. The post-mortem analysis of deformed samples, coupled with digital image correlation methods, using both SEM and Optical acquisition devices, provide a better understanding of the role of the heterogeneities, identified at different scales, in the strain localization and their potential impacts on permeability changes at the sample scale.La compréhension et la prévention des mécanismes d'endommagement ayant un impact sur les cinétiques de production et les taux de récupération reste un problème ouvert en ingénierie de réservoir. L'objectif de cette étude repose sur la caractérisation du comportement hydromécanique de roches gréseuses et carbonatés en 'condition de réservoir' et l'identification des mécanismes locaux responsables des évolutions de perméabilité mesurables macroscopiquement. L'essentiel du travail expérimental s'est articulé autour d'un dispositif triaxial original, permettant de mesurer en cours de chargement les évolutions de perméabilité dans les directions principales du tenseur de contraintes. La validation de notre protocole effectuée, une première campagne d'essais en compression hydrostatique et uniaxiale sur les grès de Fontainebleau et de Bentheim permet, d'une part, une analyse plus fine du comportement intrinsèque de ces matériaux par la mise en évidence et la prise en compte d'effets expérimentaux, et d'autre part d'étudier l'impact de l'endommagement fragile sur les perméabilités directionnelles. Une seconde série d'expériences de compression, selon différents chemins de chargement, a été réalisée sur un carbonate, le calcaire d'Estaillades. En régime de déformation élastique, la modélisation de la réduction de perméabilité a été appréhendée par une approche Réseau de Pores, basée sur une représentation simplifiée de l'espace poreux et extraite des reconstructions 3D par µ-tomographie RX. Si la rupture fragile des échantillons ne perturbe que très peu la perméabilité initiale, à plus haute pression effective, les diminutions de perméabilité peuvent atteindre 90% et sont alors associées aux mécanismes de pore collapse. L'analyse des échantillons déformés, couplée aux techniques de corrélation d'images acquises in-situ sur des dispositifs de compression simple sous MEB et sous Optique, permet une meilleure prise compte du rôle des échelles d'hétérogénéités dans la localisation des déformations et de mieux définir leur impact sur les évolutions de perméabilités à l'échelle de l'échantillon

Comportement hydromécanique de roches réservoir sous contraintes (relations entre évolution de perméabilité des mécanismes d'endommagement)

La compréhension et la prévention des mécanismes d'endommagement ayant un impact sur les cinétiques de production et les taux de récupération reste un problème ouvert en ingénierie de réservoir. L'objectif de cette étude repose sur la caractérisation du comportement hydromécanique de roches gréseuses et carbonatés en 'condition de réservoir' et l'identification des mécanismes locaux responsables des évolutions de perméabilité mesurables macroscopiquement. L'essentiel du travail expérimental s'est articulé autour d'un dispositif triaxial original, permettant de mesurer en cours de chargement les évolutions de perméabilité dans les directions principales du tenseur de contraintes. La validation de notre protocole effectuée, une première campagne d'essais en compression hydrostatique et uniaxiale sur les grès de Fontainebleau et de Bentheim permet, d'une part, une analyse plus fine du comportement intrinsèque de ces matériaux par la mise en évidence et la prise en compte d'effets expérimentaux, et d'autre part d'étudier l'impact de l'endommagement fragile sur les perméabilités directionnelles. Une seconde série d'expériences de compression, selon différents chemins de chargement, a été réalisée sur un carbonate, le calcaire d'Estaillades. En régime de déformation élastique, la modélisation de la réduction de perméabilité a été appréhendée par une approche Réseau de Pores, basée sur une représentation simplifiée de l'espace poreux et extraite des reconstructions 3D par -tomographie RX. Si la rupture fragile des échantillons ne perturbe que très peu la perméabilité initiale, à plus haute pression effective, les diminutions de perméabilité peuvent atteindre 90% et sont alors associées aux mécanismes de pore collapse. L'analyse des échantillons déformés, couplée aux techniques de corrélation d'images acquises in-situ sur des dispositifs de compression simple sous MEB et sous Optique, permet une meilleure prise compte du rôle des échelles d'hétérogénéités dans la localisation des déformations et de mieux définir leur impact sur les évolutions de perméabilités à l'échelle de l'échantillon.The Understanding and the prevention of damage mechanisms, which have an impact on the kinetics of production and the rate of recovery, remain an outstanding issue in reservoir engineering. The aim of this study, based on the characterization of the hydromechanical behaviour of sandstone and carbonate rocks in 'reservoir conditions', is the identification of the local mechanisms responsible for changes in permeability measured macroscopically. The experimental work was performed with an original triaxial set-up, which allows measurements of the stress-induced permeability evolutions in the principal directions of the stress tensor. A first experimental campaign, consisting in hydrostatic and uniaxial compression tests, has been performed on Fontainebleau and Bentheim sandstones. In one hand, we showed that experimental end-effects might affect significantly the classical' axial permeability measurements, and in the other hand, we have determined the impact of brittle failure on directional permeabilities. Compression experiments, following different stress-paths, were also carried out on a carbonate, the Estaillades limestone. In elastic deformation regime, the reduction of permeability was modelized by pore network simulations, based on 3D reconstructions of -tomography RX and a simplified representation of the pore space. While brittle fracture of carbonate samples induced slight permeability evolutions, at higher effective pressure, permeability drops can reach 90% to the initial values and are associated with mechanisms of pore collapse. The post-mortem analysis of deformed samples, coupled with digital image correlation methods, using both SEM and Optical acquisition devices, provide a better understanding of the role of the heterogeneities, identified at different scales, in the strain localization and their potential impacts on permeability changes at the sample scale.PALAISEAU-Polytechnique (914772301) / SudocSudocFranceF

Hydromechanical behavior of heterogeneous carbonate rock under proportional triaxial loadings

International audienc

Comportement mécanique du carbonate d’Estaillades en condition drainée: influence du chemin de chargement

On réalise sur des échantillons cylindriques des essais drainés en appliquant divers chemins de chargement, caractérisés par le rapport des contraintes radiale et axiale (K). Le mode d’endommagement et son influence sur la perméabilité dépendent surtout de K, mais dans des régimes intermédiaires, les hétérogénéités jouent un rôle majeur. Nous proposons une normalisation des données expérimentales qui permet de définir sans autre ajustement l’enveloppe de rupture dans le plan des contraintes

Axial and Radial Permeability Evolutions of Compressed Sandstones: End Effects and Shear-band Induced Permeability Anisotropy

International audienceThe influence of hydrostatic and uniaxial stress states on the porosity and permeability of sandstones has been investigated. The experimental procedure uses a special triaxial cell which allows permeability measurements in the axial and radial directions. The core sleeve is equipped with two pressure samplers placed distant from the ends. They provide mid-length axial permeability measure as opposed to the overall permeability measure, which is based on the flow imposed through the pistons of the triaxial cell. The core sleeve is also equipped to perform flows in two directions transverse to the axis of the sample. Two independent measures of axial and complementary radial permeability are thus obtained. Both Fontainebleau sandstone specimens with a porosity of about 5.8% to 8% and low permeability ranging from 2.5 mD to 30 mD and Bentheimer sandstone with a porosity of 24% and a high permeability of 3D have been tested. The initial axial permeability values obtained by each method are in good agreement for the Fontainebleau sandstone. The Bentheimer sandstone samples present an axial mid-length permeability 1.6 times higher than the overall permeability. A similar discrepancy is also observed in the radial direction, also it relates essentially to the shape of flow lines induced by the radial flow. All the tested samples have shown a higher stress dependency of overall and radial permeability than mid-length permeability. The effect of compaction damage at the pistons/sample and radial ports/sample interfaces is discussed. The relevance of directional permeability measurements during continuous uniaxial compression loadings has been shown on the Bentheimer sandstone until the failure of the sample. We can efficiently measure the influence of brittle failure associated to dilatant regime on the permeability: It tends to increase in the failure propagation direction and to decrease strongly in the transverse direction

Acoustic signature of fluid substitution in reservoir rocks

International audienc

Application of 2D and 3D Digital Image Correlation on CO2-like altered carbonate

International audienc

Mechanical evolution of an altered limestone using 2D and 3D digital image correlation (DIC)

International audienceIn the context of long term CO 2 sequestration, we present an original laboratory workflow which allows a multiscale analysis of the effect of a chemical alteration on both petrophysical and mechanical properties of a carbonate. The relation between the mechanical behavior and the microstructural evolutions is investigated using the digital image correlation techniques. For this purpose we imaged intact and altered samples during uniaxial and triaxial mechanical tests using optical microscopy, scanning electron microscopy and 3D X-ray microtomography. The porosity increase induced by homogeneous alteration leads to lower elastic moduli and critical stress at sample failure. Furthermore at a same confining pressure we observed a transition from brittle to ductile failure mechanisms with the increase of alteration level. These observations are evidenced by digital image correlation. Altered samples evidenced early strain localization which was not observed in the intact ones. These early localization events induce more compaction at the macroscopic scale and tend to initiate early fracturing

Localized deformation induced by heterogeneities in porous carbonate analysed by multi−scale digital image correlation

International audienceThe understanding and the prevention of damage mechanisms, which have an impact on the hydrocarbon production and recovery rates, are of paramount interest for reservoir engineers. The modelling of such coupled processes relies essentially on experimentally obtained data, which characterize the macroscopic mechanical and transport properties. This approach however cannot account for the multi-scale structural heterogeneities of the considered rocks, in spite of their fundamental importance. The microstructural characterization of damage is usually based on ‘post-mortem’ observations of the samples, which provide both qualitative and quantitative information about the effects of the mechanisms activated at the grain scale and at intermediate scales, at a pervasive stage of damage after sample unloading. New techniques provide more quantitative and direct methods to follow the deformation history and the eventual development of localization and damage. In this study, the 2D Digital Image Correlation (DIC) technique has been applied to sequences of images taken from carbonate samples during uniaxial compression tests. Several scales have been considered, ranging from the centimetric scale of the samples to the local scale of their microstructure. For this purpose both optical observations and Scanning Electron Microscopy (SEM) were used. Although the macroscopic strain at failure was very small (< 0.2%), the DIC technique has proven to be reliable, provided one selects carefully image acquisition conditions and DIC parameters, as highlighted in our discussion on the uncertainties and the evaluation of errors. This technique has allowed us to quantify both the global and local strain fields during the deformation process. We have thus been able to precisely identify the localizations of damage and the local compaction mechanisms, and to relate them to the characteristic structural heterogeneities of the tested carbonate