Consistent experimental investigation of the applicability of Biot-Gassmann’s equation in carbonates

International audienceCarbonate formations are characterized by multiscale heterogeneities which control theirflow and acoustic responses. At the laboratory scale, carbonate rocks already do not show astrong correlation between P- and S-wave velocities and porosity. The velocity disparitiesbetween carbonates of similar mineralogy and porosity result from different microstructuresderived from their sedimentary facies and subsequent diagenetic transformations. The stilldiscussed applicability of Biot-Gassmann’s equation for fluid substitution in carbonate rocksremains another key issue. We propose an integrated experimental workflow that allows aconsistent checking of the applicability of Biot-Gassmann’s equation and provide key geologicaland microstructural information to understand the petroacoustic signature of carbonate rocks.This approach is implemented on samples representative of two different carbonate formations.The obtained results demonstrate the applicability of Biot-Gassmann's equation for the twostudied carbonate families and show the interrelation of mineralogy and porosity distribution intheir acoustic response

Combined controls of sedimentology and diagenesis on seismic properties in lacustrine and palustrine carbonates (Upper Miocene, Samos Island, Greece)

International audienceFor the subsurface characterization of carbonates, linking physical properties (e.g. porosity and seismic reflectors) with their geological significance (e.g. sedimentary facies and diage-nesis) is of primary importance. To address this issue, we study the lacustrine and palustrine carbonates on Samos Island through a geological and geophysical characterization of a sed-imentary succession. The microstructures of the samples are described, and the samples' physical properties are measured (porosity, P-wave velocity and density). The results show that the identification of only the primary (i.e. sedimentary) microstructure is not sufficient for explaining the huge variations in porosity and P-wave velocity. Hence, we highlight two early diagenetic processes that strongly impact the microstructures and control the physical properties: (i) neomorphism occludes porosity and increases the P-wave velocity of mud-and grain-supported microstructures, which implies a mineralogical stabilization of the grains; (ii) conversely, the dissolution process creates porosity and decreases the P-wave velocity of grain-supported microstructures if the mineralogical composition of the grains is not previously stabilized. These two diagenetic processes thus depend on the primary microstructures and mineralogy of the sediments. This work aims to explain the variations in porosity and P-wave velocity for each defined primary microstructure. A 1-D seismogram is then built to highlight seismic reflectors with a metre-scale resolution. These reflectors are associated with several geological contrasts. Hard kicks (positive amplitude reflectors) match well with exposure surfaces related to palaeosols. They correspond to contrasts between non-modified primary microstructures and highly neomorphosed microstructures. Conversely, soft kicks (negative amplitude reflectors) are linked with diagenetic contrasts (e.g. neomorphosed mi-crostructures versus non-modified primary microstructures) and sedimentary contrasts that can be overprinted by diagenesis (e.g. neomorphosed mud-supported microstructures versus dissolved grain-supported microstructures). This study highlights that high-resolution seismic reflectors of lacustrine and palustrine carbonates are strongly related to the spatial contrasts of primary microstructures overprinted by early diagenesis

Effective medium modeling of diagenesis impact on the petroacoustic properties of carbonate rocks

International audienceCarbonate formations are highly heterogeneous, and the velocity-porosity relationships are controlled by various microstructural parameters, such as the types of pores and their distribution. Because diagenesis is responsible for important changes in the microstructure of carbonate rocks, we have extended the standard effective medium approach to model the impact of diagenesis on the carbonate elastic properties through a step-by-step effective medium modeling. Two different carbonate rocks deposited, respectively, in lacustrine and marine environments are considered in this study. The first key step is the characterization of the diagenesis, which affected the two studied carbonate sample sets. Effective medium models integrating all of the geologic information accessible from petrographic analysis are then built. The evolution of the microstructural parameters during diagenesis is thoroughly constrained based on an extensive experimental data set, including X-ray diffraction analysis, different porosimetry methods, and ultrasonic velocity measurements. A new theoretical approach including two sources of compliance is developed to model the specific behavior of carbonates. A compliant interface is introduced around the main carbonate grains to represent grain contacts and the different pore scales are taken into account through multiscale modeling. Finally, direct calculations with the model provide elastic wave velocities representative of the different diagenetic stages. An extrapolation to permeability evolution is also introduced. This approach allows the identification of the acoustic signature of specific diagenetic events, such as dolomitization, dissolution, or cementation, and the assessment of their impact on the elastic properties of carbonates

Improved unconventional reservoir characterization using multi-azimuth stratigraphic inversion, case study on the Fort Worth Basin.

International audienceThe development of the exploration and exploitation of unconventional hydrocarbons requires innovative skills to allow a better characterization of natural fracture networks and fracability of the rocks at depth. Within this context, we bring into focus a multi-azimuth inversion methodology to give clues about the "sweet spots" characterization through the Barnett Shales formation. To achieve our goal we used an isotropic model-based stratigraphic seismic inversion, considering a series of azimuthal sectors to build partial seismic stacks required as input of the workflow. The initial seismic survey was recorded over a surface of approximately 100 square miles to image the Barnett Shales formation, of about 150 ft thickness only and localized in a transition zone between the oil and gas window. A preliminary processing task was to define limits of azimuthal and incidence angle sectors to build several multi-azimuth post-stack and pre-stack seismic datasets. Two exploration wells with a set of log data are available in the interest area helping in the model a priori building and in the well-to-seismic calibration, both crucial steps to apply the inversion workflow. We first perform full post-stack and pre-stack usual inversion to help in finding sets of best inversion parameters, then we do the multi-azimuth post-stack and pre-stack inversion workflow using the same previous sets of parameters for each azimuthal sector. Consequently a series of acoustic impedance, P- and S-wave elastic impedance cubes are computed for each azimuthal sector. Another aspect of the study is the quantification of azimuthal variations using an ellipse fitting algorithm giving, for each bin of the interest volume, the ratio (between major and minor axes) and tilt (i.e. azimuth from the East) of the ellipse. Results showed up to 10% of anisotropy for impedances in the Barnett Shales. We go a step further computing geomechanical features like Poisson's ratio and brittleness coefficient within the Barnett Shales and we quantified their azimuthal variations. Some areas present lower Poisson's ratio and higher brittleness than other parts. It may suggest that the latter are "sweet spots" that could be more easily stimulated by induced fracturing

Du terrain au laboratoire, étude des propriétés élastiques du basalte

Nearly 60 % of the Earth surface is composed by basaltic rocks which form the upper part of the oceanic crust. These PhD works focus on the elastic properties of basalt at several spatial scales : field scale, sample scale and porosity microstructure scale. With its active hydrothermalism Iceland appears as the ideal natural laboratory to study the interactions existing between basalts, fluids and fracturing at a large scale. We first begin by the study of a paleosite of hydrothermal fluid flow which took place in a basaltic pile. Some evidence of hydrofracturing are identified. We also investigate the porosity of the different materials associated to the hot fluid flow. The second part of the field study is based on microseismicity surveys on the Reykjanes Peninsula which is an active area in Iceland. The most important results is those deduced from inverting the tomography data obtained by C. Dorbath in 2005. By applying an effective medium theory to seismic velocities we have attempted to estimate the crack density and aspect ratio of the Icelandic crust at this place. We have show that areas characterized by a strong P-waves velocities anomaly were characterized by high crack density and low aspect ratio at 6 km depth. We have also investigated the effect of the fluid compressibility on the crack parameters with depth. The sample scale is investigated through two studies. The first one is the investigation of three different modes of deformation in an Icelandic basalt by using laboratory seismological tools (elastic waves and acoustic emissions) and scan imaging. first of all we show that at low effective pressure (5 MPa) an axial loading induces a shear failure in the basalt with a classical angle of about 45°. On the contrary at higher effective pressures (75 MPa and more) an increasing of the axial stress induces a localization of the deformation in the centre part of the sample. Focal mechanisms of the acoustic emissions reveal an important part of compression events (mode I rupture) suggesting pore collapse mechanisms. Such compaction structures are usually obtained for porous rocks (20-25 %). And yet the investigated basalt has an intact total porosity of about 10 %. Then the size of the equant porosity and its ratio with grain size can be involved for explaining the pore collapses. Finally the third triaxial experiment is an induced fluid pressure failure from a high confining pressure state (80 MPa). A large shear plane failure is formed due to pore pressure increasing the local porosity of about 1 %. The second sample scale study is based on the porosity scale in order to investigate the frequency effect on elastic moduli. To obtain experimental data we performed hydrostatic experiments on an Icelandic basalt specimen under both dry and saturated conditions. This basalt is characterized by a bimodal porosity, i.e., cracks and equant pores. The elastic properties -bulk moduli in our case- were investigated under high pressure through two experimental methods : (1) a classical one using ultrasonic P- and S-waves velocities (frequency 106 Hz), (2) and a new one, using oscillation tests (frequency 102 Hz). In dry condition, experimental data show no significant difference between high (HF) and low (LF) frequency bulk moduli. However, in saturated conditions, two effects are highlighted : a physico-chemical effect emphasized by a difference between drained and dry moduli, and a squirt-flow effect evidenced by a difference between HF and LF undrained moduli. The experimental approach was completed by a theoretical study. The HF moduli are derived from a new effective medium model with an isotropic distribution of pores and cracks with idealized geometry, respectively spheres and ellipsoids. LF moduli are obtained by taking HF dry moduli from the model and substituting into Gassmann's equations. In the case of a porosity only supported by equant pores, the calculated dispersion in elastic moduli is equal to zero. In the case of a crack porosity, no bulk dispersion is predicted but a shear dispersion appears. Finally in the general case of a mixed porosity (pores and cracks), dispersion in bulk and in shear is predicted. Our results show that the maximum dispersion is predicted for a mixture of spheroidal pores and cracks with a very small aspect ratio (<10-3). Our theoretical predictions are compared to experimental data and a good agreement is observed. We also used our theoretical model to predict elastic waves velocities and Vp/Vs ratio dispersion. We show that the P-waves dispersion can reach almost 20 % and the Vp/Vs dispersion a maximum value of 9 % for a crack porosity of about 2 %. Since laboratory data are ultrasonic measurements and field data are obtained at much lower frequencies, these results are useful for geophysicists to interpret seismic data in terms of fluid and rock interactions.Les roches basaltiques constituent près de 60 % de la surface terrestre formant en particulier la couche supérieure de la croûte océanique. Afin de mieux appréhender la capacité de rétention en fluides des basaltes, une meilleure connaissance de leurs propriétés mécaniques et élastiques est indispensable. Au cours de ce travail nous avons choisi d'étudier ces propriétés à trois échelles spatiales distinctes. L'approche grande échelle porte sur la caractérisation du lien entre basalte, fluides et fracturation en Islande. Cette île, née de l'accrétion océanique et d'une anomalie mantellique chaude, constitue, avec son hydrothermalisme intense, un laboratoire naturel idéal. Une étude géologique d'un ancien site de circulation de fluides chauds dans un encaissant basaltique a été réalisée permettant de mettre en évidence certaines caractéristiques intéressantes :modification des paramètres hydrodynamiques à proximité des zones de circulation, altération hydrothermale puissante et figures de fracturation hydraulique. Par ailleurs, l'approche grande échelle s'appuie sur l'étude de la microsismicité actuelle au niveau de la péninsule de Reykjanes, zone particulièrement active de l'île. A partir des données de tomographie acquises en 2005 et en utilisant un modèle de milieu effectif adéquat, nous avons réinterprété les anomalies de vitesse en terme de paramètres de fissuration de la croûte islandaise à cet endroit. Les résultats d'inversion montrent une diminution de la densité de fissure avec la profondeur excepté au niveau d'une zone localisée interprétée comme un réservoir de fluide supercritique. Les propriétés élastiques du basalte ont également été étudiées à l'échelle de l'échantillon grâce à des expériences en presse triaxiale. Le dispositif utilisé est celui de la presse ST100 du laboratoire de géologie de l'ENS spécialement conçue pour l'écoute des émissions acoustiques (EA) qui sont les analogues expérimentaux des séismes. Trois séries d'expériences avec augmentation de la contrainte axiale ont été menées sur un basalte de Reykjanes : (1) à bas confinement, 5 MPa de pression effective, (2) à haut confinement, 75 et 95 MPa de pression effective et (3) à haut confinement avec augmentation de la pression de pore. Les mesures de déformation, la localisation des EA, leur mécanisme et l'imagerie scanner des échantillons déformés ont montré différents modes de déformation du basalte. Pour les séries (1) et (3) la rupture se localise macroscopiquement par un plan de cisaillement oblique. Dans le cas (2) la déformation se localise dans la partie centrale de l'échantillon sous forme de bandes associant compaction (collapse de pores sphériques) et cisaillement (glissement inter-grains). Enfin, nous nous sommes intéressés aux propriétés élastiques du basalte à l'échelle microscopique en étudiant la dispersion des modules élastiques avec la fréquence. En effet, il est généralement connu que dans un milieu saturé en fluide, les modules basse fréquence (BF) sont plus faibles que les modules haute fréquence (HF), une partie de l'énergie des ondes élastiques se retrouvant dissipée par des mécanismes d'écoulement de fluides à différentes échelles. au cours de ce travail, nous n'avons considéré que le mécanisme dispersif du squirt-flow (écoulement local entre inclusions). Nous avons pour cela utilisé une approche couplant théorie et expérimentation. A l'aide d'un modèle de milieu effectif et des équations de Biot-Gassmann, les dispersions théoriques des modules d'incompressibilité K et de cisaillement G ont été calculées en fonction de la microstructure de porosité. On a montré que la dispersion est maximale quand le milieu est composé de pores sphériques et de fissures aplaties (facteur de forme < 5.10-3). Un nouveau protocole expérimental en presse a été développé au cours de cette thèse permettant d'acquérir des données de dispersion de K avec la fréquence. Les résultats expérimentaux sont venus confirmer les prédictions théoriques. Un effet de squirt-flow a ainsi été mis en évidence dans le basalte tant que les fissures restent ouvertes

Du terrain au laboratoire, étude des propriétés élastiques du basalte

Les roches basaltiques constituent près de 60 % de la surface terrestre formant en particulier la couche supérieure de la croûte océanique. Afin de mieux appréhender la capacité de rétention en fluides des basaltes, une meilleure connaissance de leurs propriétés mécaniques et élastiques est indispensable. L'originalité de ce travail de thèse réside dans l'étude de certaines de ces propriétés à differentes échelles. L'approche grande échelle porte sur l'étude des propriétés de la croûte islandaise sous la péninsule de Reykjanes. A partir des données de tomographie acquises en 2005 et en utilisant un modèle de milieu effectif adéquat, nous avons réinterprété les données de vitesses des ondes sismiques en terme de variations des paramètres de fissuration et de l'état du fluide avec la profondeur. Globalement, les résultats d'inversion montrent une diminution de la densité de fissures et du facteur de forme avec la profondeur dans la région étudiée. A l'inverse, au niveau des zones d'anomalies localisées en tomographie, la densité de fissures tend à augmenter avec la profondeur. L'existence d'un ou plusieurs réservoir(s) de fluide supercritique sous pression pourrait être à l'origine de ce constat. A l'échelle de l'échantillon, nous avons étudié les modes de déformation d'un basalte de Reykjanes en presse triaxiale. A des pressions effectives faibles, le basalte soumis à un chargement axial se casse selon un plan de cisaillement oblique. Pour des pressions effectives plus importantes (supérieures à 75 MPa), le même type de chargement entraîne une localisation de la déformation sous forme de bandes subhorizontales. Une étude fine des émissions acoustiques émises au cours du chargement associée à celle de la microstructure des échantillons déformés a permis de montrer que ces bandes étaient le siège de processus associant cisaillement et compaction. C'est la première fois qu'un tel mode de déformation est observé expérimentalement sur des roches basaltiques à porosité faible (8 %). Enfin, nous nous sommes intéressés aux propriétés élastiques du basalte à l'échelle microscopique en étudiant la dispersion des modules élastiques avec la fréquence. Il est généralement connu que les vitesses des ondes élastiques émises à basse fréquence (BF) sont plus faibles que celles des ondes émises à haute fréquence (HF). Une partie de leur énergie est dissipée par des mécanismes d'écoulement de fluide à differentes échelles. Nous nous sommes intéressés au mécanisme dispersif du squirt-flow (écoulement local entre inclusions). Nous avons pour cela utilisé une approche couplant théorie et expérimentation. A l'aide d'un modèle de milieu effectif et des équations de Biot-Gassmann, les dispersions théoriques des modules d'incompressibilité K et de cisaillement G ont été calculées en fonction de la microstructure de porosité. On a montré que la dispersion était maximale quand le milieu était composé de pores sphériques (facteur de forme 1) et de fissures aplaties (facteur de forme 5.10-3). Un nouveau protocole expérimental en presse triaxiale a été développé au cours de cette thèse permettant d'acquérir des données de dispersion de K avec la fréquence. Les résultats expérimentaux sont venus confirmer les prédictions théoriques. Un effet de squirt-flow a ainsi été mis en évidence dans le basalte tant que les fissures sont ouvertes.Nearly 60 % of the Earth surface is composed by basaltic rocks which form the upper part of the oceanic crust. These PhD works focus on the elastic properties of basalt at several spatialscales : field scale, sample scale and porosity microstructure scale. With its active hydrothermalism Iceland appears as the ideal natural laboratory to study the interactions existing between basalts, fluids and fracturing at a large scale. We tried to approach the properties of the Icelandic crust beneath the Reykjanes Peninsula by inverting seismic data. We refer to a mechanical framework and develop an effective medium model for estimating velocities in porous media including both pores and cracks. It makes it possible to derive the distribution of crack density from accurate tomography data. Outside the active hydrothermal areas, crack density is shown to decrease with depth. However, locally beneath the southwestern part of the Kleifarvatn lake, crack density increases with depth. This can be consistent with the presence of a deep reservoir with supercritical uids under pressure, which would trigger off hydrofracturing processes. At sample scale, we investigated modes of deformation of a Reykjanes basalt by triaxial tests. At low confining pressure, an axial loading induces failure in basalt according to an inclined shear plane. On the contrary, for higher confining pressures (more than 75 MPa), the deformation is localized in subhorizontal bands. Focal mechanisms of the acoustic emissions reveal the coexistence of two types of event in these bands : compaction and shear events. Moreover, microstructure has been studied through SEM. Pore collapse processes associated to grain sliding and shear cracks have also been revealed on pictures. It is the first time that such a mode of deformation is experimentally observed on basaltic rocks which have a lower porosity than sedimentary rocks on which compaction bands are usually observed (some sandstones or carbonates for instance). Finally we studied elastic properties of basalt at microscopic scale by investigating the dispersion of elastic moduli with the strain frequency. It is known that velocities of elastic waves generated at high frequency (HF) are usually higher than velocities of waves generated at lower frequency (LF). A part of their energy is lost by wave-induced fluid flows at different scale. In this study we focus on the squirt-flow effect which corresponds to a fluid flow between porosity inclusions. First, we developed a theoretical approach to calculate frequency dispersion of bulk (K) and shear (G) moduli as a function of the porosity microstructure. We shown that the dispersion was maximal when the effective medium was composed by both spherical pores and spheroidal cracks. Moreover, the flatter the cracks, the higher the dispersion. Second, we developed a new experimental set-up to investigate the K dispersion during a hydrostatic loading. We shown that as long as cracks are open in the basalt, a frequency dispersion in K occurs. We interpreted this observation as a squirt-flow between cracks and pores. Experimental results fit very well with theoretical predictions.LE MANS-BU Sciences (721812109) / SudocSudocFranceF

Crack Features and Shear-Wave Splitting Associated with Fracture Extension during Hydraulic Stimulation of the Geothermal Reservoir in Soultz-sous-Forêts

International audienceThe recent tomography results obtained within the scope of the Enhanced Geothermal System (EGS) European Soultz project led us to revisit the meso-fracturing properties of Soultz test site. In this paper, we develop a novel approach coupling effective medium modeling and shear-wave splitting to characterize the evolution of crack properties throughout the hydraulic stimulation process. The stimulation experiment performed in 2000 consisted of 3 successive injection steps spanning over 6 days. An accurate 4-D tomographic image was first carried out based upon the travel-times measured for the induced seismicity [Calò M., Dorbath C., Cornet F.H., Cuenot N. (2011) Large-scale aseismic motion identified through 4-D P-wave tomography, Geophys. J. Int. 186, 1295-1314]. The current study shows how to take advantage of the resulting compressional wave (Calò et al., 2011) and shear-wave velocity models. These are given as input data to an anisotropic effective medium model and converted into crack properties. In short, the effective medium model aims to estimate the impact of cracks on velocities. It refers to a crack-free matrix and 2 families of penny-shaped cracks with orientations in agreement with the main observed geological features: North-South strike and dip of 65°East and 65°West [Genter A., Traineau H. (1996) Analysis of macroscopic fractures in granite in the HDR geothermal well EPS-1, Soultz-sous-Forêts, France, J. Vol. Geoth. Res. 72, 121-141], respectively. The resulting output data are the spatial distributions of crack features (lengths and apertures) within the 3-D geological formation. We point out that a flow rate increase results in a crack shortening in the area imaged by both compressional and shear waves, especially in the upper part of the reservoir. Conversely, the crack length, estimated during continuous injection rate phases, is higher than during the increasing injection rate phases. A possible explanation for this is that cracks remain large because the system has time to relax. We also calculate the extension and opening rates during all hydraulic stimulation sets. While the opening rate is unchanged, the extension rate varies depending on the stimulation phase. It is also shown to be higher around and above the open-hole section than below. This can indicate a potential upward path that makes fluid percolation easier within the granite formation, this path being induced by the temperature gradient. We also compare the evolution of crack extension during injection with shear-wave splitting. Split shear waves were recorded at 2 stations during hydraulic stimulation and processed in terms of splitting parameters. The fast shear-wave polarization remains constant and parallel to the maximum horizontal stress orientation while the amplitude of splitting varies with time. We observe a good agreement between travel-time differences and crack extension rates during the first 4 days of the stimulation experiment. Afterwards, these two , which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. parameters depart from each other. This study emphasizes the added value of the coupling between effective medium modeling and shear-wave splitting to monitor meso-scale cracks in reservoirs submitted to hydraulic stimulation.Résumé — Caractérisation et évolution des fissures au cours de la simulation hydraulique du réservoir géothermique de Soultz-sous-Forêts — Les récents résultats obtenus en tomographie sismique dans le cadre du projet européen EGS (Enhanced Geothermal System) offrent un nouvel éclairage sur les propriétés de fissure du site expérimental de Soultz-sous-Forêts. Dans cet article, une nouvelle approche associant modélisation par milieu effectif et bi-réfringence des ondes S a été développée afin de caractériser les propriétés des fissures au cours d'une stimulation hydraulique. Le test hydraulique mené en 2000 comporte 3 phases d'injection réparties sur 6 jours. Une première tomographie 4D précise a été réalisée à partir des temps de trajet mesurés sur la sismicité induite [Calò M., Dorbath C., Cornet F.H., Cuenot N. (2011) Large-scale aseismic motion identified through 4-D P-wave tomography, Geophys. J. Int. 186, 1295-1314]. La présente étude s'attache à utiliser les modèles de vitesse obtenus en ondes P (Calò et al., 2011) mais également en ondes S. Ces vitesses sont les données d'entrée d'un modèle anisotrope de milieu effectif que l'on construit pour traduire les vitesses en propriété de fissures. Le modèle est fait d'une matrice saine et de 2 familles de fissures modélisées sous forme de pièces de monnaie. Leurs orientations géométriques sont établies à partir des données géologiques : orientation Nord-Sud et pendages respectifs de 65° Est et 65° Ouest [Genter A., Traineau H. (1996) Analysis of macroscopic fractures in granite in the HDR geothermal well EPS-1, Soultz-sous-Forêts, France, J. Vol. Geoth. Res. 72, 121-141]. Les résultats en sortie du modèle de milieu effectif sont les longueurs et les ouvertures de fissures dans le réservoir géologique. Nous mettons en évidence qu'une augmentation du débit d'injection entraine un rétrécissement en longueur des fissures dans la zone tomographique couverte en vitesses P et S, notamment dans la partie supérieure du réservoir. Inversement, les longueurs des fissures calculées au cours des phases d'injection continue sont supérieures à celles calculées au cours des phases d'augmentation brutale du débit. Une explication possible réside dans le fait que les fissures restent ouvertes sur une longueur importante au cours des phases de plateau car le système a le temps de se relâcher. Nous avons également calculé les taux d'ouverture et d'allongement des fissures au cours des différentes phases de l'injection. Alors que le taux d'ouverture reste globalement le même, le taux d'allongement est très variable. Il est plus important autour du puits ouvert et au-dessus qu'en dessous. Ceci pourrait être le signe d'un chemin préférentiel de drainage des fluides vers le haut au sein de la formation granitique, peut-être induit par un gradient de température. Nous avons également comparé les évolutions des taux d'allongement durant les phases d'injection avec les résultats de bi-réfringence des ondes S. Deux types d'ondes S ont été enregistrées à deux stations au cours de l'injection et ont donc pu être interprétées en terme de paramètres de bi-réfringence. L'orientation du plan de polarisation de l'onde S rapide reste constante et parallèle à l'orientation de la contrainte horizontale maximale alors que l'amplitude de la réfringence varie au cours du temps. On observe une bonne cohérence entre les différences de temps de trajet entre les ondes S lentes et rapides et les taux d'allongement des fissures au cours des 4 premiers jours de la stimulation hydraulique. Au-delà, ces deux paramètres divergent. Finalement, cette étude met en évidence l'importance de coupler différentes méthodes issues de la géophysique et de la géomécanique, ici la modélisation en milieu effectif et la bi-réfringence des ondes S, dans le but de mieux caractériser la fracturation dans des réservoirs géologiques

Seismic characterization of Cenomanian–Turonian carbonate platform based on sedimentological and geophysical investigation of onshore analogue outcrop (northern Lebanon)

International audienceAbstract Outcrop analogues play a key role in the characterization of subsurface carbonate platforms. The lack of well data and relevant outcrop analogues can result in the misinterpretation of seismic data. To address this issue, we apply an integrated workflow based on sedimentology, geophysics and petrophysics on outcrop analogues present onshore Lebanon, to constrain the carbonate platform's properties on onshore seismic data. A thorough sedimentary description is completed for a 400‐m‐thick Cenomanian–Turonian carbonate platform located in Kfarhelda, northern Lebanon. P‐wave velocity is acquired directly on the outcrop, and the petrophysical properties are measured on 44 samples. A 1D synthetic seismogram is computed with Ricker wavelet 25 Hz resembling seismic resolution. The resulting reflectors are mainly (1) high amplitude reflectors at the limit between two facies with contrasting physical properties enhanced by diagenesis, (2) moderate amplitude reflectors corresponding to stratigraphic limits at the transition between facies and (3) very low amplitude reflectors in karstified units. The integration of outcrop and seismic data is based on the generation of the synthetic seismogram to identify the geological origin of reflectors. The best fit between the synthetic seismic and seismic profile is used to interpret a seismic facies representing bedded limestones of Sannine and Maameltain formations (Cenomanian–Turonian). Two other distinctive reflectors are identified at the boundary of the Marly Limestone Zone, and the Channel facies unit characterized by bioclastic packstone to floatstone. This study highlights the importance of using outcrop analogues to identify the seismic signal of stratigraphic sequences and improve the interpretation of onshore seismic data

Methane Occurrence and Quantification in a Very Shallow Water Environment: A Multidisciplinary Approach

International audienceIn the last decades, geological degassing in shallow marine environments has been recognized as a significant contributor to atmospheric methane, hence to the global-warming potential. Especially in shallow water environments, a proper assessment of the distribution, quantification and migration pathways of methane within the sediments is fundamental to help forecasting the amount that could leak and eventually reach the atmosphere. Traditionally, velocity anomalies from seismic data are the ones used to assess the occurrence of gas and its concentration. However, in shallow-waters (<30 m), the post-critical conditions make the near-surface velocity estimation from P-wave reflections extremely challenging, requiring an integrated approach. Here, we propose an original joint analysis of seismic data and geophysical logs, together with information from drilling reports, with the aim of characterizing and quantifying the gas along two crossing multichannel seismic profiles in the Northern Adriatic Sea, a very shallow marine basin where methane occurrence within the sedimentary succession is widespread. We estimated the gas distribution from resistivity anomalies, which are correlated with the seismic response associated with the presence of gas through the signal frequency content. Our results show a different concentration pattern in the two seismic profiles, revealing that gas is both diffuse ad concentrated in local accumulations, in agreement with the gas-related features already identified on the seismic data. Gas concentration appears to be locally associated to the tectonic features identified in the area, indicating that faults act as preferential conduits for gas migration, locally reaching the seafloor and seeping in the water column