Physical properties of Icelandic basalts: field observations and experimental data

see Abstract VolumeIstituto Nazionale di Geofisica e Vulcanologia, Italy (INGV) Centre National de la Recherche Scientifique (CNRS) ExxonMobil Upstream Research CompanyUnpublishedErice, Italyope

Multiscale reservoir geological modeling and advanced geostatistics

ABSTRACT This presentation will discuss new methodologies an

Seismic-refraction field experiments on Galapagos Islands: A quantitative tool for hydrogeology

International audienceDue to their complex structure and the difficulty of collecting data, the hydrogeology of basaltic islands remains misunderstood, and the Galapagos islands are not an exception. Geophysics allows the possibility to describe the subsurface of these islands and to quantify the hydrodynamical properties of its ground layers, which can be useful to build robust hydrogeological models. In this paper, we present seismic refraction data acquired on Santa Cruz and San Cristobal, the two main inhabited islands of Galapagos. We investigated sites with several hydrogeological contexts, located at different altitudes and at different distances to the coast. At each site, a 2D P-wave velocity profile is built, highlighting unsaturated and saturated volcanic layers. At the coastal sites, seawater intrusion is identified and basal aquifer is characterized in terms of variations in compressional sound wave velocities, according to saturation state. At highlands sites, the limits between soils and lava flows are identified. On San Cristobal Island, the 2D velocity profile obtained on a mid-slope site (altitude 150 m), indicates the presence of a near surface freshwater aquifer, which is in agreement with previous geophysical studies and the hydrogeological conceptual model developed for this island. The originality of our paper is the use of velocity data to compute field porosity based on poroelasticity theory and the Biot-Gassmann equations. Given that porosity is a key parameter in quantitative hydrogeological models, it is a step forward to a better understanding of shallow fluid flows within a complex structure, such as Galapagos volcanoes

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

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

The 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 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

Physical properties of Icelandic basalts: field observations and experimental data

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Frequency and fluid effects on elastic properties of basalt: Experimental investigations

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From laboratory scale to field scale: experimental evidence of the frequency effects in basalt

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