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Seismic signature of hydraulic interconnectivity of fractures

English title Seismic signature of hydraulic interconnectivity of fractures
Applicant Quintal Beatriz
Number 172691
Funding scheme Project funding (Div. I-III)
Research institution Institut des sciences de la Terre Université de Lausanne
Institution of higher education University of Lausanne - LA
Main discipline Geophysics
Start/End 01.01.2018 - 28.02.2022
Approved amount 501'928.00
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All Disciplines (2)

Discipline
Geophysics
Geology

Keywords (11)

attenuation; fractures; seismic; dispersion; fluid-saturated rocks; squirt flow ; fluid pressure diffusion ; hydraulic conductivity; fracture connectivity ; forced oscillation; numerical modeling

Lay Summary (French)

Lead
La méthode géophysique de sismique par réflexion est un moyen efficace pour obtenir des informations sur les roches à quelques kilomètres de profondeur dans la croûte terrestre. En général, cette méthode fournit une image des interfaces entre les couches de différentes roches. Pour obtenir plus d'informations à partir des données sismiques, il faut mieux comprendre la physique de la propagation des ondes sismiques dans les roches saturées. Les études théoriques montrent que la perte d'énergie, ou l'atténuation, des ondes sismiques peut être sensible à l'inter-connectivité hydraulique des fractures. Cependant, aucune mesure d’atténuation sismique n’a été faite à ce jour en laboratoire ou sur le terrain pour confirmer les prédictions théoriques.
Lay summary

L'atténuation sismique dans les roches fracturées et saturées en liquide sera quantifiée en laboratoire et sur ordinateur à travers des simulations numériques du squirt flow. Dans le mécanisme squirt flow, quand les ondes déforment le milieu, un flux de fluide est induit d'une fracture à l'autre si les fractures se croisent. Les mesures en laboratoire utiliseront des échantillons de verre fracturé que seront imagées avec la tomodensitométrie par rayons X. Des modèles numériques créés à partir de ces images et les résultats des simulations correspondantes seront utilisés pour interpréter les données de laboratoire. Les objectifs sont d’identifier les mécanismes d'atténuation prédominants sur ces données, d’identifier les motifs d'atténuation associés à une augmentation de l'inter-connectivité hydraulique des fractures, et de développer un protocole pour le traitement d'image des solides fracturés ainsi qu'un algorithme capable de simuler le squirt flow dans des réseaux de fractures en trois dimensions.

Notre travail générera des informations inédites et essentielles que contribueront à améliorer le monitoring des déchets radioactifs et du gaz CO2 dans la croûte terrestre, ainsi que l'exploration de l'énergie géothermique et des hydrocarbures. 

Direct link to Lay Summary Last update: 27.10.2017

Responsible applicant and co-applicants

Employees

Associated projects

Number Title Start Funding scheme
156593 Identification of dominant attenuation mechanisms in fluid-saturated rocks 01.10.2014 Project funding (Div. I-III)

Abstract

The hydromechanical characterization of a fractured and fluid-saturated subsurface domain has a number of important applications, such as the sustainable use of groundwater, the optimized production of geothermal energy and hydrocarbons, as well as the safe geological storage of CO2 and disposal of nuclear waste. Assessing the hydromechanical properties (permeability and storage capacity) of a fractured rock indirectly, for example, by using seismic data, has been speculated but not yet proven feasible. The most promising speculation is based on recent studies showing that the attenuation of seismic waves is sensitive to the degree of hydraulic interconnectivity of fractures in extremely idealized fracture models. Further research is thus necessary to quantify this sensitivity for realistic fracture models. This is the target of our proposed research. Our plan is divided into two subprojects to be conducted in parallel in two universities: University of Lausanne and University of Stuttgart. Both subprojects aim at the same target but use different methodologies. The subproject Lausanne involves numerical modeling while the subproject Stuttgart is based on laboratory measurements. With this proposal, we request funding for: •two 3-year PhD students within the subproject Lausanne; •one 3-year PhD student within the subproject Stuttgart.The PhD students in Lausanne will extend and optimize existing numerical methodologies and apply them to fracture models. One of them will study large models of idealized fracture geometries and the other will study smaller models of realistic fracture geometries based on micro-CT images of rock samples. In the study considering idealized fracture geometries, statistical studies of the representative elementary volume (REV) will be carried out, so that the results can be upscaled to field dimensions. In the study based on micro-CT images, an existing segmentation workflow will be adapted to fractured rocks, and numerical modeling will be performed to calculate attenuation in image-based models of the fractured samples, considering full water saturation. The PhD student in the University of Stuttgart will mainly acquire micro-CT images of fractured samples under reservoir pressure conditions and perform laboratory measurements of seismic attenuation on the same or equivalent fractured rock samples that were used in the numerical studies of one of the PhD students from Lausanne. For these laboratory measurements, fluids of different viscosities will be used, so that the frequency-dependent attenuation can be observed in the frequency range of measurements (1-500 Hz). A constant exchange of information and comparison between numerical and laboratory results will be required from the PhD students. An important aspect of this comparison is that, in the laboratory, the measured attenuation results from a superposition of all possible mechanisms which are relevant under the adopted conditions. On the other hand, numerically we are able to study the isolated effect of a single physical mechanism. Therefore, this exchange allows for definite answers with respect to what causes the observed attenuation and is an essential step for reaching our aimed target.The physical mechanism for attenuation, on which our numerical studies are based, is pressure diffusion in fluid-saturated interconnected fractures. The fact that a wave deforms differently two interconnected fractures, because they are differently positioned with respect to the direction of propagation, induces fluid pressure differences between them. This results in pressure diffusion from one fracture into the other one, here referred to as squirt flow, and in consequent viscous dissipation (attenuation) of the wave energy.
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