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Seismic characterization of fractured rocks based on a porolastic approach

English title Seismic characterization of fractured rocks based on a porolastic approach
Applicant Holliger Klaus
Number 178946
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 01.05.2019 - 30.04.2023
Approved amount 523'928.00
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All Disciplines (2)

Discipline
Geophysics
Geology

Keywords (8)

seismic wave propagation; poroelasticity; fractured rocks; upscaling; velocity dispersion; permeability; attenuation; anisotropy

Lay Summary (German)

Lead
Geophysikalische Informationen in Bezug auf die hydraulischen Eigenschaften des Untergrunds sind wertvoll für die Beantwortung einer Vielzahl von wissenschaftlich und praktisch relevanten Fragen. Der Hauptgrund hierfür ist, dass die entsprechenden Methoden nicht-invasiv und vergleichsweise billig sind und dass sie das das Potenzial haben, die für klassische hydrogeologische Ansätze bestehende Lücke in Bezug auf Auflösung und Abdeckung überbrücken zu können.
Lay summary

Die Permeabilität ist eine der wichtigsten aber gleichzeitig auch eine der am schwierigsten zu bestimmenden gesteinsphysikalischen Eigenschaften. Entsprechende geophysikalische Informationen sind daher besonders wertvoll. Der zur Zeit wohl am ehesten erfolgversprechende Ansatz dieser Art basiert auf der poroelastischen Interpretation von seismischen Daten. Die realistische numerische Simulation der seismischen Wellenausbreitung und der damit verbundenen Fliessbewegungen des Porenfluids in heterogenen porösen Medien ist daher von grosser Bedeutung. Aus technischer Sicht ist dieser Ansatz aber sehr anspruchsvoll und der wohl effektivste Weg dieses Problem zu lösen, beruht auf der numerischen Simulation von Kompressibilitäts- und Scherversuchen wie sie, in ähnlicher Weise, auch im Labor durchgeführt werden. Unsere wichtigsten Ziele hierbei sind die Gültigkeit von bestehenden seismischen Charakterisierungsmethoden für heterogen geschichtete poröse und klüftige Gesteine zu prüfen sowie die Empfindlichkeit von seismischen Messungen in Bezug auf die Permeabilität derartiger Medien systematisch zu erforschen.

Direct link to Lay Summary Last update: 26.11.2018

Responsible applicant and co-applicants

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Associated projects

Number Title Start Funding scheme
149240 Seismic Characterization of Heterogeneous Sedimentary Sequences and Fractured Rocks Based on a Poro-Elastic Approach 01.10.2013 Project funding (Div. I-III)
153889 Exploration and characterization of deep underground reservoirs 01.10.2014 NRP 70 Energy Turnaround

Abstract

The seismic characterization of fractured rocks is widely regarded as a frontier with a vast range of important applications, such as, for example, the sustainable use of groundwater, the optimized production of hydrocarbons and geothermal energy, and the safe storage of carbon dioxide and nuclear waste. While it is generally recognized that seismic data can provide valuable information with regard to fractured media, including fracture density and orientation, the relations between seismic observables and parameters controlling the hydraulic properties of fractured formations remain enigmatic. This may be due to the fact that, so far, most works concerned with the effective seismic properties of fractured rocks were based on the assumption of linear elasticity, which does not account for the effects of wave-induced fluid flow (WIFF) between the fractures and the embedding background as well as between connected fractures. Due to the commonly observed characteristics of fractures, notably their strong compressibility, WIFF effects may have a profound impact on the effective seismic properties. Indeed, based on the theory of poroelasticity and numerical upscaling based thereon, our recent work provides evidence to suggest that the phase velocity and attenuation of seismic waves are expected to be sensitive to the size and connectivity of fractures. More recently, we were also able to demonstrate that the seismic velocity anisotropy, which represents a robust observable commonly used to characterize fractured formations, is expected to decrease with increasing degree of fracture connectivity. In the context of this proposal, we plan to extend and complement our previous research efforts on fractured media. We seek to address some major unresolved questions, notably, the importance of 3D effects and the links between microscopic characteristics of fractures and their poroelastic representation. To explore the role played by the dimensionality of fracture networks on the effective seismic signatures, we will extend our 2D generalized numerical upscaling approach to 3D. This will allow us to determine whether and to what extent our previous findings need be quantitatively reassessed. To study the links between the microscopic characteristics of fractures and their poroelastic representations, we will follow a learning path of increasing complexity. First, we will use published analytical theories for quantifying the mechanical and hydraulic properties of fractures conceptualized as arrangements of cracks and contact areas, which will allow us to assign effective properties to the porous media used to represent the fractures. We will then study how the properties of these equivalent poroelastic materials change as functions of the microscopic characteristics of the fractures, that is, aperture, contact area, and number of cracks, and we will evaluate the corresponding impacts on WIFF effects. Subsequently, we will develop a novel numerical upscaling procedure that represents the fractures as acoustic media embedded in a porous background. This will allow us to explore the importance of different microscopic characteristics of fractures in the context of WIFF and to assess the limitations of simpler representations of fractures. In addition to the methodological work outlined above, our research will also focus on some major unresolved questions of practical importance. Notably, we will analyze the information content of typical seismic recordings in fractured environments. To this end, we will perform numerical simulations of seismic wave propagation using upscaled equivalent anisotropic viscoelastic solids that account for WIFF effects. We will also explore the seismic response of vertically fractured stratified media, which are commonly found in the shallower parts of the Earth's crust. Finally, we propose to model the changes of the mechanical and hydraulic properties of individual fractures in response to variations of pore fluid pressure as well as the corresponding effects on the effective seismic properties. This will allow us to assess the variations of seismic properties of fractured rocks during fluid injection and extraction processes, which in turn may help to improve the interpretation of time-lapse seismic data acquired during such operations. All these tasks will be performed in the framework of Biot's theory of poroelasticity, which naturally accounts for WIFF effects. Since WIFF effects are particularly prominent in the presence of fractures, the proposed research has the potential of providing fundamentally new insights. The proposed research is expected to clarify whether and to what extent seismic data contain information on the geometrical, mechanical, and hydraulic characteristics of fracture networks and it may contribute the development of workflows for extracting this information from observed seismic data. It ideally complements related work of our group associated with our involvement in the SCCER-SOE and NRP70 programs and as such has the potential for generating significant added value through synergies and cross-fertilizations.
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