Project

Discipline

Krauklis waves; Rock physics; Fractured rocks; Fluid reservoir; Low-frequency tremor; Micro-seismicity

Lead
Das SNF-Projekt “UPseis” beschäftigt sich mit zerklüfteten Reservoirgesteinen. Im Speziellen wird das seismische Phänomen der sogenannten Krauklis Wellen im Detail untersucht. Dieser Wellentyp ist für vulkanisches Grollen oder für mikroseismische Anwendungen in Fluidreservoirs von Bedeutung.
Lay summary
Die detailierte Beschreibung und das Verständnis von Fluidreservoirgesteinen ist von grosser ökonomischer, umweltbezogener und wissenschaftlicher Bedeutung. Beispiele dafür sind die geologische Kohlenstoffbindung, die Erdöl- und Gasindustrie, die geologische Tiefenlagerung von radionuklearen Abfällen oder die Reinigung von verschmutzten Grundwasserleitern. All diese Fluidreservoire werden üblicherweise mit seismischen Methoden aufgespürt und untersucht. Da ein Grossteil der Porosität und Permeabilität solcher Fluidreservoirs aufgrund von Klüften im Gestein auftreten kann, ist es unumgänglich die Effekte von Klüften auf seismischen Wellen zu kennen um das Gestein realitätsnah zu charakterisieren. Das Projekt “UPseis” beschäftigt sich mit einem ganz speziellen seismischen Phänomen in zerklüfteten Gesteinen, der sogenannten Krauklis Welle. Dieser Wellentyp ist an Fluid-gesättigte Klüfte gebunden. Dadurch kann die Krauklis Welle entlang einer Kluft hin und her laufen und ein seismisches Signal mit einer charakteristischen Frequenz aussenden. Wenn dieses Signal an der Erdoberfläche empfangen wird, können mögliche Aussagen über die Kluftdichte, Kluftorientierung oder über die Art des Fluids im Reservoirgestein getroffen werden. Zu Beginn des Projekts wird die Krauklis Welle mittels numerischen Wellenausbreitungs-Simulationen auf der Skala einer einzelnen Kluft im Detail studiert. Die dabei gewonnen Erkenntnisse werden dann auf eine grössere Skala hochskaliert, welche ein geanzes zerklüftetes Fluidreservoir umfasst. So kann das effektive seismische Verhalten eines zerklüfteten Fluidreservoirgesteins besser verstanden werden.

Direct link to Lay Summary

Last update: 11.03.2013

Lead
The project “UPseis” focuses on fractured fluid reservoir rocks. In particular, the seismic phenomenon of the so-called Krauklis wave is investigated in detail. This wave phenomenon is relevant for volcanic tremor generation or micro-seismicity applications in fluid reservoir settings.
Lay summary
A detailed description and understanding of fluid reservoir rocks is of great economic, environmental, and scientific interest, for example for CO2-sequestration, hydrocarbon exploration, nuclear waste disposal, or groundwater aquifer remediation. Such fluid reservoirs are commonly probed using seismic investigation methods. Because a large part of the reservoir porosity and permeability can be due to fractures, understanding their effects on the seismic response of a rock is therefore essential for a reliable rock characterization. The project “UPseis” investigates a particular seismic phenomenon in fractured rocks, the so-called Krauklis wave. This special wave mode is bound to fluid-filled fractures. When propagating back and forth along a fracture, a Krauklis wave can emit a seismic signal to the surrounding rock with a characteristic frequency. When picked up at the Earth’s surface, such seismic signals may contain information on the fracture density, fracture orientation, or fluid type in the subsurface reservoir. In a first phase, the Krauklis wave phenomenon will be studied in detail by applying numerical wave-propagation simulations on the scale of individual fractures. In a second phase, these results will be upscaled to the reservoir-scale to understand the bulk seismic response of fractured fluid reservoirs.

Direct link to Lay Summary

Last update: 11.03.2013

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The presence of fluids in reservoir rocks has a major effect on the propagation of seismic waves, e.g., dispersion and frequency-dependent attenuation (Biot, 1962; White, 1975; Bourbie et al., 1987; Carcione, 2001; Quintal et al., 2011c). Research on fluid-related seismic effects faces some scientific challenges and is significant for various industrial applications. One particular challenge is the interaction of simultaneous physical processes on different length-scales. Because it is not possible to model a whole reservoir and resolve all the microscale details at the same time, microscale processes have to be upscaled and their macroscale effects are described in effective medium models (e.g., Frehner et al., 2009; Frehner et al., 2010). For the case of porous rocks, such as sandstone, this is done successfully, e.g., in the Biot-theory (Biot, 1962), the squirt-flow theory (Mavko and Jizba, 1991; Dvorkin et al., 1995), or the patchy-saturation model (White, 1975; White et al., 1975).Despite including many effects in porous rocks, existing effective medium models do not describe very well fracture-related phenomena, one of which is the propagation of Krauklis waves (Ferrazzini and Aki, 1987; Chouet, 1988; Korneev, 2008; Frehner and Schmalholz, 2010). This wave mode is bound to and propagates along fluid-filled fractures and may also influence seismic body waves (e.g., from earthquakes or seismic surveys). Krauklis waves can propagate back and forth along a fracture and emit a periodic signal (Aki et al., 1977; Chouet, 1988; Chouet, 1996). Seismic data may contain this characteristic frequency and eventually reveal fracture-related petrophysical parameters of the reservoir. Krauklis waves are a truly multi-scale phenomenon (Frehner and Schmalholz, 2010). Their wavelength may be similar to body waves propagating through a fractured reservoir, but they propagate along fractures that are orders of magnitude thinner than this wavelength.Although Krauklis waves are well described mathematically for the theoretical case of infinitely long and straight fractures (Ferrazzini and Aki, 1987; Ashour, 2000; Korneev, 2008; Korneev, 2010), many of the key processes in real reservoir rocks are only speculated in the existing literature. An in-depth investigation is necessary to understand these processes and their significance in nature. Fundamental questions raised in this proposal are:- Can Krauklis waves be initiated by a passing body wave?This determines if Krauklis waves are an important microscale process for seismic surveys or earthquake measurements. It is known that Krauklis waves can be initiated by a seismic source within the fracture (e.g., hydrofracturing; Chouet, 1986; Frehner and Schmalholz, 2010). Preliminary studies by the applicant (Frehner and Quintal, 2011) suggest that initiation by body waves is possible at the fracture tips.- Can Krauklis waves propagate several times back and forth a fracture and emit a periodic seismic signal?This determines if fracture-related petrophysical properties could be extracted from the frequency-content of seismic data. Preliminary studies by the applicant indicate that a characteristic frequency can indeed be emitted, at least for low-viscosity fluids.- How do Krauklis waves propagate in interconnected and unconnected fracture networks and how does this influence a passing or incident body wave?Understanding this difference may allow extracting information about the percolation threshold and the fracture-controlled permeability from seismic signals.- How can all of these effects be upscaled and integrated into an effective medium model?Upscaling the microscale processes to their macroscopic effects is essential for integrating these effects into a macroscale model, which is the ultimate goal of this research.The stated scientific questions are faced mainly from a theoretical and numerical modeling point of view, and will be addressed during one three-year PhD-project, whose main focus and tasks are:- Study the initiation of Krauklis waves by a passing or incident body wave with the existing 2D numerical model (Frehner and Schmalholz, 2010).- Conduct simulations of Krauklis waves propagating back and forth a fracture and emitting a periodic seismic signal.- Investigate Krauklis wave-effects in interconnected and unconnected fracture-networks.- Expand the model to 3D and include the Biot-equations for poro-elasticity to simulate Krauklis waves in more realistic fractured porous rocks.- Include micro-computed-tomography data of real fractured reservoir rocks into the model.- Upscale the observed effects and include them into an effective medium model.Besides the individual fundamental research results related to Krauklis waves in fractures, which will be generated during this project, the ultimate goal of the project is two-fold:- Establishing the significance of Krauklis waves in real fractured reservoir rocks for seismic body waves propagating through such rocks.- Upscaling the microscale Krauklis wave-process and including its macroscopic effects into an effective medium model to access the full seismic signature of waves propagating through fractured reservoir rocks.The proposed project focuses on the theoretical aspects of Krauklis waves, but the ETH Zurich offers the unique opportunity to combine this project with laboratory studies being conducted in related on-going projects. Experimental data allow calibrating the numerical models by comparing theoretical and numerical results with laboratory data.