Frequency-dependent mechanical properties of geomaterials : laboratory experiments and digital rock physics




Ikeda, Ken, 1993-

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Geoscientists often model the subsurface by studying the propagation of seismic waves. As a seismic wave propagates through a medium, its amplitude and phase change according to the medium physical properties. Geoscientists often simplify the rheology of geomaterials to elastic media where elastic properties are frequency-independent. However, geomaterials possess frequency-dependent properties. Such oversimplification can create inaccurate subsurface models. Therefore, often geomaterials should be modeled as frequency-dependent materials using appropriate attenuation models. In this dissertation, I explore the elastic properties of rocks at low-frequencies (~0.1–100 Hz) and ultrasonic frequencies (10⁵-10⁶ Hz). I use laboratory measurements to estimate elastic properties of rocks. These measurements are then compared to Digital Rock Physics (DRP) results, where the same laboratory measurements are numerically simulated on Computed-Tomographic (CT) images. The first part of this dissertation explores elastic properties of a quartz-rich sandstone at ultrasonic frequencies. I demonstrate that the laboratory-measured elastic properties could be efficiently predicted using a new DRP based technique called Segmentation-Less withOut Targets (SLOT). The SLOT method uses the local variation of X-ray attenuation in CT-images to map the density distribution of the corresponding material. Numerical simulations of wave propagations are used to estimate the elastic properties of the sample, and show a small mismatch to the laboratory measurements. The second part of the dissertation focuses on estimating low-frequency elastic properties using the SLOT technique, which has been extended to accommodate the characteristics of a polymineralic carbonate. The modified-SLOT combines segmentation-based DRP with segmentation-less DRP to create elastic property distribution maps. The DRP predictions agree with the laboratory measurements. The last part of the dissertation shows the development of a new apparatus to measure low-frequency attenuation: the Low-Frequency Module (LFM). The mechanical and electronic design of the LFM is carefully chosen to accommodate a decametric sample that can be tested at reservoir pressure-temperature conditions. The design and the limitation of the apparatus are discussed. The newly developed DRP techniques and the state-of-the-art apparatus will help geoscientists exploring the elastic properties of rocks at different bandwidths. Improved estimations of elastic properties will help to better capture subsurface features


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