Search

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

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

Methane hydrate comprises a significant piece of the global carbon cycle and is an important potential energy resource. Thick marine sands around the world contain free gas beneath and high concentrations of methane hydrate far above the base of the hydrate stability zone. The mechanisms controlling gas transport and hydrate formation within the region where hydrate is stable remains an important research question. I developed a new experimental method to investigate the fundamental behaviors associated with hydrate formation during gas flow into the hydrate stability zone. First, I performed a set of experiments at the same experimental conditions, to determine the repeatability of this behavior. I compared these results to those from an experiment performed outside the hydrate stability zone to elucidate the change in intrinsic gas flow behavior due to hydrate formation. Second, I performed additional experiments at a range of flow rates as well as several shut-in experiments, where I observed long-term hydrate formation after flow took place. I analyzed the bulk and core-scale behaviors of these experiments using a combination of mass balance and computed-tomography analyses. I found that many of my experimentally observed behaviors are not accurately described by previous models and that the mechanism for gas transport was fundamentally different than typically assumed. I proposed that hydrate formation at the gas-brine interface separates the gas and brine phases and limits hydrate formation to methane transport through the hydrate. This behavior produced temporary flow blockages that were mitigated when the hydrate skin fails due to pressure gradients across the sample. This behavior also produced different thermodynamics states on either side of the hydrate that could persist for hundreds to thousands of years. These results provide an alternative mechanism for gas transport and hydrate formation through the hydrate stability zone that does not require the gas, hydrate, and brine to be at three-phase equilibrium. This mechanism provides a first-order connection between experimentally observed micro-scale phenomena and field-scale gas transport and hydrate formation behaviors in these reservoirs.

Methane hydrate comprises a significant piece of the global carbon cycle and is an important potential energy resource. Thick marine sands around the world contain free gas beneath and high concentrations of methane hydrate far above the base of the hydrate stability zone. The mechanisms controlling gas transport and hydrate formation within the region where hydrate is stable remains an important research question. I developed a new experimental method to investigate the fundamental behaviors associated with hydrate formation during gas flow into the hydrate stability zone. First, I performed a set of experiments at the same experimental conditions, to determine the repeatability of this behavior. I compared these results to those from an experiment performed outside the hydrate stability zone to elucidate the change in intrinsic gas flow behavior due to hydrate formation. Second, I performed additional experiments at a range of flow rates as well as several shut-in experiments, where I observed long-term hydrate formation after flow took place. I analyzed the bulk and core-scale behaviors of these experiments using a combination of mass balance and computed-tomography analyses. I found that many of my experimentally observed behaviors are not accurately described by previous models and that the mechanism for gas transport was fundamentally different than typically assumed. I proposed that hydrate formation at the gas-brine interface separates the gas and brine phases and limits hydrate formation to methane transport through the hydrate. This behavior produced temporary flow blockages that were mitigated when the hydrate skin fails due to pressure gradients across the sample. This behavior also produced different thermodynamics states on either side of the hydrate that could persist for hundreds to thousands of years. These results provide an alternative mechanism for gas transport and hydrate formation through the hydrate stability zone that does not require the gas, hydrate, and brine to be at three-phase equilibrium. This mechanism provides a first-order connection between experimentally observed micro-scale phenomena and field-scale gas transport and hydrate formation behaviors in these reservoirs.