Diss. (sammanfattning) Stockholm : Karolinska institutet, 2010.

Thesis--University of Oklahoma. / Bibliography: leaves 97-101.

Thesis--University of Oklahoma. / Bibliography: leaves 76-79.

Diss. (sammanfattning) Stockholm : Karolinska institutet, 2010. / Härtill 4 uppsatser.

Open and connected fractures, where present, control fluid flow and dominate solute transport. Flow through fractures has major implications for water resource management, underground waste repositories, contaminant remediation, and hydrocarbon exploitation. Complex fracture morphology makes it difficult to quantify and predict flow and transport accurately. The difficulty in usefully describing the complex morphology of a real fracture from a small 3-D volume or 2-D profile sample remains unresolved. Furthermore, even when complex fracture morphology is measured across three-dimensions, accurate prediction of discharge remains difficult. High resolution x-ray computed tomography (HXRCT) data collected for over 20 rock surfaces and fractures provide a useful dataset to study fracture morphology across scales of several orders of magnitude. Samples include fractured rock of varying lithology, including sandstone, volcanic tuffs and crystalline igneous and metamorphic rocks. Results suggest that the influence of grain size on surface roughness is not readily apparent due to other competing variables such as mechanics, skins and coatings, and weathering and erosion. Flow tests of HXRCT-scanned fractures provide real discharge data allowing the hydraulic aperture to be directly measured. Scale-invariant descriptions of surface roughness can produce constrained estimates of aperture variability and possibly yield better predictions of fluid flow through fractures. Often, a distinction is not made between the apparent and true fracture apertures for rough fractures measured on a 2-D topographic grid. I compare a variety of local aperture measurements, including the apparent aperture, two-dimensional circular tangential aperture, and three-dimensional spherical tangential aperture. The mechanical aperture, the arithmetic mean of the apparent local aperture, is always the largest aperture. The other aperture metrics vary in their ranking, but remain similar. Results suggest that it may not be necessary to differentiate between the apparent and true apertures. Rock fracture aperture is the predominant control on permeability, and surface roughness controls fracture aperture. A variety of surface roughness characterizations using statistical and fractal methods are compared. A combination of the root-mean-square roughness and the surface-to-footprint ratio are found to be the most useful descriptors of rock fracture roughness. Mated fracture surfaces are observed to have nearly identical characterizations of fracture surface roughness, suggesting that rock fractures can be sampled by using only one surface, resulting in a significantly easier sampling requirement. For mated fractures that have at least one point in contact, a maximum potential aperture can be constrained by reflecting and translating a single surface. The maximized aperture has a nearly perfect correlation with the RMS roughness of the surface. These results may allow better predictions of fracture permeability thereby providing a better understanding of subsurface fracture flow for applications to contaminant remediation and water and hydrocarbon management. Further research must address upscaling fracture morphology from hand samples to outcrops and characterizing entire fracture networks from samples of single fractures. / text

The effects of porous medium heterogeneity on methane hydrate formation, water flow through the heterogeneous hydrate-bearing sand, and hydrate dissociation were observed in an experiment using a heterogeneous sand column with prescribed heterogeneities. X-ray computed tomography (CT) was used to monitor saturation changes in water, gas, and hydrate during hydrate formation, water flow, and hydrate dissociation. The sand column was packed in several segments having vertical and horizontal layers with two distinct grain-size sands. The CT images showed that as hydrate formed, the water and hydrate saturations were dynamically redistributed by variations in capillary strength of the medium (the tendency for a material to imbibe water), which changed with the presence and saturation of hydrate. Water preferentially flowed through fine sand near higher hydrate-saturation regions where the capillary strength was elevated relative to the lower hydrate saturation regions. Hydrate dissociation initiated by depressurization varied with different grain sizes and hydrate saturations.

Background: The use of pure oxygen during preoxygenation and induction of general anaesthesia is a major cause of atelectasis. The interaction between reduced lung volume, resulting in airway closure, and varying inspiratory fractions of oxygen (FIO2) in determining the risk of developing atelectasis is still obscure. Methods: In this thesis, computed tomography (in studies I and II during anaesthesia, in studies III and IV postoperatively) was used to investigate the area of atelectasis in relation to FIO2 and varying levels of continuous positive airway pressure (CPAP) or positive end-expiratory pressure (PEEP). Study I investigated the short-term influence of reducing FIO2 during preoxygenation and induction of general anaesthesia, and the time to hypoxia during apnoea. Study II focused on the long-term effect of an FIO2 of 0.8 for preoxygenation. Study III applied CPAP/PEEP with an FIO2 of 1.0 or 0.8 for pre- and postoxygenation until extubation. After extubation, CPAP with an FIO2 of 0.3 was applied before the end of mask ventilation. Study IV compared two groups given CPAP/PEEP during anaesthesia and an FIO2 of 1.0 or 0.3 during postoxygenation, but without CPAP after extubation. Results: Study I showed a reduction in atelectasis with an FIO2 of 0.8 or 0.6, compared with 1.0, but the time to hypoxia decreased. In study II, atelectasis evolved gradually after preoxygenation. In study III, atelectasis was reduced with an FIO2 of 1.0 and CPAP/PEEP compared with an FIO2 of 1.0 without CPAP/PEEP. The intervention failed in the group given an FIO2 of 0.8, this group had more smokers. Atelectasis and age were correlated. In study IV, no difference was found between the groups. Post hoc analysis showed that smoking and ASA class increased the risk for atelectasis. Conclusion, the effect of reducing FIO2 during preoxygenation to prevent atelectasis might be short-lived. A lower FIO2 shortened the time to the appearance of hypoxia. Increasing lung volume by using CPAP/PEEP also decreased the risk of atelectasis, but the method might fail; for example in patients who are heavy smokers. In older patients care must be taken to reduce a high FIO2 before ending CPAP.

Polymer gel dosimetry, in conjunction with x-ray computed tomography (x-ray CT) imaging, is a three-dimensional dosimetric tool that shows promise in the verification of complex radiation therapy treatments. Previous studies have shown that x-ray CT imaging of gel dosimeters is robust, easy-to-use, and has wide clinical accessibility. The effects of x-ray CT dose imparted to the gel dosimeter, during imaging of the delivered therapy dose distributions, is not well understood. This thesis quantifies the effects of CT dose on normoxic polyacrylamide gel (nPAG) dosimeters. The investigation is comprised of four parts. First, quantification of the x-ray CT dose given during CT imaging of nPAG gels was measured using ion chamber measurements and filmed dose profiles for a range of typical gel dosimetry imaging protocols (200 mAs (current-time), 120-140 kVp (peak potential energy of photons), 2-10 mm slice thickness). It was found that CT doses ranged from 0.007 Gy/slice (120 kVp, 2 mm) to 0.021 Gy/slice (140 kVp, 10 mm) for volumetric phantoms. Second, Raman spectroscopy was used to determine the effect of photon energy on the dose response of nPAG dosimeters exposed to photon energies from a CT scanner (140 kVp photons) and from a Linac (6 MV photons). A weaker response was exhibited within the gels irradiated with kV photons than MV photons. Thirdly, the measurements of the given x-ray CT dose as established in the first study and the dose response of the polymer gel to different photon energies in the second study were correlated to estimate the induced changes of the nPAG CT number ("NCT ), caused by x-ray CT imaging of the polymer gel. (CT number is defined to be the measured attenuation coefficient normalized to water.) For typical gel imaging protocols (as above with 16-32 image averages), it was found that "NCT <0.2 H is induced in active nPAG gel dosimeters. This "NCT is below the current threshold of detectability of CT nPAG gel dosimetry. Finally, the traditional method of chemically fixing the dose response mechanism of nPAG gels by passive oxygenation of the gel, is investigated to determine if oxygenation would mitigate the changes caused by x-ray CT imaging of the gels. It was determined that oxygen diffusion was too slow to cause fixation of nPAG dosimeters, as the diffusion constant was 1.2 ± 0.2 × 10−6cm2/s, or 25% of the diffusion constant for anoxic PAG gel dosimeters. In conclusion, it was found that x-ray CT dose in polymer gel dosimeters is not a concern for standard gel imaging protocols. X-ray CT dose can potentially be a concern when large numbers of image averages (e.g. >60 image averages) are utilized, as in gel imaging protocols for high-resolution scans.

Current oil production technologies recover only about one‐third to one‐half of the oil originally present in an oil reservoir. Given current oil prices, even a modest increase in oil recovery efficiency is fiscally attractive. One novel approach to increase oil recovery efficiency is a process called microbial enhanced oil recovery (MEOR), where microorganisms are either used as a clogging agent to redirect flow or to produce biosurfactant that reduces interfacial tension. This dissertation aims to understand the MEOR pore‐scale mechanisms relevant to oil recovery by taking a two‐fold approach where transparent 2‐dimensional micromodel experiments imaged with stereo microscopy and 3‐dimensional column experiments imaged with x‐ray computed microtomography (CMT) are utilized. Micromodel experiments allow for direct visualization of the biological phase (i.e. biofilm), however, only 2‐dimensional information is provided. Conversely, CMT experiments provide 3‐dimensional pore‐scale information, but lack the ability to image the biological phase. With this two‐fold approach, it is possible to distinguish multiple fluid interfaces, quantify fluid phase saturations, measure oil blob size distributions, and visualize the biological phase. Furthermore, a method to measure interfacial curvature from 3‐dimensional images is developed, providing researchers a new perspective from which to study multiphase flow experiments. Overall, the presented research utilizes pore‐scale imaging techniques to study the interfacial interactions occurring during MEOR in an effort to better explain the physics, and thus, increase the efficacy of MEOR. / Graduation date: 2012

Previous research has shown that the sample preparation method used to reconstitute specimens for granular materials can have a significant impact on its mechanistic behavior. As the Discrete Element Method becomes a more popular choice for modeling multiphysics problems involving granular materials, the sample heterogeneity should be correctly characterized in order to obtain accurate results. In order to capture the effect of sample preparation on the homogeneity of the sample, standard procedures were used to reconstitute samples composed of a homogeneous granular material. X-ray computed tomography and image analysis techniques were then used to characterize the spatial heterogeneity of a typical sample. The sample preparation method was modeled numerically using the Discrete Element program PFC3D. The resulting microstructure of the numerical sample was compared to the results of the image analysis to determine if the heterogeneity of the sample could be reproduced correctly for use in Discrete Element Modeling.