published_or_final_version / Mechanical Engineering / Master / Master of Philosophy

Dissertation (Ph. D.)--University of Akron, Dept. of Mechanical Engineering, 2007. / "December, 2007." Title from electronic dissertation title page (viewed 03/27/2008) Advisor, Xiaosheng Gao; Committee members, T.S. Srivatsan, Fred Choy, Wieslaw Binienda, Kevin Kreider; Department Chair, Celal Batur; Dean of the College, George K. Haritos; Dean of the Graduate School, George R. Newkome. Includes bibliographical references.

The dependence of fracture strength upon the time of loading is commonly termed static fatigue or delayed failure. This has been attributed to the growth of subcritical flaws under stress. Hence the study of subcritical crack growth is important in predicting the life expectancy of a material when it is subjected to a stress. Subcritical crack growth of glass and glass-ceramics, at room temperature and in two different environments (toluene and water) was studied. Glass containing 17.8 wt% Li₂O - 82.2 wt% SiO₂ and crystallized glasses (glass-ceramics) were chosen. The double torsion technique was used to determine crack velocity at various stress intensity factors. It was shown that the slopes of the velocity-stress intensity factor diagrams for glass and glass-ceramics (having different volume fractions of crystalline phase) tested in water, remained constant. However, these plots shifted to the higher stress-intensity region, as the degree of crystallinity in the glass increased. The crack velocity-stress intensity factor plots of glass and glass-ceramics tested in toluene have shown a similar behaviour but the slope of these plots increased as the degree of crystallinity in the glass increased. A modification of the stress- corrosion model of Hillig and Charles²³ is proposed. Crack velocity data of glass and glass-ceramics tested in water agreed well with the proposed model. Crack velocity data of glass and glass-ceramics tested in toluene are discussed.in terms of the "lattice trapping theory". An equation is presented to predict.the life expectancy under stress of these materials from crack growth data. The transverse rupture test was used to determine the fracture strength of glass and glass-ceramics. These results have shown that the fracture strength of glass-ceramics is increased mainly due to the increase in the fracture surface energy. The critical stress intensity factor of glass-ceramics increases as the degree of crystallinity increases. The fracture surface. energy of these materials was calculated from the knowledge of the critical, stress intensity factor and it was shown that the fracture surface energy of glass-ceramics containing up to 0.5 volume fraction of. crystalline phase is related to the inter-particle spacing. This observation is further substantiated by fracto-graphic examination. The kinetics of crystallization of lithium disilicate from 17.8wt% Li₂0 - 82.2 wt % Si0₂ glass was studied by crystallizing this glass at 530°C for various lengths of time. It was shown that the crystallization of lithium disilicate is a diffusion controlled reaction. It was found that the diffusivity for this process is much lower than the diffusion coefficient of lithium ion. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

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published_or_final_version / Civil and Structural Engineering / Doctoral / Doctor of Philosophy

published_or_final_version / Mechanical Engineering / Doctoral / Doctor of Philosophy

published_or_final_version / Medical Sciences / Master / Master of Medical Sciences

published_or_final_version / Mechanical Engineering / Doctoral / Doctor of Philosophy

Determining fracture characteristics at the laboratory scale is a major challenge. It is known that fracture characteristics are scale dependent; as such, the minimum sample size should be deduced in order to scale to reservoir dimensions. The main factor affecting mechanical and hydrological characteristics of natural fractures is aperture distribution, which is a function of scale and confining pressure, rather than roughness of one fracture surface. Scale and pressure dependencies of artificial and natural fractures were investigated in this study using an X-Ray CT Scanner. Fractal dimension, D, and amplitude parameter, A, of fracture aperture approaches a constant value with increased sampling area, similar to the behavior of fracture roughness. In addition, both parameters differ under different confining pressures for a reference sampling area. Mechanical properties of fracture-fracture deformation behavior and fracture normal stiffness were obtained from CT scan data as well. Matrix porosity is relatively easy to measure and estimate compared to fracture porosity. On the other hand, fracture porosity is highly heterogeneous and very difficult to measure and estimate. When matrix porosity of naturally fractured reservoirs (NFR) is negligible, it is very important to know fracture porosity to evaluate reservoir performance. Since fracture porosity is highly uncertain, fractal discrete fractal network (FDFN) generation codes were developed to estimate fracture porosity. To reflect scale dependent characteristics of fracture networks, fractal theories are adopted. FDFN modeling technique enables the systematic use of data obtained from image log and core analysis for estimating fracture porosity. As a result, each fracture has its own fracture aperture distribution, so that generated FDFN are similar to actual fracture systems. The results of this research will contribute to properly evaluating the fracture porosity of NFR where matrix porosity is negligible.

Low-permeability gas wells often produce less than predicted after a fracture treatment. One of the reasons for this is that fracture lengths calculated after stimulation are often less than designed lengths. While actual fracture lengths may be shorter due to fracture growth out of zone, improper proppant settling, or proppant flowback, short calculated fracture lengths can also result from incorrect analysis techniques. It is known that fracturing fluid that remains in the fracture and formation after a hydraulic fracture treatment can decrease the productivity of a gas well by reducing the relative permeability to gas in the region invaded by this fluid. However, the relationships between fracture fluid cleanup, effective fracture length, and well productivity are not fully understood. In this work I used reservoir simulation to determine the relationship between fracture conductivity, fracture fluid production, effective fracture length, and well productivity. I simulated water saturation and pressure profiles around a propped fracture, tracked gas production along the length of the propped fracture, and quantified the effective fracture length (i.e., the fracture length under single-phase flow conditions that gives similar performance as for multiphase flow conditions), the "cleanup" fracture length (i.e., the fracture length corresponding to 90% cumulative gas flow rate into the fracture), and the "apparent" fracture length (i.e., the fracture length where the ratio of multiphase to single-phase gas entry rate profiles is unity). This study shows that the proppant pack is generally cleaned up and the cleanup lengths are close to designed lengths in relatively short times. Although gas is entering along entire fracture, fracturing fluid remains in the formation near the fracture. The water saturation distribution affects the gas entry rate profile, which determines the effective fracture length. Subtle changes in the gas rate entry profile can result in significant changes in effective fracture length. The results I derived from this work are consistent with prior work, namely that greater fracture conductivity results in more effective well cleanup and longer effective fracture lengths versus time. This study provides better explanation of mechanisms that affect fracturing fluid cleanup, effective fracture length, and well productivity than previous work.