Browsing by Author "Zhang, Xin 1987-"
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Item Field Scale Permeability Estimation Based on Microseismic Monitoring(2015-08) Zhang, Xin 1987-; Chesnokov, Evgeni M.; Castagna, John P.; Goloshubin, Gennady M.; Li, Xin-GongThe 3-dimension diffusion equation which describes fluid flow during hydraulic fracturing is interpreted in a statistical way. For all diffusing particles in a pumping procedure, the root-mean-square average of diffusing distance, which evaluates the fluctuation of diffusing particles as time evolves, is proportional to the square root of the product of diffusivity and elapsed time. The diffusivity is obtained from the spatial-temporal distribution of located microseismic events as a function of the distance between these events and pumping points, as well as elapsed time from injection inception. The upper-limit diffusivity of the original formation is characterized by the curve which fits the outermost located events on a distance-time plot. Similarly, diffusivity of the formation after hydraulic fracturing is obtained by curve fitting innermost located events induced by fluid flow back after injection stops. The theoretical expression between the diffusivity tensor and permeability tensor is obtained based on an isothermal condition and assumed incompressible slurry. The diffusivity tensor is found to be equal to the permeability tensor divided by a scalar which is the product of dynamic viscosity, connected formation porosity, and formation compressibility. Application of these equations to microseismic data acquired in the Barnett Shale Formation yields, with assumed hydraulic fracture geometry, initial permeability of 0.16 to 3.21 milliDarcy in the assumed dominant direction of fracturing and normal to it, with an increase to 12.1 milliDarcy along the dominant direction of fracturing after hydraulic fracturing. Numerical simulation results of fluid flow in synthetic media demonstrate: (1) If the flow domain size is not much larger than the part influenced by the entrance effect, the variables in Darcy’s law are inter-dependent. If so, the obtained permeability, no matter by experiment or simulation, cannot be upscaled, even under homogeneous condition. (2) The volume or area influenced by the entrance effect inside the flow domain depends on the geometry of the flow domain, fluid properties, and in-situ parameters. The more viscous fluid flow in a lower rate, the smaller volume or area influenced by entrance effect. (3) The flow field is difficult to be fully developed in a periodic domain. The wide throat zones store fluid as reservoirs.Item Organic photovoltaics: An investigation of interface layers and space-charge effects(2013-12) Zhang, Xin 1987-; Curran, Seamus A.; Wood, Lowell T.; Stokes, Donna W.; Bensaoula, Abdelhak; Clarke, Mark S. F.The research described in this thesis includes interface study of organic photovoltaics, and the study of the space-charge effect on organic electronics from a photonics perspective. First, dynamic spin coating was used as a deposition technique in the fabrication of p-n bilayer organic photovoltaics, solvents used in the fabrication were carefully selected to generate different p-n interface morphologies from low level inter-digitated to flat, and it was found that the interface morphology can significantly affect the device performance. Stability optimization was carried out by using electro-polymerized PEDOT:PSS (EPEDOT:PSS) as a replacement for the spin coated PEDOT:PSS (SPEDOT:PSS) as hole transport layer (HTL). The X-ray photoelectron spectroscopy study shows a decrease of PSS to PEDOT ratio as well as a lower Indium atomic concentration in the electro-polymerized film. The device fabricated with EPEDOT:PSS showed an appreciable improvement in the stability property. The highly conductive PEDOT:PSS was designed to replace the Indium Tin Oxide (ITO)/PEDOT:PSS junction as the new anode to make the device fully flexible. The ITO-free device showed higher power conversion efficiency than the ITO-based device. However, the light-intensity study suggests that due to the less anode conductivity in the ITO-free device, more space-charge effects can be observed and in turn causes a higher bimolecular recombination rate in the flexible device. Using the flexible substrates with alternative anodes, examining their current voltage performance under different light intensity, the correlation of open circuit voltage (VOC) with anode transport property was studied. A simple model on VOC as a function of space charge density and photo-generated charge density was applied and successfully explained how space-charge affects the changing trend of VOC.