496 research outputs found
Towards Realistic Facial Expression Recognition
Automatic facial expression recognition has attracted significant attention over the past decades. Although substantial progress has been achieved for certain scenarios (such as frontal faces in strictly controlled laboratory settings), accurate recognition of facial expression in realistic environments remains unsolved for the most part. The main objective of this thesis is to investigate facial expression recognition in unconstrained environments. As one major problem faced by the literature is the lack of realistic training and testing data, this thesis presents a web search based framework to collect realistic facial expression dataset from the Web. By adopting an active learning based method to remove noisy images from text based image search results, the proposed approach minimizes the human efforts during the dataset construction and maximizes the scalability for future research. Various novel facial expression features are then proposed to address the challenges imposed by the newly collected dataset. Finally, a spectral embedding based feature fusion framework is presented to combine the proposed facial expression features to form a more descriptive representation. This thesis also systematically investigates how the number of frames of a facial expression sequence can affect the performance of facial expression recognition algorithms, since facial expression sequences may be captured under different frame rates in realistic scenarios. A facial expression keyframe selection method is proposed based on keypoint based frame representation. Comprehensive experiments have been performed to demonstrate the effectiveness of the presented methods
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Study of brittle/ductile layering effect on fracture geometry and mechanical behavior by tri-axial testing
textHydraulic fracturing has been a widely used technology to produce hydrocarbon from shale plays. A better understanding of the fracturing process is needed to improve oil and gas production. Understanding fracture height growth is one of the main concerns and fracture systems are usually influenced by the presence of layers with contrasting mechanical properties. This study uses a tri-axial test to investigate the fracture geometry and mechanical behavior of brittle/ductile layered samples. Synthetic hydrostone is used as brittle rock, and uncemented sand is used to mimic ductile rock. A series of experiments evaluate the effect of loading speed, confining stress, and layer thickness on the mechanical behavior and fracture geometry of the layered samples. A discrete element method is also used to calculate the mechanical behavior of layered samples and investigate the layering effect. The tri-axial test results show that the ductile/brittle multilayer becomes more brittle by increasing the number of layers. According to the results, the loading rate has less effect on thicker layer samples, and the samples are more ductile under higher confining stress. A sensitivity analysis using the discrete element method includes interface properties, number of layers, layer thickness, boundary conditions and edge effects. The results show that the mechanical behavior of brittle/ductile layered samples is highly dependent on the interface properties as well as on the number of layers. The layered samples become stronger and more brittle by increasing interface roughness and friction as well as the number of layers. This work will help better understand brittle ductile behavior of rocks and provide guidelines for the investigation of the brittle ductile layering effect on fracture height containment.Petroleum and Geosystems Engineerin
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Height containment of hydraulic fractures in layered reservoirs
Oil and gas production from unconventional reservoirs generally requires hydraulic fracturing within layered reservoirs, which are usually stratified with layers of different mechanical properties. Vertical height growth of hydraulic fractures is one of the critical factors in the success of hydraulic fracturing treatments. Among all the factors, modulus contrast between adjacent layers is generally considered of secondary importance in terms of direct control of fracture height containment. However, arrested fluid-driven fractures at soft layers are often observed in outcrops and hydraulic fracture diagnostics field tests. Furthermore, conventional hydraulic fracturing models generally consider planar fracture propagation in the vertical direction. However, this ideal scenario is rather unsatisfactory and fracture offset at bedding planes was widely observed in experimental testing and outcrops. Once the offset is created, the reduced opening at the offset may result in proppant bridging or plugging and may also act as a barrier for fluid flow, and thus fracture height growth is inhibited compared to a planar fracture.
In order to illustrate the effect of modulus contrast on fracture height containment, this study proposed a new approach, which is based on the effective modulus of a layered reservoir. In this study, two-dimensional finite element models are utilized to evaluate the effective modulus of a layered reservoir, considering the effect of modulus values, fracture tip location, height percentage of each rock layer, layer thickness, layer location, the number of layers, and the mechanical anisotropy. Then, the effect of modulus contrast on fracture height growth is investigated with an analysis of the stress intensity factor, considering the change of effective modulus as the fracture tip propagates from the stiff layer to the soft layer. The results show the effective modulus is mainly dependent on the modulus values, fracture tip location, and height percentage of rock layers. This study empirically derived two types of effective modulus depending on fracture tip location, namely the modified height-weighted mean and the modified height-weighted harmonic average. By combining linear elastic fracture mechanics with the appropriate effective modulus approximations, the results indicate that hydraulic fracture propagation will be inhibited by the soft layer due to a reduced stress intensity factor.
A two-dimensional finite element model was utilized to quantify the physical mechanisms on fracture offset at bedding planes under the in-situ stress condition. The potential of fracture offset at a bedding plane is investigated by examining the distribution of the maximum tensile stress along the top surface of the interface. A new fracture is expected to initiate if the tensile stress exceeds the tensile strength of rocks. The numerical results show that the offset distance is on the order of centimeters. Fracture offset is encouraged by smaller tensile strength of rocks in the bounding layer, lower horizontal confining stress and higher rock stiffness in the bounding layer, weak interface strength, higher pore pressure, lower reservoir depth, and larger fracture toughness.Petroleum and Geosystems Engineerin
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