Recent Submissions

  • Characterizing an unconventional reservoir with conventional seismic data: a case study using seismic inversion for the Vaca Muerta Formation, Neuquen Basin, Argentina

    Davis, Thomas L. (Thomas Leonard), 1947-; Fernandez-Concheso, Jorge E.; Sonnenberg, Stephen A.; Larner, Kenneth L.; Benson, Robert D.; Curia, David (Colorado School of Mines. Arthur Lakes Library, 2015)
    Reservoir characterization for unconventional shale plays ideally requires multi-component, wide-azimuth, long-offset surface seismic data. These data are generally not available, espe- cially in exploration or pre-development stages. Furthermore, it is common to have only a few wells over a large area, along with non-existent or scarce microseismic, engineering and production data. This thesis presents a methodology and workflow to deal with these cir- cumstances of limited data availability. By using a narrow-azimuth, regional P-wave seismic volume and integrating it with wireline logs, cuttings and PLT data, the variability in the geomechanical properties of the Vaca Muerta Formation in Argentina’s Neuquen Basin, and their relationships with lithology, stress state and total organic content, were analyzed. Post-stack and pre-stack inversions were performed on the seismic volume. The un- certainties inherent from limited well control in the estimation of elastic properties were investigated using blind well testing. Sensitivity and error analysis was conducted on post- stack vs pre-stack derived P-impedance, the choice of the inversion algorithm (model-based vs sparse-spike) and the definition of the low frequency model (simple kriging model vs com- plex model derived from multi-attribute stepwise regression) were examined. Also, the use of isotropic AVA equations to approximate the anisotropic (VTI) behaviour of the reservoir was evaluated, using estimates of Thomsen parameters and simple AVA modelling. The integration of the inversion results with the petrophysical analysis and the mechanical stratigraphy work by Bishop (2015), suggests that the rock composition has the largest influence on the geomechanical behaviour of the reservoir. Overpressure is also a major driving factor in that it controls changes in elastic properties. Bishop’s cluster analysis was used to identify good quality rock classes. The probabilistic interpretation of these rock classes from seismic inversion provides a quantitative measure of uncertainty and guides the selection of potential drilling targets.
  • Long-term fouling and performance of forward osmosis membranes treating activated sludge and oil and gas produced water

    Cath, Tzahi Y.; Bell, Elizabeth A.; Spear, John R.; Bellona, Christopher (Colorado School of Mines. Arthur Lakes Library, 2015)
    The thesis is comprised of two studies that compared the performance and fouling of cellulose triacetate (CTA) and polyamide thin film composite (TFC) forward osmosis (FO) membranes. The first study used CTA and TFC membranes to treat high-salinity and low-salinity activated sludge for 100 days, and the second study used CTA and improved TFC membranes to treat oil and gas produced water for three weeks. Both studies evaluated virgin and fouled membrane performance by measuring water flux, reverse salt flux (RSF), and specific reverse salt flux (SRSF). Membrane autopsies were performed to understand the importance of physiochemical membrane properties on fouling potential and to determine the organic and inorganic composition of fouling layers. The second study expanded on the first study and used chemically enhanced osmotic backwashing for membrane cleaning, streaming potential analysis of membrane surfaces, and gas chromatography-mass spectroscopy for evaluating the rejection of volatile and semi-volatiles hydrocarbons by the FO membranes. The studies found that CTA membranes outperformed TFC membranes with higher water flux, lower RSF, and lower SRSF. TFC membranes had more ideal virgin membrane properties (e.g., smooth, hydrophilic, and neutrally charged surfaces), but were more prone to fouling and demonstrated that conventional characterization techniques do not accurately predict fouling potential. In both studies, the organic composition of the fouling was similar for both membrane types, but the inorganic fouling varied according to membrane type, wastewater composition, and cleaning. The second study found over 90% organic compound rejection for both membranes, in which TFC membranes had superior rejection of neutral hydrophobic compounds.
  • Experimental study and numerical modeling of cryogenic fracturing process on laboratory-scale rock and concrete samples

    Wu, Yu-Shu; Yin, Xiaolong; Yao, Bowen; Abass, Hazim H.; Tutuncu, Azra; Cha, Minsu (Colorado School of Mines. Arthur Lakes Library, 2015)
    Cryogenic fracturing is a new fracturing concept that uses cryogenic fluids as fracturing fluids. Its mechanism rests on the effect of a thermal shock introduced by cryogen on the hot surface of reservoir rock. The thermo-mechanical properties of rock and failure criteria play very important roles during the cryogenic fracturing process. The objective of this research is to conduct cryogenic fracturing experiments on concrete and rock samples and build a simulation tool. For the experimental aspect, a tri-axial stress confining system with capability of injecting liquid nitrogen or other cryogenic fluid is built. It is capable of conducting cryogenic fracturing treatment on concrete and rock samples under tri-axial stresses. For modeling, the experimental process is simulated and matched with the actual experiment results. The simulation tool can also predict the distribution of artificial fractured samples. The influences of different confining stress, injection pressure and failure criteria are identified by comparing results from modeling and experiments. The experiment setup and modeling tool developed can also provide valuable guidance to field applications of cryogenic fracturing technology to select most efficient operation factors.
  • Wavefield imaging using the energy norm

    Sava, Paul C.; Rocha, Daniel Carvalho; TSvankin, I. D.; Nissen, Edwin; Martin, P. A.; Mooney, Michael A. (Colorado School of Mines. Arthur Lakes Library, 2015)
    For various wavefield types, one can formulate a measurement of the mechanical energy that satisfies conservation laws. Based on this formulation, one can derive an{\it energy norm} that is applicable to wavefield imaging. Extending the concept of the norm to an inner product, one can compare two related wavefields. Therefore, an imaging condition can be defined as the inner product between the source and receiver wavefields at every spatial location. In this regard, the imaging condition based on the energy inner product accounts for wavefield directionality in space and time, overcoming some problems present in the conventional imaging condition. I exploit the wavefield directionality information from the energy imaging condition to attenuate unwanted events in reverse time migrated (RTM) images. For acoustic wavefields, these unwanted events are characterized by the collinearity of the source and receiver raypaths, and they are described as RTM backscattering artifacts. For elastic wavefields, these events are characterized by the fact that source and receiver displacement fields have the same polarization and wave propagation directions. In both acoustic and elastic cases, one can to attenuate these artifacts and produce high quality images. Another application that uses the wavefield directionality is to enhance the full waveform inversion (FWI) gradient for acoustic wavefields. By enhancing wave events that are collinear and suppressing all other wave events, I am able to compute gradients that are more suitable for the inversion process. Numerical experiments show the efficacy of these applications for synthetic models that emulate the complexity of subsurface structures found in exploration seismology, such as salt bodies, diffractors, dipping layers and faults.
  • Development of metal matrix composite powder cored tubular wire for electron beam freeform fabrication

    Liu, Stephen; Gonzales, Devon Scott; Bourne, Gerald; Yu, Zhenzhen; Domack, Marcia S.; Hafley, Robert A. (Colorado School of Mines. Arthur Lakes Library, 2015)
    Electron Beam Freeform Fabrication (EBF3) is a technique, developed at NASA Langley Research Center (NASA-LaRC), which uses a computerized numerical controlled electron beam welder and a wire feed system to fabricate large scale aerospace parts. Advantages of using EBF3 as opposed to conventional manufacturing methods include decreased deign-to-product time, decreased wasted material, and the ability to adapt controls to produce geometrically complex parts. The EBF3 process is compatible with a range of aerospace alloys and material properties of parts produced by this process have been shown to be comparable to wrought products. However, to fully exploit the potential of the EBF3 process, development of materials tailored for the process is required. This research used alloy theory, as well as powder cored tubular wire technology, to develop aluminum based metal matrix composite feedstock for the EBF3 system. Five iterations of powder cored tubular wire were made using silicon carbide particulates with different surface conditions. Uncoated particles, as well as particles with high and low amounts of copper and nickel coatings, were incorporated into the powder core. Single and multiple layer deposits were made using each wire. Beam conditions were varied for each wire to determine the optimal combination of feedstock material and electron beam parameters required to create a uniformly distributed metal matrix composite deposit using EBF3. Completely uniform dispersion of the reinforcement particles was not achieved in the matrix, however, it was determined that nickel additions enhanced particle dispersion as well as mitigated solidification cracking and secondary carbide formation. It was also determined that using a lower energy beam also promotes dispersion and mitigates secondary carbide formation. Computational models were created to predict phase transformations and particle dispersion in the matrix for various conditions. The thermodynamic and fluid dynamic models were able to describe trends observed through characterization of EBF3 depositions. Results collected from characterization of the deposits as well as trends observed in the models can be used to plan future iterations of powder cored tubular wire feedstock, as well as deposition parameters, to create homogeneous metal matrix composite structures by Electron Beam Freeform Fabrication.
  • Tectono-hydrothermal evolution of the Neoarchean Abitibi greenstone belt, Canada

    Monecke, Thomas; Hufford, Gregory; Hitzman, Murray Walter; Kuiper, Yvette; Plink-Björklund, Piret (Colorado School of Mines. Arthur Lakes Library, 2015)
    The Timiskaming assemblage represents the youngest supracrustal assemblage of the Neoarchean Abitibi greenstone belt of Ontario and Quebec, Canada. The deposits of the Timiskaming assemblage include deep-water turbidites, alluvial-fluvial conglomerates and sandstones, as well as alkalic volcanic and intrusive rocks. The sedimentary and volcanic rocks of the Timiskaming assemblage record a special time in the greenstone belt evolution as they represent the first deposits in the Abitibi greenstone belt that were not exclusively deposited in a deep submarine setting. The Timiskaming assemblage is thought to have formed in response to a period of Neoarchean mountain building and crustal thickening. To better constrain the depositional setting of the Timiskaming assemblage, high-resolution lithofacies mapping was conducted on key outcrops. The mapping highlights the presence of graywacke turbidite deposits, alluvial-fluvial conglomerate-sandstone facies, near-vent pyroclastic surge and fallout deposits, and syenite intrusions within the Timiskaming assemblage. The clastic facies contain evidence for high rates of deposition and paleoseismic activity. The detrital material appears to be of varied provenance and may not only be derived from local sources. The abundance of near-vent pyroclastic deposits in Kirkland Lake is consistent with a subaerial depositional setting of parts of the Timiskaming assemblage and indicates that Timiskaming sedimentation was accompanied by at least local extension. The documented characteristics are consistent with deposition in a variety of tectonic settings, although they are most consistent with sedimentation in continental rift or strike slip basins. The alluvial-fluvial conglomerates and sandstones of the Timiskaming assemblage contain abundant quartz clasts and igneous clasts containing quartz veins. To better constrain the origin of these clasts, representative quartz samples were collected from exposures of the Timiskaming assemblage in the Timmins-Porcupine, Kirkland Lake, Duparquet, and Rouyn-Noranda mining camps. Microscopic investigations revealed that most quartz clasts have been affected by extensive recrystallization, hampering identification of the quartz origin. However, primary textural relationships, fluid inclusion inventories, and CL signatures are preserved in several of the sampled quartz clasts. Although some of the clasts probably originate from pegmatites, most of the clasts appear to have been sourced from hydrothermal veins, including veins comparable to those found in modern epithermal deposits or shallow orogenic deposits. The finding provides new important insights into the metallogenic evolution of the Abitibi greenstone belt. The presence of quartz vein material in the conglomerates and sandstones of the Timiskaming assemblage suggests that hydrothermal systems present in modern convergent plate settings already existed during the early phase of Neoarchean mountain building and crustal thickening.
  • Use of diffusion multiples to explore the Co-Cr-Fe-Mn-Ni high entropy system, The

    Kaufman, Michael J.; Wilson, Paul Nathaniel; Field, Robert; Bourne, Gerald (Colorado School of Mines. Arthur Lakes Library, 2015)
    High entropy alloys (HEAs) or Multi-principal element alloys (MEAs) are a relatively new class of alloys. These alloys are defined as having at least five major alloying elements in atomic percent from 5% to 35%. There are hundreds of thousands of equiatomic compositions possible and only a fraction have been explored. This project examines diffusion multiples as a method to accelerate alloy development in these systems. The system chosen for this experiment is the Co-Cr-Fe-Mn-Ni system. The methodology developed for creating these diffusion multiples involved a two-step process. In the first step two binary alloys (50at-% Fe-Mn and 50 at%- Ni-Co ) were diffusion bonded together. In the second step, under uniaxial compression, was used to bond Cr to diffusion couple prepared in Step I. Successful diffusion multiples were created by this method. An auxiliary method named differential melting liquid impingement (DMLI) was developed that created diffusion multiples using liquid processing methods that will be described. After creation of these multiples, the ternary and quinary interface regions were examined using scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), and nanoindentation. The Cr/NiCo region experienced interdiffusion but no intermediate phase formation retaining the FCC / BCC interface at the hot-pressing temperature (1200 °C). However, upon cooling from 1200 °C, the BCC region adjacent to the interface decomposed into BCC + σ. In contrast, the Cr/FeMn interface region developed a layered structure of FCC/σ/BCC suggesting that σ is stable at 1200 °C in contradiction to the published 1200 °C ternary phase diagram. Upon cooling, the σ present at 1200 °C decomposed into FCC + σ, except in samples that were contaminated with C; in those cases, FCC + M23C6 was observed as the decomposition product. The quinary regions were evaluated using the various HEA parameters, namely, ΔSmix, ΔHmix, Ω, Δχ, and δ. No strong correlations with phase stability were found using these parameters in contrast to expectations based on the literature. It was found that Cr solubility in the quinary disordered FCC varied linearly between the two ternary system endpoints (Co-Cr-Ni and Cr-Fe-Mn) Additionally, while nano-hardness maps did not support the severe lattice distortion hypothesis proposed for HEAs, a comparison of different solid solution strengthening mechanisms suggests that elastic modulus mismatch and a change in the lattice friction stress were the most likely contributors to strengthening.
  • Mechanical stratigraphy of the Vaca Muerta Formation, Neuquén Basin, Argentina

    Sonnenberg, Stephen A.; Davis, Thomas L. (Thomas Leonard), 1947-; Bishop, Kyla; Karg, Harald (Colorado School of Mines. Arthur Lakes Library, 2015)
    The Vaca Muerta offers an excellent opportunity to study field development at an early stage. The particularities of how best to characterize this play are developing, and various techniques and processes must be tested. Even in parts of the basin where wells are being drilled in rapid succession, production history is relatively short, and the dynamics of how the production may change with time are still to be anticipated (Rimedio Et al., 2015). With limited core data for calibration, a comparison of several predictive methods was made to estimate rock properties for the Vaca Muerta Formation in a study area consisting of 5 wells. Borehole image logs were interpreted to study pre-existing natural fractures, and a stability assessment was conducted to estimate the reactivation potential of the identified natural fractures. The comparison results in the characterization of an interval with clear vertical trends between an upper, middle, and lower section. In the lower section, organic and siliciclastic contents are highest. In the upper, organic content is lowest, and carbonate content is highest. The middle zone between them is a transitional interval in which the organic matter continuously decreases upwards in depth, in relative proportion with the increase in carbonate content. Borehole image log data were interpreted for the presence of natural fractures, horizontal discontinuity surfaces, concretions, and other heterogeneities. Documented is the presence of a naturally fractured zone in the middle Vaca Muerta and abundant calcareous concretions in the Lower Vaca Muerta. The identified natural fractures are likely candidates for reactivation during hydraulic stimulation based on a stability assessment. The middle of the interval is suitably brittle, organic rich, and has now been documented to contain multiple natural fractures. Although this conclusion was reached independently, it is corroborated by regional literature (Fantin Et al., 2014; Williams Et al., 2014). In addition, geochemical analysis on the quality of the liquid hydrocarbons in the Vaca Muerta indicates that the middle interval has high volumes, with a moderate gas-to-oil ratio, and moderate viscosities (Williams Et al., 2014).
  • Characterization of novel microstructures in Al-Fe-V-Si and Al-Fe-V-Si-Y alloys processed at intermediate cooling rates

    Kaufman, Michael J.; Marshall, Ryan; Midson, Stephen; Field, Robert (Colorado School of Mines. Arthur Lakes Library, 2015)
    Samples of an Al-Fe-V-Si alloy with and without small Y additions were prepared by copper wedge-mold casting. Analysis of the microstructures developed at intermediate cooling rates revealed the formation of an atypical morphology of the cubic α-Al12(Fe/V)3Si phase (Im3¯ space group with a = 1.26 nm) in the form of a microeutectic with α-Al that forms in relatively thick sections. This structure was determined to exhibit promising hardness and thermal stability when compared to the commercial rapidly solidified and processed Al-Fe-V-Si (RS8009) alloy. In addition, convergent beam electron diffraction (CBED) and selected area electron diffraction (SAD) were used to characterize a competing intermetallic phase, namely, a hexagonal phase identified as h-AlFeSi (P6/mmm space group with a = 2.45 nm c = 1.25 nm) with evidence of a structural relationship to the icosahedral quasicrystalline (QC) phase (it is a QC approximant) and a further relationship to the more desirable α-Al12(Fe/V)3Si phase, which is also a QC approximant. The analysis confirmed the findings of earlier studies in this system, which suggested the same structural relationships using different methods. As will be shown, both phases form across a range of cooling rates and appear to have good thermal stabilities. Additions of Y to the alloy were also studied and found to cause the formation of primary YV2Al20 particles on the order of 1 μm in diameter distributed throughout the microstructure, which otherwise appeared essentially identical to that of the Y-free 8009 alloy. The implications of these results on the possible development of these structures will be discussed in some detail.
  • Developing a social network to increase involvement in a computer science academic community

    Camp, Tracy; Rader, Cyndi A. (Cyndi Ann); Robison, Kolten; Claussen, Stephanie; Holles, Cortney (Colorado School of Mines. Arthur Lakes Library, 2015)
    In order to address a lack of involvement and participation by computer science (CS) students in their academic communities, CS CONNECT, a social networking site for the computer science students at the Colorado School of Mines, aims to increase interaction between students and strengthen their sense of belonging to a community. Students that develop a stronger sense of community within the CS academic environment tend to have higher completion rates in CS programs. This thesis focuses on further development of CS CONNECT, in order to give it more aesthetic appeal, as well as to introduce new features that may have a positive impact on site usage. It remains unclear whether CS CONNECT directly influenced students' feelings of the CS community; however, motivating methods and features were shown to have an impact on site participation and content contribution.
  • Elevated temperature mechanical properties of line pipe steels

    Matlock, David K.; Jacobs, Taylor Roth; Findley, Kip Owen; De Moor, Emmanuel (Colorado School of Mines. Arthur Lakes Library, 2015)
    The effects of test temperature on the tensile properties of four line pipe steels were evaluated. The four materials include a ferrite-pearlite line pipe steel with a yield strength specification of 359 MPa (52 ksi) and three 485 MPa (70 ksi) yield strength acicular ferrite line pipe steels. Deformation behavior, ductility, strength, strain hardening rate, strain rate sensitivity, and fracture behavior were characterized at room temperature and in the temperature range of 200-350 °C, the potential operating range for steels used in oil production by the steam assisted gravity drainage process. Elevated temperature tensile testing was conducted on commercially produced as-received plates at engineering strain rates of 1.67 x 10 4, 8.33 x 10 4, and 1.67 x 10 3 s 1. The acicular ferrite (X70) line pipe steels were also tested at elevated temperatures after aging at 200, 275, and 350 °C for 100 h under a tensile load of 419 MPa. The presence of serrated yielding depended on temperature and strain rate, and the upper bound of the temperature range where serrated yielding was observed was independent of microstructure between the ferrite-pearlite (X52) steel and the X70 steels. Serrated yielding was observed at intermediate temperatures and continuous plastic deformation was observed at room temperature and high temperatures. All steels exhibited a minimum in ductility as a function of temperature at testing conditions where serrated yielding was observed. At the higher temperatures (>275 °C) the X52 steel exhibited an increase in ductility with an increase in temperature and the X70 steels exhibited a maximum in ductility as a function of temperature. All steels exhibited a maximum in flow strength and average strain hardening rate as a function of temperature. The X52 steel exhibited maxima in flow strength and average strain hardening rate at lower temperatures than observed for the X70 steels. For all steels, the temperature where the maximum in both flow strength and strain hardening occurred increased with increasing strain rate. Strain rate sensitivities were measured using flow stress data from multiple tensile tests and strain rate jump tests on single tensile samples. In flow stress strain rate sensitivity measurements, a transition from negative to positive strain rate sensitivity was observed in the X52 steel at approximately 275-300 °C, and negative strain rate sensitivity was observed at all elevated temperature testing conditions in the X70 steels. In jump test strain rate sensitivity measurements, all four steels exhibited a transition from negative to positive strain rate sensitivity at approximately 250-275 °C. Anisotropic deformation in the X70 steels was observed by measuring the geometry of the fracture surfaces of the tensile samples. The degree of anisotropy changed as a function of temperature and minima in the degree of anisotropy was observed at approximately 300 °C for all three X70 steels. DSA was verified as an active strengthening mechanism at elevated temperatures for all line pipe steels tested resulting in serrated yielding, a minimum in ductility as a function of temperature, a maximum in flow strength as a function of temperature, a maximum in average strain hardening rate as a function of temperature, and negative strain rate sensitivities. Mechanical properties of the X70 steels exhibited different functionality with respect to temperature compared to the X52 steels at temperatures greater than 250 ºC. Changes in the acicular ferrite microstructure during deformation such as precipitate coarsening, dynamic precipitation, tempering of martensite in martensite-austenite islands, or transformation of retained austenite could account for differences in tensile property functionality between the X52 and X70 steels. Long term aging under load (LTA) testing of the X70 steels resulted in increased yield strength compared to standard elevated temperature tensile tests at all temperatures as a result of static strain aging. LTA specimen ultimate tensile strengths (UTS) increased slightly at 200 °C, were comparable at 275 °C, and decreased significantly at 350 °C when compared to as-received (standard) tests at 350 °C. Observed reductions in UTS were a result of decreased strain hardening in the LTA specimens compared to standard tensile specimens. Ideal elevated temperature operating conditions (based on tensile properties) for the X70 line pipe steels in the temperature range relevant to the steam assisted gravity drainage process are around 275-325 °C at the strain rates tested. In the temperature range of 275-325 °C the X70 steels exhibited continuous plastic deformation, a maximum in ductility, a maximum in flow stress, improved strain hardening compared to intermediate temperatures, reduced anisotropic deformation, and after extended use at elevated temperatures, yield strength increases with little change in UTS.
  • Design and behavior of a mid-rise cross-laminated timber building

    Pei, Shiling; Lenon, Conor; Kingsley, Greg; Tabares-Velasco, Paulo Cesar; Kiousis, Panagiotis Demetrios, 1956- (Colorado School of Mines. Arthur Lakes Library, 2015)
    Cross-Laminated Timber (CLT) is a new engineered wood material that was introduced in the past decade as a promising candidate to build structures over 10 stories. So far, a handful of tall CLT buildings have been built in low seismic regions around the world. Full-scaled seismic shaking table tests revealed the vulnerability of this building type when resisting seismically-induced overturning. This study proposes a new analysis and design approach for developing overturning resistance for platform CLT buildings. New structural detailing is proposed to alter the moment-resisting mechanism and enable coupled action through the floor system. The method is applied to the design of a 12-story CLT building, which was evaluated numerically to assess the conservativeness of the design through system level finite element model simulations.
  • Study of thermoelectric properties of graphene materials, A

    Wu, Zhigang; Twombly, Chris; Lusk, Mark T.; Wood, David M. (Colorado School of Mines. Arthur Lakes Library, 2015)
    Graphene has very beneficial charge transport properties which make it an interesting potential thermoelectric material, but its thermoelectric efficiency is limited by large thermal conductivity. Nanostructuring graphene by incorporating periodic holes in the crystal structure produces graphene nanomesh with reduced thermal conductivity due to increased phonon scattering. The goal of this study was to investigate the thermoelectric properties of graphene nanomeshes and defected graphene using Density Functional Theory and semi-classical Boltzmann Transport Theory. We computed the Seebeck coefficient, electrical conductivity, and the electrical component of thermal conductivity from first principles. We first developed and verified the accuracy of our techniques using silicon. We then examined the properties of silicon nanowires in order to study systems with more complex geometry and to show that nanostructuring can improve thermoelectric properties. Our results agreed closely with previous experimental and theoretical studies of silicon systems. We then employed this suite of methods to study graphene, graphene nanomeshes, and periodically defected graphene. Our calculations for pristine graphene agreed closely with experimental measurements, proving that our methods work well with 2D systems. Our calculations suggest that there is up to a one order of magnitude increase in Seebeck coefficient for graphene nanomeshes compared to pristine graphene. This increase was found to be strongly dependent on a previously predicted geometrically based semimetal to semiconductor transition. We estimated a maximum ZT of 0.15-0.4 for graphene nanomeshes based on a simple scaling law for the thermal conductivity in these systems. The ZT value is strongly dependent on the purity and the quality of the graphene crystal lattice, which affects the relaxation time of charge carriers in these systems. We then studied defected graphene with partial hydrogen passivation and boron-nitride (BN) doping to further demonstrate the importance of the semimetal to semiconductor transition. We concluded that the geometrically based semimetal to semiconductor transition in graphene systems is responsible for improved thermoelectric properties, and helps explain strong disorder based reduction in efficiency reported in previous computational studies. Our study suggests that with further optimization nanostructured graphene could be a potential thermoelectric material.
  • Assessing computational thinking in Computer Science Unplugged activities

    Camp, Tracy; Rader, Cyndi A. (Cyndi Ann); Rodriguez, Brandon R.; Bridgman, Terry; Painter-Wakefield, Christopher (Colorado School of Mines. Arthur Lakes Library, 2015)
    There is very little research on assessing computational thinking without using a programming language, despite the wide adoption of activities that teach these concepts without a computer, such as CS Unplugged. Measuring student achievement using CS Unplugged is further complicated by the fact that most activities are kinesthetic and team-oriented, which contrasts traditional assessment strategies designed for lectures and individual tasks. To address these issues, we have created an assessment strategy that uses a combination of in-class assignments and a final project. The assessments are designed to test different computational thinking principles, and use a variety of problem structures. The assessments were evaluated using a well-defined rubric along with notes from classroom observations to discover the extent CS Unplugged activities promote computational thinking. The results from our experiment include several statistically significant shifts supporting the hypothesis that students are learning computational thinking skills from CS Unplugged. Student performance across all of the worksheets gave insight into where problems can be improved or refined such that a greater number of students can reach proficiency in the subject areas.
  • Improving time synchronization protocols in wireless sensor networks

    Camp, Tracy; Gonzalez, Santiago; Jaffrès-Runser, Katia; Wakin, Michael B. (Colorado School of Mines. Arthur Lakes Library, 2015)
    As the world moves towards greater ubiquity of wireless sensor networks, the need for lightweight, resilient time synchronization protocols is apparent. Wireless nodes' internal clocks are subject to drift over time due to manufacturing imperfections and environmental changes. Such drift can be detrimental for many systems, especially for those where accurate data timestamping is required. Time synchronization protocols introduce a means by which two or more nodes can wirelessly synchronize their internal clocks. We have implemented and compared two existing time synchronization protocols, Reference Broadcast Synchronization (RBS) and Simple Synchronization Protocol (SISP), on Atmel ATMega328p based microcontroller platforms with IEEE 802.15.4 Xbee radio modules. We have found that SISP is able to achieve much higher synchronization performance than RBS. Our goal, however, is to improve upon current time synchronization protocols even further. Thus, we have developed a new protocol, the Sticking Heartbeat Aperture Resynchronization Protocol (SHARP), that aims to reduce synchronization error and resolve shortcomings of existing protocols. We show that SHARP exhibits a higher level of synchronization than SISP, while requiring significantly fewer messages. Additionally, to assist in developing accurate time synchronization protocols, we have performed message transmission / reception latency measurement experiments on the Xbee radios using a logic analyzer. Latency was found to be consistent across test runs, exhibiting a low standard deviation under 100µs. Such latency was also found to strongly follow a Gaussian distribution. These results will be useful in a future implementation of SHARP.
  • Performance study of an implementation of the push-relabel maximum flow algorithm in Apache Spark's GraphX, A

    Mehta, Dinesh P.; Langewisch, Ryan P.; Wu, Bo; Ravindran, Arun; Joshi, Bharat (Colorado School of Mines. Arthur Lakes Library, 2015)
    GraphX is an API for graph computation built upon Apache Spark, a fast and generalized engine for large-scale data processing in the cloud. While the popularity of Spark and GraphX is growing, the relatively young technology has yet to explore the breadth of graph problems that exist in the field. In order to examine and gain insights into the capabilities of GraphX, this thesis approaches the framework with the intention of implementing a solution to the Maximum Flow Problem, a complex graph problem without a trivial distributed approach. Specifically, the implementation is to be based on the serial Push-Relabel algorithm. An original MapReduce-based approach to the problem is presented, as well as an implementation of the approach in GraphX. In addition to the implementation, experimentation and deployment to an Amazon EC2 cluster allowed observations on caching and checkpointing intervals to be made.
  • Three-dimensional stratigraphic architecture and evolution of an ancient river-dominated delta, Iles Formation, Book Cliffs, Colorado

    Pyles, David R.; Andresen, Matthew A.; Kirschbaum, Mark A.; Santi, Paul M. (Paul Michael), 1964- (Colorado School of Mines. Arthur Lakes Library, 2015)
    The architecture and evolution of an ancient river-dominated delta in the eastern Book Cliffs is documented in a three-dimensional analysis of one parasequence in the Iles Formation. The mouth bars of parasequence 3 (PS 3) have three distinct regions, which are, from updip to downdip: proximal, medial, and distal. The proximal mouth bar is characterized by convex-up units of trough cross-stratified sandstone. The medial mouth bar region is characterized by steeply-dipping (ca. 5°) clinoforming units of amalgamated, planar-laminated sand that transfer downdip to the distal mouth bar region, which is characterized by shallowly-dipping (less than 0.5°), heterolithic strata with partial Bouma sequences. This downdip transition in facies is interpreted to reflect a transition in depositional process, from bedload and traction in the proximal mouth bar to sediment gravity flow in the medial and distal mouth bar. The stacking of mouth bars is hierarchical: beds build bedsets, bedsets build stories, and stories build elements. Mouth bars stack in different directions according to hierarchical scale: stories stack basinward whereas elements stack laterally. The stratigraphic architecture of PS 3, which was deposited in a minimum of 15 meters of water, contrasts with that of a shallower-water (ca. 4 meters) delta documented in the Neslen Formation by a companion study. There is significant aggradation in PS 3 of the silt-rich bottomset, which reduces the water depth in which subsequent mouth bars are deposited. The maximum clinoform angle is 5° and mouth bars are predominantly deposited by sediment gravity flow. In contrast, there is little bottomset aggradation in the shallower-water delta. The maximum clinoform angle is 3° and mouth bars are predominantly deposited by bedload and traction. Mouth bar elements stack more compensationally in the shallower-water delta than in PS 3. Finally, this study suggests that bottomset aggradation should be accounted for in numerical stratigraphic models of river-dominated deltas.
  • Impact of texture heterogeneity on elastic and viscoelastic properties of carbonates

    Prasad, Manika; Sharma, Ravi; Graves, Ramona M.; Kazemi, Hossein; Mooney, Michael A.; Vega, Sandra; Young, Terence K. (Colorado School of Mines. Arthur Lakes Library, 2015)
    This thesis discusses the impacts of fabric heterogeneity, fluids and fluid saturations, effective pressures, and frequency of investigation on the elastic and viscoelastic properties of calcite-rich limestone and chalk formations. Carbonate reservoirs have been analyzed either with empirical relations and analogs from siliciclastic reservoirs or using simplistic models. However, under the varying parameters mentioned above, their seismic response can be very different. The primary reason is because these rocks of biochemical origins readily undergo textural changes and support heterogeneous distribution of fluid flow and elastic properties. Thus, many current rock physics models are unable to predict the time-lapse elastic response in these reservoirs. I have measured elastic properties of calcite rich rocks in the seismic frequency range of 2 to 2000 Hz and at the ultrasonic frequency of 800 kHz. The samples selected for this study represent the typical heterogeneities found in carbonate formations. These measurements covering a large frequency range provide an understanding of the dispersion and attenuation mechanisms during seismic wave propagation in the subsurface. I find that a heterogeneous formation shows significant velocity dispersion and attenuations when saturated with brine, and even more on saturation with CO2. I also show that the shear modulus of carbonate rocks changes significantly (from 8% for brine saturation to 70% for CO2 saturation) upon fluid saturation with polar fluids. I evaluated rock physics models, such as Gassmann’s and with uniform and patchy fluid substitution, and Hashin-Shtrikman to predict saturated elastic properties in carbonates. Fluid sensitivity is directly related to the initial stiffness of the rock instead of porosity, as normally assumed. The Gassmann model can predict elastic properties for uniform saturations - mostly in homogenous rocks. Heterogeneous rocks, however, are better modeled using a patchy fluid saturation model. The results of this study provide valuable information on modeling the elastic response of saturated carbonate rocks currently lacking in fluid substitution models. It also provides data for reservoir simulation models to incorporate heterogeneity effects for realistic rock property variation to honor the textural complexities in carbonate reservoirs instead of using the simplified Gassmann model.
  • Perfluoroalkyl acids and microorganisms: implications for subsurface transport and microbial processes

    Sharp, Jonathan O.; Weathers, Tess Suzanne; Benson, David A.; Higgins, Christopher P.; McCray, John E.; Spear, John R. (Colorado School of Mines. Arthur Lakes Library, 2015)
    Perfluoroalkyl acids (PFAAs) are contaminants of emerging concern found throughout the environment. The interactions between subsurface microbiological process and PFAAs are largely unknown. Similarly, the effects of active microorganisms on PFAA transport are not well understood. This work explores these interactions by assessing co-contaminant biodegradation in the presence of PFAAs, shifts in microbial ecosystems, and stress-related effects on select microorganisms. Additionally, transport characteristics of PFAAs in the presence of pure cellular material and active microbiology are addressed. PFAAs are often found in aqueous film-forming foams (AFFF) used for fire suppression and often co-occur in groundwater with chlorinated solvents and BTEX compounds (benzene, toluene, ethylbenzene, and xylene). Here we show that reductive dechlorination by a methanogenic, mixed culture was significantly inhibited when exposed to concentrations representative of PFAA source zones (>66 mg/L total of 11 PFAA analytes, 6 mg/L each). Significant repression (8-fold decrease in abundance) of the pivotal reductive dechlorinator Dehalococcoides corresponded to an enhancement of methane-generating Archaea within the community (9-fold increase). Growth and dechlorination by axenic cultures of Dehalococcoides mccartyi strain 195, which can completely dechlorinate TCE to non-toxic ethene, were similarly repressed under these conditions. These results suggest that enhanced reductive dechlorination of chlorinated solvents could be impeded in subsurface environments. This work also addresses the effects of PFAAs on biodegradation of toluene. No effect on toluene degradation rate or induction time was observed when active cells of Rhodococcus jostii strain RHA1 were exposed to toluene and a mixture of PFAAs at concentrations of 110 mg/L total PFAAs. However, exposure to aqueous PFAA concentrations above 2 mg/L each was associated with enhanced aggregation of bacterial cells and extracellular polymeric substance production. This behavior was accompanied by two- to three-fold upregulation of stress-associated genes, sigF3 and prmA, during growth of this Rhodococcus in the presence of PFAAs. These results suggest that biological responses, such as microbial stress and biofilm formation, could be more prominent than suppression of BTEX biodegradation in subsurface locations where PFAAs occur with hydrocarbon fuels. To address the impacts of microbiological presence of PFAA transport, this dissertation evaluated aqueous sorption coefficients for PFAAs onto cellular material. Calculated logarithmic distribution coefficients (logKd) generally increase with increasing carbon chain length within the range of 2.3 to 4.7, exceeding the published sorption coefficients for sorption to cellular organic matter by nearly an order of magnitude. Microcosms containing soil amended with inactivated bacterial cells at quantities representative of subsurface growth in a biostimulated zone revealed changes in organic carbon normalized distribution coefficients as a function of biomass and analyte. These results demonstrate that PFAAs preferentially sorb to intact bacterial cells over soil-associated organic carbon and that traditional normalization techniques to bulk organic carbon may not accurately predict sorption of all PFAAs in microbially active zones. This phenomenon is a function of carbon chain length: … This information may have significant effects on our ability to predict subsurface fate and transport of PFAAs. Future research is proposed that focuses on upscaling sorption behavior and community ecology by assessing controlled flow-through column scenarios. These systems will evaluate changes in PFAA sorption and desorption as a function of biomass growth, coupled with the monitoring of biofilm production and corresponding advective shifts as a function of PFAA concentration.
  • Multiscale computational simulation of multidimensional chemically reacting flow over washcoated heterogeneous catalysts

    Kee, R. J.; Blasi, Justin M.; Bogin, Gregory E.; Dean, Anthony M.; Sullivan, Neal P.; Vincent, Tyrone (Colorado School of Mines. Arthur Lakes Library, 2015)
    This work addresses three distinct but interrelated topics: The simulation and evaluation of a novel ceramic microchannel heat exchanger; the adaptation of in situ adaptive tabulation for the acceleration of transient, heterogeneous chemistry in large CFD models; and the development of catalytic washcoat performance relations informed by 3D reconstructions of actual, commercial washcoats. Ceramic microchannel heat exchangers/reactors have several distinct advantages. They are cost effective, chemically inert, are capable of operating at high temperatures, and integrate well with internal catalysts. In this work, a particular design for a ceramic microchannel reactor is evaluated. A 3D computational fluid dynamics model with conjugate heat transfer and detailed kinetics for methane steam reforming was built. The model is used to evaluate the kinetic and hydraulic performance of the reactor as well as explore potential catalyst degradation mechanisms. In situ adaptive tabulation (ISAT) is a technique for accelerating the simulation of detailed kinetic mechanisms. The present work adapts this method, which was originally developed for homogeneous chemistry, to accelerate transient, heterogeneous kinetics. The approach is demonstrated on a series of transient simulations of methane steam reforming within the ceramic microchannel reactor model. For the particular case studied, 10 - 20 times speed-up factors were observed over the internal kinetics solver of ANSYS Fluent with no appreciable decrease in accuracy. Data collected from FIB-SEM imaging provided nanoscale phase information for actual, commercial washcoat structures. Algorithms were developed to process, filter, and discriminate phase information in the FIB-SEM images to facilitate 3D reconstruction. Reconstructed washcoat pores were cast in a dimensionless context and simulated to predict their catalytic performance. Performance relations, first for pores alone, and then for washcoats as a whole, were developed based on good agreement between reconstructed pores and an idealized 2D cylindrical pore model. The generalized relations suggest optimum washcoat depths based on effective kinetic rates and diffusion coefficients. Additionally, a modified washcoat factor was developed to relate the often used geometric multiplicative factor to a particular reaction-diffusion regime. These have important implications not only on the accurate simulation of washcoat performance but also for washcoat manufacturing.

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