bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of c-Si1-xGex Thickness Grown by LPCVD on the Performance of Thin-Film a-Si/c-Si1-xGex/c-Si Heterojunction Solar Cells
Authors: Hadi Sabina Abdul , Hashemi Pouya , DiLello Nicole , Nayfeh Ammar , Hoyt Judy L.
Published: November 2012

In this paper the effect of Si1-xGex absorber layer thickness on thin film a-Si:H/crystalline-Si1-xGex/c-Si heterojunction solar cells (HIT cells) is studied by simulation and experiment. Cells with 1, 2 and 4 μm-thick epitaxial cap layers of p-type Si0.59Ge0.41 on top of 5 μm Si1-xGex graded buffer layers are fabricated and compared to study the effect of the absorber layer thickness. The results show no change in Voc (0.41V) and that Jsc increases from 17.2 to 18.1 mA/cm2 when the Si0.59Ge0.41 absorber layer thickness is increased from 1 to 4 μm. The effect of thickness on Jsc is also observed for 2 and 4 μm-thick Si and Si0.75Ge0.25 absorber layers. Experiments and simulations show that larger Ge fractions result in a higher magnitude and smaller thickness dependence of Jsc, due to the larger absorption coefficient that increases optical carrier generation in the near surface region for larger Ge contents.
Optimization of Graded CIGS Solar Cells Using TCAD Simulations
Authors: Lee Mankoo , Pramanik Dipankar , Liang Haifan , Korczynski Ed , van Duren Jeroen
Published: November 2012

To understand paths towards higher efficiency (η) for copper-indium-gallium-(sulfur)-selenide [CIG(S)Se] solar cells, we investigated a variety of absorber composition grading schemes for various back-side gallium (Ga), front-side sulfur (S), and double-graded Ga composition depth profiles in TCAD 1D/2D simulations. We fitted experimental results of a Back-Side Graded (BSG) solar cell with our TCAD models, prior to investigating other grading and interface schemes. The BSG solar cell was fabricated on a High Productivity Combinatorial (HPC™) platform based on sputtering Cu(In,Ga) followed by selenization. Our TCAD simulation methodology for optimizing CIG(S)Se solar cells started with a sensitivity analysis using 1D Solar-cell CAPacitance Simulator (SCAPS) [1] by selecting a typical range of key model parameters and analyzing the impact on η. We then used a 2D commercially-available Sentaurus simulation tool [2] to incorporate wavelength-dependent optical characteristics. As a result, we provide insight in the impact of grading schemes on efficiency for a fixed ‘material quality’ equal to an in-house BSG solar cell. We also quantify the effects of interface layers like MoSe2 at the Mo/CIG(S)Se interface, and an inverted surface layer at the CIG(S)Se/CdS interface.
Addressing Bottlenecks in Dye-sensitized Solar Cell Manufacture Using Rapid Near-infrared Heat Treatments
Authors: Watson Trystan , Charbonneau Cecile , Carnie Matthew , Mabbett Ian , Cherrington Martyn , Worsley David
Published: November 2012

A primary challenge to the industrial uptake of dye-sensitized solar cells (DSC) is the ability to improve manufacturing efficiency. New thinking is required in terms of lowering cost, improving the process steps and increasing throughput. The typical manufacture of a DSC contains a number of long process steps; the sintering and dyeing of the TiO2 are prime examples. The current solution is to batch process on rigid substrates or use long energy intensive convection ovens for flexible metal substrates. Here we present a method for reducing some of the bottlenecks in the manufacturing process using near infra red radiation to speed up the thermal treatment of TiO2 and silver inks reducing their processing times to 12 and 2 seconds from normal process times of 30 and 10 minutes respectively.
Does Thermal Annealing Affect the Mechanical Properties of Silkworm (Bombyx mori) Cocoon Silk?
Authors: Reed Emily J. , Viney Christopher
Published: November 2012

It has been reported [1] that microwave radiation can enhance many of the mechanical properties of Bombyx mori silkworm cocoon silk, as measured in constant strain rate tensile tests to failure and in stress relaxation tests. The consequences of microwave radiation will affect decisions about the use of silk in settings subjected to significant microwave exposure – for example, as a reinforcing fiber in an epoxy matrix composite that may be microwave cured, or as a component in aircraft radomes.

There are two possible mechanisms by which microwave radiation may affect a material [2]: (i) the radiation may enable chemical and/or microstructural changes – and therefore property changes – in the same way that conventional heating would, or (ii) the high heating rates that are achievable by microwaving may selectively favor changes that would be masked under conventional conditions, where heating rates are low enough to give preference to changes that have a lower activation energy. Here we explore the former possibility for silk.

We characterized several mechanical properties of degummed and subsequently annealed B. mori silk, and compared them to the corresponding properties of degummed B. mori silk that was not annealed. The annealing treatment was carried out at 140 °C for 7 hours (conditions that optimally increased crystal size in an unrelated study of B. mori silk [3]), and then the fibers were allowed to cool gradually to room temperature over the course of an hour. Comparison of mechanical properties revealed no differences between the materials that we tested. Thus, for annealed silk, we do not observe the enhancements that can be achieved by microwaving. We conclude that in cases where microwaving affects the properties of silk, those changes are not a simple consequence of annealing by the microwaves.
Ab initio studies of the crystal structure of cellulose triacetate I
Authors: Kobayashi Takanori , Hayakawa Daichi , Khishigjargal Tegshjargal , Ueda Kazuyoshi
Published: November 2012

We have investigated crystal structure of cellulose triacetate I (CTA I) by using first principal density functional theory (DFT) calculation. The results are in good agreement with the experimentally obtained crystal structure when we used the cutoff energy higher than 70 Ry. However, the cell parameters calculated without dispersion correction are overestimated the results compared to the experimental value. Contrary, with the inclusion of dispersion correction, the cell parameters were calculated slightly smaller than the experimental one. The smaller cell parameter can be considered to be reasonable because the effect of the thermal expansion is not included in the density functional calculation. That is, inclusion of the dispersion term is important in the calculation of this crystal structure of CTA I.
Molecular Dynamics Based Study on Ductility Enhancement Effect of Nano-scale Void in Fine-grained Metallic Materials
Authors: Taniguchi Shin , Kameda Toshihiro , Fujita Toshiyuki
Published: November 2012

In fine-grained metallic materials, the dominant grain boundary (GB) process, such as dislocation emission, dislocation absorption, and dislocation pile-up, causes non-uniform deformation, which results in high yield stress and low ductility. When a nano-scale void is introduced, the dislocation activity enhancement around the void could inhibit GB fracture and enhance ductility. In this study, by considering nanocrystalline Cu models, the influence of an intragranular nano-scale void on the fracture process has been investigated through molecular dynamics simulation. The dependence of ductility enhancement on the grain size and void size has especially been discussed at low and room temperatures. Sufficient dislocation activity enhancement accompanied by optimal void growth causes a fracture mode transition from GB fracture to transgranular fracture. While the ductility enhancement strongly depends on the void size at low temperature, it depends on the grain size at room temperature. The strong dependence of ductility enhancement on the temperature is found in the case of relatively small grains.
Collaborative Preparation of K-12 Teachers and Graduate Students in Nanoscience Research and Education.
Authors: Williams Myron N.V. , Kohlhorst Drew , Reed James L. , Khan Ishrat , Marsteller Pat , Rose Jordan
Published: November 2012

A 2010 Report to the President from the Council of Advisors on Science and Technology calls for “research and development to create “well-designed and validated examples of comprehensive, integrated instructional materials” for K-12 education. The Center for Functional Nanoscale Materials (CFNM) at Clark Atlanta University (CAU) and the Center for Science Education at Emory University have partnered in a program that provides a collaborative experience between CAU graduate students and Atlanta area high school and middle school teachers. The partnership expands Emory’s PRISM (Problems and Research to Integrate Science and Mathematics) program, a NSF Graduate Teaching Fellows in K-12 Education (GK-12) program. We believe that personal and experiential collaboration between these stakeholders in materials education provides even more substantial and tangible benefits.

PRISM aims to stimulate reflection, by providing teachers and graduate students (PRISM Fellows) with an opportunity to lead in producing knowledge about pedagogical practice in STEM areas. By direct linkage with pedagogic theory, teaching practice can be subjected to continuous improvement, and it is anticipated that participants will catalyze change in both the educational and research communities.

CFNM/PRISM Program ensures that both categories of Fellows participate in professional development activities designed to propagate active learning pedagogies and reflective practice during an annual Summer Institute. Teachers are immersed in a content-rich nano- and materials science research environment, while the graduate fellows have the opportunity during the subsequent academic year to assist with instruction in local schools. Thirteen graduate students and seventeen teachers have participated in the CFNM/PRISM Program over the course of five years. Teams were formed that comprised a teacher, a CAU faculty researcher and a graduate student. Each team tackled a nanoscience research problem during a summer project, developed problem-based learning (PBL) and investigative case-based learning strategies that integrate grade-appropriate science and math content, and implemented the cases in middle school and high school classrooms.

The program has been preliminarily evaluated using online written attitudinal surveys and interviews with participants. Most striking among our observations is that teachers report an enhancement of their science process skills, including an increased ability to design and implement experiments for their students. Correspondingly, graduate students report a better understanding of the importance and practice of mentoring, as well as improved ability to articulate complex scientific concepts to lay audiences. Finally, since the student body at CAU and in these Atlanta area school systems is predominantly of African American heritage, the project also contributes to diversification of the Nation’s scientific enterprise.
Development of an Integrated Research, Curricular, Historically-Informed and Extracurricular Learning Environment
Authors: Cross William M. , Kellar Jon J. , Crawford Grant A. , Howard Stanley M. , Medlin Dana J. , West Michael K.
Published: November 2012

The Department of Materials and Metallurgical Engineering faculty at the South Dakota School of Mines and Technology (SDSM&T) has developed a unique undergraduate program that integrates research, extracurricular activities, and outreach experiences. A common thread throughout the program is an introduction to the artistic and historical background of metallurgical engineering. These activities use kinesthetic learning to promote student learning of metallurgical engineering, aspects often not traditionally included in engineering curricula. These programs are similar to those envisioned by the National Academy of Engineering in response to the changing needs of engineering. These are described in two visionary books published by the National Research Council.

A major focus of the program integrates blacksmithing activities with curricular, extracurricular, and outreach activities. All SDSM&T students are invited to a weekly blacksmithing activity called Hammer-in. Blacksmithing-related laboratories were added to the curriculum. Additionally, students developed a portable blacksmithing laboratory with faculty supervision. The laboratory has been taken to K-12 schools, including Native American schools on reservations, to reach out to regional students, thereby promoting interest in STEM careers. The success of these activities led to their incorporation into a National Science Foundation Research Experience for Undergraduate (REU) at SDSM&T called Back to the Future that focuses on understanding new technologies through historical antecedents. The SDSM&T students who participated in this REU used this experience as part of their junior/senior design courses. This program has increased enrollment in the department and has led to better learning outcomes for the students.
Colloidal Stability in Ionic Liquids and Relevant Soft Materials
Authors: Ueno Kazuhide , Watanabe Masayoshi
Published: November 2012

Ionic liquids (ILs) are receiving a great deal of attention as synthetic and dispersion media for colloidal systems, as well as alternatives to organic solvents and electrolyte solutions. Colloidal stability is an essential factor for determining the properties and performance of colloidal systems combined with ILs. The remarkable properties of ILs primarily originate from their highly ionic nature. While such high ionic strength often causes colloidal aggregation in aqueous and organic dispersions, certain colloidal particles can be well dispersed in ILs without any stabilizers. First, we will discuss the colloidal stability of bare and polymer-grafted silica nanoparticles and the surface force between silica substrates in ILs. Three different repulsions between colloidal particles—electrostatic, steric, and solvation forces—will be highlighted. A possible interpretation of the stabilization mechanism in ILs, both in the presence and in the absence of stabilizers, will be proposed. Next, we will provide an overview of our recent studies on colloidal soft materials with ILs. On the basis of dispersed states of the silica colloids, two different soft materials, colloidal gel and colloidal glass in ILs, were fabricated. Their functional properties (such as ionic transport, rheological properties, and optical properties) and the microstructure of the colloidal materials will also be presented.
Tissue-specific mercury concentrations in two catfish species from the Brazilian coast
Authors: Azevedo Juliana de Souza , Sarkis Jorge Eduardo de Souza , Oliveira Talita Aparecida , Ulrich João Cristiano
Published: November 2012

Some regions of Brazil have particularly high levels of mercury (Hg) emissions due to industrial activities, and their fish species may therefore suffer high levels of contamination through bioaccumulation. In this paper we assess Hg contamination in the muscle, liver, gills and blood of two different species of catfish collected from the Cananéia and Santos-São Vicente estuaries, São Paulo State, Brazil. Fish from the most polluted site (Santos-São Vicente) showed higher median Hg concentrations in both species (Cathorops spixii - liver: 1530 µg Kg-1, muscle: 327 µg Kg-1 and gill: 101 µg Kg-1; Genidens genidens - liver: 2617 µg Kg-1, muscle: 393 µg Kg-1 and gill: 118 µg Kg-1). Multivariate analysis revealed the importance and influence of key biological variables (size, condition, etc.) in determining the overall level of Hg and its distribution within different tissues.
Parameters of Matrix/Fracture Immiscible-Fluids Transfer Obtained by Modeling of Core Tests
Authors: Civan F.. , Rasmussen M.L.. L.
Published: November 2012

Methodology is presented and proved for determination of the best-estimate parameter values affecting the matrix/fracture-interface fluid transfer in naturally fractured reservoirs. Fracture/surface-hindered interface transfer of immiscible fluids is considered between matrix blocks and surrounding natural fractures. Improved matrix/fracture-transfer models are applied on the basis of presumed matrix-block shapes. Analytical solutions and the limiting isotropic-matrix long-time shape factors developed for special boundary conditions are used for interpretation of typical laboratory tests conducted using rectangular- and cylindrical-shaped rock samples. Workable equations and straight-line data-plotting schemes are developed for effective analysis and interpretation of laboratory data obtained from various-shaped oil-saturated reservoir-rock samples immersed into brine. Applications concerning the water/air and water/decane systems in laboratory core tests are also presented. The present approach allows rapid determination of the characteristic parameters of the matrix/fracture-transfer models for various-shaped matrix blocks, which are essential for prediction of petroleum recovery from naturally fractured reservoirs. The methodology is verified using various experimental data, and the values of the characteristic parameters (e.g., the average diffusion-coefficient and the interface-skin-mass-transfer coefficient) are determined. The Arrhenius (1889) equation is shown to represent the temperature dependency of these parameters effectively.
Scale Prediction and Inhibition for Oil and Gas Production at High Temperature/High Pressure
Authors: Fan Chunfang , Kan Amy T. , Zhang Ping , Lu Haiping , Work Sarah , Yu Jie , Tomson Mason B.
Published: November 2012

With the advance of new exploration and production technologies, oil and gas production has gone to deeper and tighter formations than ever before. These developments have also brought challenges in scale prediction and inhibition, such as the prevention of scale formation at high temperatures (150–200°C), pressures (1,000–1,500 bar), and total dissolved solids (TDS) (>300,000 mg/L) commonly experienced at these depths. This paper will discuss (1) the challenges of scale prediction at high temperatures, pressures, and TDS; (2) an efficient method to study the nucleation kinetics of scale formation and inhibition at these conditions; and (3) the kinetics of barite-crystal nucleation and precipitation in the presence of various scale inhibitors and the effectiveness of those inhibitors. In this study, nine scale inhibitors have been evaluated at 70–200°C to determine if they can successfully prevent barite precipitation. The results show that only a few inhibitors can effectively inhibit barite formation at 200°C. Although it is commonly believed that phosphonate scale inhibitors may not work for high-temperature inhibition applications, the results from this study suggest that barite-scale inhibition by phosphonate inhibitors was not impaired at 200°C under strictly anoxic condition in NaCl brine. However, phosphonate inhibitors can precipitate with Ca2+ at high temperatures and, hence, can reduce efficiency. In addition, the relationships of scale inhibition to types of inhibitors and temperature are explored in this study. This paper addresses the limits of the current predition of mineral solubility at high-temperature/high-pressure (HT/HP) conditions and sheds light on inhibitior selection for HT/HP application. The findings from this paper can be used as guidelines for applications in an HT/HP oilfield environment.
Why Monitoring With a Single Downhole Microseismic Array May Not Be Enough: A Case For Multiwell Monitoring of Cyclic Steam in Diatomite
Authors: Murer Anthony S. , McNeish Greg R. , Urbancic Ted , Prince Marc , Baig Adam M.
Published: November 2012

Nine wells were drilled to test cyclic steam stimulation as a recovery mechanism in the diatomite reservoir in the Belridge field. Micro-seismic monitoring was proposed to evaluate steam-chest and fracture growth. A series of models were constructed to determine both microseismic-event detectability and locatability. The modeling indicated that poor signal/noise ratios would constrain the ability to locate events using a single array. As a result, three microseismic-monitoring wells (MOWs) were installed. The purpose of this paper is to evaluate the microseismic-event-location results obtained from the three-well solution and compare them with the solutions obtained when turning off one or two of the arrays. This first phase of investigation was performed on the sand-propped hydraulic-fracture stimulation before the cyclic steam operations. This study may be applied to other areas, including imaging hydraulic-fracture stimulations in shale plays, reservoir steam monitoring, or in any area where location precision in microseismic monitoring is necessary.

It has been demonstrated in the field that the monitoring range of a microseismic system can be increased and the potential for locating microseismic events improved by installing multiple arrays having overlapping radii of observation. Innovative system design, deployment techniques, operational procedures, and advanced multiwell processing strategies have all contributed to creating a growing data set with more than 4,000 events recorded during the first 13 months of operation. Results of turning off arrays showed increased location error with two-well solutions and a significant increase in error with one-well solutions, notwithstanding the reduction in event location count because of a lack of multiphase signals on a single array. Azimuth errors in single-microseismic-observation-well solutions result in dispersed interpreted fracture geometry and in misinterpretation.

Overall, our observations show that for both multi-and singlephase events, the observed detection limits and size distribution of the seisms have far exceeded those originally predicted by the earlier modeling.
A Comparison of Hydraulic-Fracture Modeling With Downhole and Surface Microseismic Data in a Stacked Fluvial Pay System
Authors: Mohammad Nur Azlinda , Miskimins Jennifer L.
Published: November 2012

This paper presents a study that combines and compares the results of hydraulic-fracture mapping and modeling using both downhole and surface microseismic arrays. There were three objectives for the study: developing detailed post-treatment models of hydraulic-fracturing treatments in the subject well, Well D1, which was monitored with downhole microseismic tools; developing detailed post-treatment models of the hydraulic-fracturing treatments in the subject well, Well S1, which was monitored with surface microseismic tools; and determining the match characteristics of the downhole and surface microseismic data to hydraulic-fracture models developed for both Wells D1 and S1.

Input data for this project were obtained from two wells in the Greater Natural Buttes field, Uinta basin, Utah. Ten fracture models were built using five stages each from the two subject wells, Wells D1 and S1, and detailed pressure matches were made. Comparisons of the match characteristics from the multiple inputs were then developed. The hydraulic-fracture-stimulation models were graphically integrated with the microseismic events using visualization software. This software allowed the final model-simulated fracture geometries to be plotted along with the microseismic events in 3D space, thus allowing the viewer to see a full integration of data in a stacked fluvial pay system.

Results from the integration process show good agreement in geometries and depth for most stages in the downhole-monitored well, whereas comparisons of surface microseismic-mapping measurements with the simulated fracture geometries yielded variable results. When combined with additional inputs such as geologic models, the integration methodology used in this project provides an excellent tool for hydraulic-fracture modeling and reservoir management in stacked pay systems.
Role of Stress Reorientation in the Success of Refracture Treatments in Tight Gas Sands
Authors: Roussel Nicolas P. , Sharma Mukul M.
Published: November 2012

The redistribution of stresses around a fractured vertical well has two sources: opening of propped fracture (mechanical effects) and production or injection of fluids in the reservoir (poroelastic effects). In this paper, the coupling of both phenomena was numerically modeled to quantify the extent of stress reorientation around fractured production wells. The results have been compared to field data from the Codell tight gas formation and analyzed for their impact on refracturing operations.

For previously fractured wells, a secondary fracture may be initiated perpendicular to the first fracture if a stress-reversal region is present. Altered-stress refracturing makes it possible to access zones of the reservoirs that are less depleted, thus increasing well production and reserves.

The results of our model quantitatively agree with previous tiltmeter measurements, confirming the existence of refracture reorientation in the Codell formation. The performance of refracturing treatments has been observed to be highly variable in the Wattenberg field (Colorado), with some wells underperforming while others are restored to initial production rates. Historical production from neighboring wells and initial fracture performance were shown to impact the potential benefits from refracturing predicted by the numerical model.

This paper introduces a 3D model, coupling mechanical and poroelastic stress reorientation used to interpret tiltmeter measurements and historical production in the Codell tight gas formation. Guidelines are drawn from the Wattenberg field case study that allow an operator to select candidate wells, choose the timing of the refracture operation in the life of the well, and evaluate the potential increase in well production after refracturing.
Liquid Entrainment in Annular Gas/Liquid Flow in Inclined Pipes
Authors: Magrini K.L.. L. , Sarica C.. , Al-Sarkhi A.. , Zhang H.-Q.. -Q.
Published: November 2012

Entrainment fraction is one of the key parameters in many applications, including wellbore and flowline design, separator design, wellbore loading, and corrosion inhibition. This study provides the first comprehensive entrainment data and their critical analysis for a full range of inclination angles ranging from horizontal to vertical in 76.2-mm-internal-diameter (ID) pipes. Experiments were conducted to investigate the effect of pipe inclination on entrainment fraction in air/water annular flow with inclination angles of 0, 10, 20, 45, 60, 75, and 90° from horizontal. Two techniques were used to measure the entrainment fraction: film removal and isokinetic sampling. The experimental results were compared with existing models and correlations, and the best predicting methods were determined for all flow orientations.

An inclination effect on entrainment fraction was observed. This effect occurred at low superficial gas velocities and was more prominent for higher superficial liquid velocities. Using the present study data, the Paleev and Filipovich (1966) correlation was found to be the most accurate in predicting entrainment fraction. On the basis of all available data, the Pan and Hanratty (2002b) correlation performed the best in predicting entrainment fraction in all pipe orientations. For vertical annular flow, the Oliemans et al. (1986) correlation predicted entrainment fraction more accurately. The Pan and Hanratty (2002b) correlation was the most accurate in predicting entrainment fraction for horizontal annular flow. The Wallis (1969) correlation and the mechanistic model developed by Mantilla (2008) most accurately predicted the entrainment fraction for inclined annular flow.
Multiphase Fluid-Loss Properties and Return Permeability of Energized Fracturing Fluids
Authors: Ribeiro L.H.. H. , Sharma M.M.. M.
Published: November 2012

With the growing interest in low-permeability gas plays, foam* fracturing fluids are now well established as a viable alternative to traditional fracturing fluids. Present practices in energized fracturing treatments remain, nonetheless, rudimentary in comparison to other fracturing-fluid technologies because of our limited understanding of multiphase fluid-loss and phase behavior occurring in these complex fluids. This paper assesses the fluid-loss benefits introduced by energizing the fracturing fluid.

A new laboratory apparatus has been specifically designed and built for measuring the leakoff rates for both gas and liquid phases under dynamic fluid-loss conditions. This paper provides experimental leakoff results for linear guar gels and for N2/guar foam-based fracturing fluids under a wide range of fracturing conditions. In particular, the effects of the rock permeability, the foam quality, and the pressure drop are investigated. Analysis of dynamic leakoff data provides an understanding of the complex mechanisms of viscous invasion and filter-cake formation occurring at the pore scale.

This study presents data supporting the superior fluid-loss behavior of foams, which exhibit minor liquid invasion and limited damage. It also shows direct measurements of the ability of the gas component to leakoff into the invaded zone, thereby increasing the gas saturation around the fracture and enhancing the gas productivity during flowback. Our conclusions not only confirm but add to the findings of McGowen and Vitthal (1996a, b) for linear gels and the findings of Harris (1985) for nitrogen foams.
Red vs. Green Scale Inhibitors for Extending Squeeze life—A Case Study From the North Sea, Norwegian Sector—Part II
Authors: Jordan M.M. , Sorhaug E. , Marlow D.
Published: November 2012

Over the years, environmental legislation has forced changes in the types of scale-inhibitor molecule that can be deployed in certain regions of the world. These regulations have resulted in changes from phosphonate scale inhibitor to polymer-based chemistry, particularly in the Norwegian and UK continental shelf where phosphonates have been either on the substitution list or phased out for many applications. Over the past 10 years, significant improvements in inhibitor properties of the so-called "green" scale inhibitors have been made. However, for one particular operator, the squeeze application of this green scale inhibitor resulted in poorer than expected treatment lifetimes and significant operating cost because of the frequency of retreatment. To overcome the increasing operating cost, an evaluation was made of the current treatment chemicals vs. the older, more-established phosphonate scale inhibitors. The results for the laboratory evaluation suggested that the older chemistry would extend treatment life and reduce operating cost. A case was made to the legislative authority, who approved the use of the phosphonate scale inhibitor, and field applications started. The squeeze lifetimes for the red phosphonate chemistry were shown to be significantly better than the existing yellow/green inhibitors. During the following months, other scale inhibitors with improved environmental characteristics were developed and evaluated. One such molecule was shown to have similar coreflood retention to that of the applied red phosphonate and presented no formation damage. This paper presents the laboratory evaluation of the new scale inhibitor, and illustrates the improvement observed with this new inhibitor through field squeeze-treatment results from a well treated with both the red and new yellow environmental profile inhibitor chemicals.

This paper outlines the challenges with environmental legislation and how it has been possible to develop technical solutions (in terms of environmental vs. safety issues and with new inhibitor chemicals) to meet the challenges of offshore scale control.
Field Study of the Physical and Chemical Factors Affecting Downhole Scale Deposition in the North Dakota Bakken Formation
Authors: Cenegy L.M.. M. , McAfee C.A.. A. , Kalfayan L.J.. J.
Published: November 2012

North Dakota Bakken oil recovery has increased nearly 100-fold over the last 5 years, driven by technological advancements in hydraulic fracturing and completion design. For one North Dakota operator with 150 Bakken producing wells, 22 of the wells have experienced at least one event of severe calcium carbonate scaling in the pump and production tubing, leading to well failure. Bakken wells are completed to a vertical depth of approximately 10,000 ft, with horizontal laterals up to 10,000 ft, and are produced by means of multizone hydraulic fracturing.

The operator initially conducted a typical scale-prediction study to reduce well failures and maintain oil production. However, the scale-prediction study was challenging to perform for these Bakken wells because of the variability of the composition of the produced water. Attention then turned to the tracking and analysis of historical field conditions. A “post-mortem” of data collected from all failed wells because of scale was conducted, considering the failure type, date, type of hydraulic-fracturing procedure, pump-intake pressure, scale-inhibitor residual, calcium carbonate scaling index, geographic failure concentration, production time to failure, and cumulative water production to failure.

Results showed that 82% of the wells failed during early production (defined as less than 20,000 bbls of water produced and 2 years' production since first oil), after which failures became increasingly rare. This correlated with transient alkalinity spikes in the water analyses attributed to fracturing-fluid flowback during this critical period. Simulated blending of fracturing and formation waters demonstrated that this was the most important period to maintain high scale-inhibitor residuals because of high deposition potentials.

This paper discusses the various field and laboratory studies conducted in an effort to understand the problem, the results obtained, and the implications. Also discussed is the evaluation of two scale inhibitors before and after laboratory aging in simulated fracturing fluids.
Accurate Cell-Centered Discretizations for Modeling Multiphase Flow in Porous Media on General Hexahedral and Simplicial Grids
Authors: Wheeler Mary F. , Xue Guangri , Yotov Ivan
Published: November 2012

We introduce an accurate cell-centered method for modeling Darcy flow on general quadrilateral, hexahedral, and simplicial grids. We refer to these discretizations as the multipoint-flux mixed-finite-element (MFMFE) method. The MFMFE method is locally conservative with continuous fluxes and can be viewed within a variational framework as a mixed finite-element method with special approximating spaces and quadrature rules. We study two versions of the method: with a symmetric quadrature rule on smooth grids and a nonsymmetric quadrature rule on rough grids. The framework allows for handling hexahedral grids with nonplanar faces defined by trilinear mappings from the reference cube. Moreover, the MFMFE method allows for local elimination of the velocity, which leads to a cell-centered pressure system. Theoretical and numerical results demonstrate first-order convergence on rough grids. Second-order superconvergence is observed on smooth grids. We also discuss a new splitting scheme for modeling multiphase flows that can treat higher-order transport discretizations for saturations. We apply the MFMFE method to obtain physically consistent approximations to the velocity and a reference pressure on quadrilateral or hexahedral grids, and a discontinuous Galerkin method for saturations. For higher-order saturations, we propose an efficient post-processing technique that gives accurate velocities in the interior of the gridblocks. Computational results are provided for flow in highly heterogeneous reservoirs, including different capillary pressures arising from different rock types.
The Effect of Fracture-Face Matrix Damage on Productivity of Fractures With Infinite and Finite Conductivities in Shale-Gas Reservoirs
Authors: Li J.. , Guo B.. , Gao D.. , Ai C..
Published: November 2012

Producing natural gas from shale-gas reservoirs presents a great challenge to petroleum engineers owing to the low-permeability nature of this type of gas reservoir. Large-scale and expensive hydraulic-fracturing operations are often required for enhancing gas well productivity. Because of the shaly characteristics of the reservoir rock, the hydraulically fractured gas wells are vulnerable to damage by fracturing fluids. However, the true significance of the formation damage in shale-gas reservoirs is still not clear. It is highly desirable to have a simple method for predicting the degree of fracture-face matrix damage and for optimizing fracturing treatments. This paper is meant to fill this gap.

A new mathematical model was developed in this study to predict the effect of fracture-face matrix damage on the productivity of fractured gas wells in shale-gas reservoirs. A unique feature of the new model is that it considers reservoir/fracture crossflow in finite-conductivity fractures. Results of the model analyses were sensitized to reservoir properties and facture-face matrix-skin properties determined by the fracturing-fluid properties and treatment conditions. Large ranges of possible leakoff and spurt-loss coefficients were investigated. We concluded that, in the ranges of reservoir and fluid properties used in this study, well productivity should drop by less than 15% even if the residual matrix permeability is reduced to only 5% of the virgin reservoir permeability in the damage zone. Neglecting the resistance to flow in the fracture will overestimate the effect of matrix damage on well productivity. The well-productivity drop caused by matrix damage is most sensitive to the invasion depth and damaged permeability.
A Novel Technique for Monitoring Hydrate Safety Margin
Authors: Yang Jinhai , Chapoy Antonin , Mazloum Saeid , Tohidi Bahman
Published: November 2012

A novel technique has been developed for optimizing hydrate-inhibitor dose rates by monitoring the actual hydrate safety margin. The technique is based on measuring acoustic velocity and electrical conductivity in an aqueous sample taken from the pipeline/separator. The developed system then determines salt and organic inhibitor concentration (e.g., methanol, monoethylene glycol, and kinetic hydrate inhibitors). In the case of thermodynamic inhibitors, the information is then fed to a thermodynamic model determining the hydrate stability zone (HSZ). Superimposing the pipeline operating conditions (e.g., pressure and temperature profile), the developed system can determine the hydrate safety margin. The system can provide warning if the operating conditions are inside the HSZ or if too much inhibitor is being injected.

The technique can be used in optimizing inhibitor-injection rates, reducing the impact on the environment and operational costs. It can also improve production-system reliability by constant monitoring the hydrate safety margin and protecting the system against pump malfunction and/or changes in process variables (e.g., water cut).

This technique has been developed through a joint-industry project, and its performance has been intensively evaluated using synthetic samples and real produced-water samples by the authors and the project sponsors in field trials. The results demonstrate that this system can be used for different inhibitor systems, including methanol (MeOH)/NaCl, monoethylene glycol (MEG)/NaCl, and kinetic-hydrate-inhibitor/salt systems, with good accuracy.

The developed system can have a major impact on reducing the inhibitor-injection rates and environmental impact and improving the reliability of production systems against risks associated with hydrate formation. The developed system can be laboratory-based, a mobile unit, or online, with near-real-time samples taken from live pipelines/separators.
Detailed Modeling of the Alkali/Surfactant/Polymer (ASP) Process by Coupling a Multipurpose Reservoir Simulator to the Chemistry Package PHREEQC
Authors: Farajzadeh R.. , Matsuura T.. , van Batenburg D.. , Dijk H..
Published: November 2012

Accurate modeling of an ASP flood requires detailed representation of geochemistry and, if natural acids are present, the saponification process. Geochemistry and saponification affect the propagation of the injected chemicals and the amount of generated natural soaps. These in turn determine the chemical phase behavior and, hence, the effectiveness of the ASP process.

In this paper, it is shown that by coupling a multipurpose reservoir simulator (MPRS) with PHREEQC (Parkhurst and Appelo 1999; Charlton and Parkhurst 2008), a robust and flexible tool is developed to model ASP floods. PHREEQC is used as the chemical-reaction engine, which determines the equilibrium state of the chemical processes modeled. The MPRS models the impact of the chemicals on the flow properties, solves the flow equations, and transports the chemicals.

The validity of the approach is confirmed by benchmarking the results with the ASP module of the UTCHEM simulator (Delshad et al. 2000). Moreover, ASP corefloods have been matched with the new tool. The functionality of the model also has been tested on a 2D sector model. The advantages of using PHREEQC as the chemical engine include its rich database of chemical species and its flexibility in changing the chemical processes to be modeled. Therefore, the coupling procedure presented in this paper can also be extended to other chemical enhanced-oil-recovery (EOR) methods.
Characterization of Filter Cake Generated by Water-Based Drilling Fluids Using CT Scan
Authors: Elkatatny S.M.. M. , Mahmoud M.A.. A. , Nasr-El-Din H.A.. A.
Published: November 2012

Filter-cake characterization is very important in drilling and completion operations. The homogeneity of the filter cake affects the properties of the filtration process such as the volume of filtrate, the thickness of the filter cake, and the best method to remove it. Various models were used to determine the thickness and permeability of the filter cake. Most of these models assumed that the filter cake was homogeneous. The present study shows that the filter cake is not homogeneous, and consists of two layers of different properties.

The objective of this study is to measure the filter-cake thickness and permeability of water-based drilling fluids by a new approach and compare the results with previous models. A high-pressure/high-temperature (HP/HT) filter press was used to perform the filtration process under static conditions (225°F and 300 psi). A computed-tomography (CT) scan was used to measure the thickness and porosity of the filter cake. Scanning electron microscopy (SEM) was used to provide the morphology of the filter cake.

The results obtained from the CT scan showed that the filter cake was heterogeneous and contained two layers with different properties under static and dynamic conditions. Under static conditions, the layer close to the rock surface had a 0.06-in. thickness, 10- to 20-vol% porosity, and 0.087-pd permeability, while under dynamic conditions, this layer had a 0.04-in. thickness, 15-vol% porosity, and 0.068-pd permeability. The layer close to the drilling fluid had a 0.1-in. and 0.07-in. thickness under static and dynamic conditions, respectively, and it had zero porosity and permeability after 30 minutes under static and dynamic conditions. SEM results showed that the two layers contained large and small particles, but there was extremely poor sorting in the layer, that was close to the drilling fluid, which led to zero porosity in this layer. Previous models underestimated the thickness of the filter cake by almost 50%. A new method was developed to measure the thickness of the filter cake, and various models were screened to identify the best model that can predict our permeability measurements.
Mechanically Induced Fracture-Face Skin—Insights From Laboratory Testing and Modeling Approaches
Authors: Reinicke Andreas , Blöcher Guido , Zimmermann Günter , Huenges Ernst , Dresen Georg , Stanchits Sergei , Legarth Björn , Makurat Axel
Published: November 2012

In the context of this work, a new formation-damage mechanism is proposed—the mechanically induced fracture-face skin (FFS). This new mechanism results from mechanical interactions between the proppants and the reservoir rock caused by the increasing stress on the rock/proppant system during production. Proppant embedment into the fracture face and proppant crushing lead to fines production and may impair the fracture performance. To achieve sustainable, long-term productivity from a reservoir, it is indispensable to understand the hydraulic and mechanical interactions in rock/proppant systems. In this study, permeability measurements on sandstones with propped fractures under stress using various flow cells were performed, allowing localization and quantification of the mechanical damage at the fracture face. The laboratory experiments identified a permeability reduction at the fracture face of up to 90%. The mechanical damage at the rock/proppant interface began immediately with loading of the rock/proppant system and for fracture-closure stresses less than 35 MPa; the damage was localized at the fracture face. Microstructure analysis identified quartz-grain crushing, fines production, and pore-space blocking at the fracture face, causing the observed mechanically induced FFS. At higher stresses, damage and embedment of the ceramic proppants reduce the fracture permeability further. Numerical modeling of the rock/proppant system identified highly inhomogeneous stress distributions in the granular system of grains and proppants. High tensile-stress concentrations beneath the area of contact between quartz grains and proppants were observed, even at small differential stress applied to the rock/proppant system. These high-stress concentrations were responsible for the early onset of damage at the fracture face. Therefore, even low differential stresses, which are expected under in-situ conditions, may affect the productivity of a hydraulically fractured well.