bluuebunny/crossref_metadata_2025_split huggingface dataset:
Fully Coupled Hydromechanical Simulation of Hydraulic Fracturing in 3D Discrete-Fracture Networks
Authors: McClure Mark W. , Babazadeh Mohsen , Shiozawa Sogo , Huang Jian 
Published: January 2016

SummaryWe developed a hydraulic-fracturing simulator that implicitly couples fluid flow with the stresses induced by fracture deformation in large, complex, 3D discrete-fracture networks (DFNs). The code is efficient enough to perform field-scale simulations of hydraulic fracturing in DFNs containing thousands of fractures, without relying on distributed-memory parallelization. The simulator can describe propagation of hydraulic fractures and opening and shear stimulation of natural fractures. Fracture elements can open or slide, depending on their stress state, fluid pressure, and mechanical properties. Fracture sliding occurs in the direction of maximum resolved shear stress. Nonlinear empirical equations are used to relate normal stress, fracture opening, and fracture sliding to fracture aperture and transmissivity. Fluid leakoff is treated with a semianalytical 1D leakoff model that accounts for changing pressure in the fracture over time. Fracture propagation is modeled with linear-elastic fracture mechanics. The Forchheimer equation (Forchheimer 1901) is used to simulate non-Darcy pressure drop in the fractures because of high flow rate. A crossing criterion is implemented that predicts whether propagating hydraulic fractures will cross natural fractures or terminate against them, depending on orientation and stress anisotropy. Height containment of propagating hydraulic fractures between bedding layers can be modeled with a vertically heterogeneous stress field or by explicitly imposing hydraulic-fracture-height containment as a model assumption. Limitations of the model are that all fractures must be vertical; the mechanical calculations assume a linearly elastic and homogeneous medium; proppant transport is not included; and the locations of potentially forming hydraulic fractures must be specified in advance. Simulations were performed of a single propagating hydraulic fracture with and without leakoff to validate the code against classical analytical solutions. Field-scale simulations were performed of hydraulic fracturing in a densely naturally fractured formation. The simulations demonstrate how interaction with natural fractures in the formation can help explain the high net pressures, relatively short fracture lengths, and broad regions of microseismicity that are often observed in the field during stimulation in low-permeability formations, and that are not predicted by classical hydraulic-fracturing models. Depending on input parameters, our simulations predicted a variety of stimulation behaviors, from long hydraulic fractures with minimal leakoff into surrounding fractures to broad regions of dense fracturing with a branching network of many natural and newly formed fractures.
Pioneering Utilization of Fluid Cryogenics in Mimicking Shutoff Barrier Characteristics
Authors: Al-Otaibi N. , Pinero Zambrano N. , Al-Obaidi I. , Mazzella C. 
Published: January 2016

Abstract
The objective of this paper is to discuss the successful application of a cryogenic freeze plug to fulfill the requirements of dual shutoffs prior to removing the production tree. This operation is presented as a substitute when conventional means are not feasible.
The Saudi Aramco policy regarding the decompletion of wells with downhole packers requires two independent shutoff barriers for the tubing and tubing casing annulus prior to the removal of the production tree. These shutoffs must be independently and simultaneously operated. Moreover, each shutoff must be pressure tested separately. A dual shutoff is easily achievable with conventional means. That is, tubing plugs and tubing hanger back pressure valves, or a combination of the two.
Prior to removing the production tree from an oil-producing well, the production tubing status was found to have collapsed and parted. With the presented conditions, the only annular barrier was the kill fluid column. The well also had leaking tubing hanger seals, which were not holding pressure. As a result, introducing an additional shutoff barrier proved challenging.
An innovative practice was presented as a second shutoff application. The proposal was to utilize the cryogenic freeze plug as a solution to this event. The kill fluid column, filling the tubing and tubing casing annulus, was frozen by a cryogenic freeze plug method. This allowed the fluid to mimic the form of a second barrier.
This application serves as a temporary secondary barrier that would withstand the required pressure test with a full column of kill fluid in the tubing and tubing casing annulus.
This paper reviews the methodology, purpose, technique, and qualification of cryogenic freezing as a second barrier. It also supports the positive trend of technical reviews of new methods or alternative techniques for nonconventional isolation barriers or shutoffs.
Overcoming Open Hole Multistage Acid Fracturing Challenges in Saudi Arabian Carbonate Reservoirs with Swellable Packers, Best Practices and Dynamic Evaluation
Authors: Malik Ataur R. , Leal Jairo A. , Asiri Mohammed A. , Ogundare Tolulope M. , Soriano Eduardo , Vega Roberto , Lopez Alfredo 
Published: January 2016

Abstract
This paper describes the evolution of Open Hole (OH) Multi-Stage Fracture (MSF) completions that have been deployed in the Saudi Arabian tight gas carbonate reservoirs for the past several years. The carbonate reservoir in Saudi Arabia is often highly stressed and heterogeneous and fracturing treatment response can vary significantly between stages across the same reservoir.
This paper also defines the best drilling and deployment practices of OH MSF completion with swellable packers, diagnostic technique of OH packer's integrity, acid fracturing strategies, and well performance of Saudi Arabian carbonate gas reservoirs. It is important to drill in the minimum stress direction so that transverse fractures can be induced. The swellable packers are to be set across good gauged section with optimum spacing between packers and fracture ports. The stimulation treatment designs need to be tailored with optimum fluid volume after evaluating the status of packer integrity between each acid fracturing treatment.
Maintaining OH packer integrity between acid fracturing stages has become the most prevailing concern in emerging independent hydraulic fractures. Evident hydraulic communication between the stages through OH packers can develop a single dominant fracture affecting the well's sustainable gas rate as well as the recovery of final cumulative gas. The loss of annular isolation during fracturing with reactive fluids is often unavoidable in the presence of severe washout and unfavorable location of packers and fracture ports. Drilling in the minimum stress direction, inclusion of swellable packers, scrutinized depth selection of packers and fracture ports, dynamic evaluation of packer integrity between injections, detailed after-closure analysis, and vibrant optimization of acid stimulation treatments have increased effectiveness of acid stimulation and enhanced production.
Even though fracturing treatments in low permeability oil and gas bearing sandstone formations are routinely executed in North America through OH Multi-Stage Fracturing completion to maximize stimulated reservoir volume, but it is not as common completion technique during acid fracturing treatments in the Carbonate gas reservoirs. This paper highlights unique challenges and recommendations for acid fracturing treatments in carbonate reservoirs through OH MSF completion.
Insights Into Mobilization of Shale Oil by Use of Microemulsion
Authors: Bui Khoa , Akkutlu I. Yucel , Zelenev Andrei , Saboowala Hasnain , Gillis John R. , Silas James A. 
Published: January 2016

Summary
Molecular-dynamics simulation is used to investigate the nature of two-phase (oil/water) flow in organic capillaries. The capillary wall is modeled with graphite to represent kerogen pores in liquid-rich resource shale. We consider that the water carries a nonionic surfactant and a solubilized terpene solvent in the form of a microemulsion, and that it was previously introduced to the capillary during hydraulic-fracturing operation. The water has already displaced a portion of the oil in place mechanically and now occupies the central part of the capillary. The residual oil, on the other hand, stays by the capillary walls as a stagnant film.
Equilibrium simulations show that, under the influence of organic walls, the solvent inside the microemulsion droplets enables not only the surfactant but also the complete droplet to adsorb to the interfaces. Hence, delivering the surfactant molecules to the oil/water interface is achieved faster and more effectively in the organic capillaries. After the droplet arrives at the interface, the droplet breaks down and the solvent dissolves into the oil film and diffuses. This process is similar to drug delivery at nanoscale.
Using nonequilibrium simulations based on the external force-field approach, we numerically performed steady-state flow measurements to establish that the solvent and the surfactant molecules play separate roles that are both essential in mobilizing the oil film. The surfactant deposited at the oil/water interface reduces the surface tension and acts as a linker that diminishes the slip at the interface. Hence, it effectively enables momentum transfer from the mobile water phase to the stagnant oil film. The solvent penetrating the oil film, on the other hand, modifies flow properties of the oil. In addition, as a result of selective adsorption, the solvent displaces the adsorbed oil molecules and transforms that portion of the oil into the free oil phase. Consequently, the fractional flow of oil is additionally increased in the presence of solvent. The results of this work are important for understanding the effect of microemulsion on flow in organic capillaries and its effect on shale-oil recovery.
Proactive Optimization of Intelligent-Well Production Using Stochastic Gradient-Based Algorithms
Authors: Sefat Morteza Haghighat , Muradov Khafiz M. , Elsheikh Ahmed H. , Davies David R. 
Published: January 2016

Summary
The popularity of intelligent wells (I-wells), which provide layer-by-layer monitoring and control capability of production and injection, is growing. However, the number of available techniques for optimal control of I-wells is limited (Sarma et al. 2006; Alghareeb et al. 2009; Almeida et al. 2010; Grebenkin and Davies 2012). Currently, most of the I-wells that are equipped with interval control valves (ICVs) are operated to enhance the current production and to resolve problems associated with breakthrough of the unfavorable phase. This reactive strategy is unlikely to deliver the long-term optimum production. On the other side, the proactive-control strategy of I-wells, with its ambition to provide the optimum control for the entire well's production life, has the potential to maximize the cumulative oil production. This strategy, however, results in a high-dimensional, nonlinear, and constrained optimization problem.
This study provides guidelines on selecting a suitable proactive optimization approach, by use of state-of-the-art stochastic gradient-approximation algorithms. A suitable optimization approach increases the practicality of proactive optimization for real field models under uncertain operational and subsurface conditions. We evaluate the simultaneous-perturbation stochastic approximation (SPSA) method (Spall 1992) and the ensemble-based optimization (EnOpt) method (Chen et al. 2009). In addition, we present a new derivation of the EnOpt by use of the concept of directional derivatives.
The numerical results show that both SPSA and EnOpt methods can provide a fast solution to a large-scale and multiple I-well proactive optimization problem. A criterion for tuning the algorithms is proposed and the performance of both methods is compared for several test cases. The used methodology for estimating the gradient is shown to affect the application area of each algorithm. SPSA provides a rough estimate of the gradient and performs better in search environments, characterized by several local optima, especially with a large ensemble size. EnOpt was found to provide a smoother estimation of the gradient, resulting in a more-robust algorithm to the choice of the tuning parameters, and a better performance with a small ensemble size. Moreover, the final optimum operation obtained by EnOpt is smoother. Finally, the obtained criteria are used to perform proactive optimization of ICVs in a real field.
A 3D Transient Model for the Multiphase Flow in a Progressing-Cavity Pump
Authors: de Azevedo Victor W. , de Lima João A. , Paladino Emilio E. 
Published: January 2016

Summary
This paper presents the development of a computational-fluid-dynamics (CFD) model for the 3D transient two-phase flow within a progressing-cavity pump (PCP). The model implementation was only possible because of the meticulous mesh-generation and mesh-motion algorithm, previously published by the authors, which is briefly described herein. In this algorithm, a structured mesh was generated by defining all nodes’ positions and connectivities, for each rotor position by means of FORTRAN subroutines, which were embodied into ANSYS CFX software. The model is capable of predicting accurately the volumetric efficiency and the viscous losses, and it provides detailed information of pressure and velocity fields and void distribution along the pump. Such information could be of fundamental importance for product development and/or optimization for field operation. In field applications, the common situation is that in which the oil comes into the pump accompanied with free gas, which characterizes a multiphase flow. Simplified models on the basis of the calculation of the backflow or “slippage,” which is subtracted from the displaced flow rate, fail to characterize the PCP performance under multiphase conditions because the slip is variable along the pump. In this model, the governing equations were solved with an element-based finite-volume method in a moving mesh. The Eulerian-Eulerian approach, considering the homogeneous model, is used to model the flow of the gas/liquid mixture. The compressibility of the gas is taken into account, which is one of the main shortcomings in positive/constant displacement pumps. The effects of the different gas-volume fractions (GVFs) in pump volumetric efficiency, pressure distribution, power, slippage flow rate, and volumetric flow rate were analyzed, and some new insights are presented about the slippage in PCPs operating in multiphase conditions. The results show that the developed model is capable of reproducing pump dynamic behavior under multiphase-flow conditions performed early in experimental works.
Direkte, indirekte und kombinierte Effekte von Wölfen auf die Waldverjüngung
Authors: Kupferschmid Andrea D. , Bollmann Kurt 
Published: January 2016

Direct, indirect and combined effects of wolves on tree regeneration
The wolf currently returns to its traditional distribution area in central Europe. As a large carnivore, he will change the interactions between the organisms there. The impact of wolves on wild ungulates and on the vegetation cover and its composition is manifold: besides direct, numeric effects of wolves on the abundance and demography of their prey populations (like chamois, roe deer and red deer), indirect functional effects through mediated behavior of ungulates are also reported. In a first phase of wolf recolonization with low population density in a region, the functional effects as changes in the spatio-temporal use of habitats and resources are considered to be dominant because the wild ungulates attempt avoiding to become a prey. Later, in phases with higher wolf densities, direct numeric effects on the abundance of prey species should become more obvious. Such interactions among predator and prey species also have an impact on the browsing of tree regeneration. Since the spatio-temporal habitat use of prey species is mediated by the occurrence and abundance of large predators, we cannot assume that the browsing level will gradually decrease in general. Wild ungulates could more often hide or retreat into steep and rocky terrain or towards human settlement. Hence, trophic interactions between predators, herbivores and forests are complex and multilayered, in particular because forest ecosystems in central Europe are strongly influenced by forestry and hunting practices. This limits the trophic cascading impact of carnivores on forest. Our review demonstrates that the equation “wolf = less ungulates = fewer browsing” is not a general rule.
Huftierbestände und Verbissintensitäten nach der Luchswiederansiedlung im Kanton St. Gallen
Authors: Schnyder Jasmin , Ehrbar Rolf , Reimoser Friedrich , Robin Klaus 
Published: January 2016

Ungulate populations and browsing intensities following the reintroduction of lynx in the canton of St. Gallen
In the context of the project “Lynx resettlement north-east Switzerland” (LUNO), the Eurasian lynx (Lynx lynx L.) was reintroduced in the canton of St. Gallen from 2001. As a consequence of reduced roe deer (Capreolus capreolus L.) and Alpine chamois (Rupicapra rupicapra L.) population sizes, one goal was the reduction of browsing intensities in young forest stands. This study investigates interactions between the levels of this trophic cascade by means of data on lynx distribution, roe deer and chamois populations (hunting statistics and population estimations) as well as browsing intensities. The results have shown significantly reduced roe deer and chamois population sizes in the years after lynx reintroduction. The effect of lynx could have worked cumulatively with factors like hunting, weather conditions and diseases (chamois). Within the chamois population, the lynx probably affected primarily the forest chamois, although before and after the lynx presence other causal reasons have been responsible for population decreases. Furthermore, there was noted a significantly positive relationship between hunting statistics (as an indicator of local ungulate population sizes) and browsing intensities, although the size of local ungulate populations was one influencing factor among many more. The browsing intensity of silver fir (Abies alba Mill.) was significantly reduced after lynx reintroduction in the core area of lynx distribution. All results have shown significant correlations and suggest an indirect influence of lynx on browsing intensities of silver fir. However, causal connections should be analysed in further studies.
Wirtschaftliche Konsequenzen waldbaulicher Strategien zur Bewältigung des Klimawandels
Authors: Pauli Bernhard , Stöckli Barbara , Rosset Christian 
Published: January 2016

Economic consequences of silvicultural strategies for tackling climate change
Climate change poses significant economic challenges for Swiss forest management. The main issue is which silvicultural strategies are best suited to ensuring economic success and minimising loss even under new climatic conditions. As part of the “Forest and Climate Change” programme, a model has been developed to calculate the monetary effects of four silvicultural strategies – a profit-oriented, a risk-reducing, a cost-avoiding and a reference strategy. The results obtained, valid for the Swiss Central Plateau, show that the various silvicultural strategies have a strong influence on economic success. A profit-oriented or a risk-reducing strategy appears to be the most promising economically. “Continue as before” represents the worst strategy. Government and the forestry sector must now initiate appropriate measures.
High Level of Virological Suppression among HIV-Infected Adults Receiving Combination Antiretroviral Therapy in Addis Ababa, Ethiopia
Authors: Mekuria Legese A , Nieuwkerk Pythia T , Yalew Alemayehu W , Sprangers Mirjam AG , Prins Jan M 
Published: January 2016

Background Plasma viral load (pVL) is a key indicator of therapeutic response in HIV-infected patients receiving combination antiretroviral therapy (cART), but is often unavailable in routine clinical care in resource-limited settings. Previous model-based simulation studies have suggested that the benefits of routine pVL monitoring among patients on first-line regimens in resource-limited settings are modest, but this needs corroboration in well-defined study populations. Methods We investigated virological suppression levels and identified predictors of detectable viraemia among 870 randomly selected patients who started cART between May 2009 and April 2012 in 10 health-care facilities in Addis Ababa, Ethiopia. A total of 656 (75.4%) patients, who were alive, were retained in HIV care and receiving cART for at least 6 months provided a blood sample for pVL measurement. Predictors of detectable viraemia were identified in a multivariate logistic regression model. Results In on-treatment analysis, 94.5% (95% CI 92.5, 96.1) of the patients achieved virological suppression below 400 copies/ml after a median (IQR) of 26 (17–35) months on cART. When patients who were lost to follow-up, dead or stopped were assumed to have had detectable viraemia, the proportion of patients with virological suppression ≤400 copies/ml decreased to 74.6% (95% CI 71.5%, 77.4%). Younger age, lower educational status, ≤95% medication adherence, lower CD4+ T-cell count at cART initiation and/ or the diagnosis of immunological failure thereafter significantly predicted detectable viraemia. Conclusions Virological suppression levels can be high in an established ART programme in a resource-limited setting, even without the availability of routine pVL monitoring. Efforts to improve treatment outcomes should focus on younger and illiterate patients, earlier detection of HIV-positive status and cART initiation before patients are severely immunocompromised, and improving retention in care.
Globes, savoir situé et éducation à la beauté : Patrick Geddes géographe et sa relation avec les Reclus
Authors: Ferretti Federico 
Published: January 2016

Cet article interroge la relation que Patrick Geddes a entretenue avec la géographie. Elle est souvent citée, mais elle n’est pas encore étudiée systématiquement à travers une analyse de sa collaboration avec le réseau des géographes anarchistes : Pierre Kropotkine, Élie, Élisée et Paul Reclus. Nos sources principales sont les archives et les travaux géographiques de Geddes. Nous suivons l’exhortation de Charles Withers, qui invite à étudier la géographie interne de l’Outlook Tower d’Édimbourg par l’analyse des objets exposés dans ce « musée géographique ». Notre problématique s’ordonne autour de deux aspects thématiques contigus. Le premier d’entre eux est l’engagement de Geddes dans l’enseignement de la géographie et la résonance de sa critique du système éducatif, qualifié de « fabrique d’imbéciles artificiels », avec la pédagogie libertaire. Le second aspect est sa critique, tout à la fois didactique et épistémologique – et inspirée par Élisée Reclus –, de la carte plane en tant qu’instrument de représentation du monde.
Gastrointestinal stromal tumor and extragastrointestinal stromal tumor: Rare pattern of recurrence of disease
Authors: Ray Amitabh , Agarwal Vikash , Khan Enam , Ghosh Saurav , Goyenka Mahesh 
Published: January 2016

ABSTRACTGIST and EGIST are known to be distinct clinico-pathological entities. The occurrence of both in the same patient as a pattern of recurrence is extremely rare. We report one such case which was managed successfully due to high index of suspicion and interdisciplinary co-ordination.