bluuebunny/crossref_metadata_2025_split huggingface dataset:
Compositional Nonlinear Solver Based on Trust Regions of the Flux Function Along Key Tie-lines
Authors: Voskov D. V. , Tchelepi H. A. 
Published: February 2011

Abstract
A new nonlinear solver that combines tie-line and fractional-flow information is developed for general-purpose compositional simulation. The solver, which takes advantage of the hyperbolic nature of the transport equations, is demonstrated for a compositional formulation that employs molar (or mass) variables. Since compositional recovery processes evolve along a few ‘key’ tie-lines, the flux functions (fractional flow curves) associated with these key tie-lines play a dominant role in the evolution of the solution. A two-stage nonlinear solver is proposed, and its applicability and efficiency for general-purpose compositional simulation is demonstrated. In the first stage, given the pressure and overall composition of the current nonlinear iteration, the key tie-lines are computed using a negative-flash procedure, and the nonlinear unknowns are updated accordingly. In the second stage, the flux functions associated with the computed tie-lines are segmented into trust regions (i.e., appearance and disappearance of a phase, and the inflection point of the S-shaped two-phase region), which are then used to guide the consistent evolution of the compositions. These trust regions delineate convex parts of the flux function, where convergence of the Newton iterations is guaranteed. Several challenging, multi-contact, miscible compositional problems, including the SPE 10 permeability field with the 6-component SPE 5 fluid, are used to test the robustness and efficiency of this new nonlinear solver. The convergence rate of the new nonlinear solver is always better than our standard safeguarded Newton method, which employs heuristics on maximum changes in the variables. We demonstrate that for aggressive time stepping (i.e., maximum throughput - so called CFL numbers - is hundreds to thousands of times larger than the IMPES limit), the new nonlinear solver converges within a few Newton iterations for the wide range of problems we studied.
Pressure Preconditioning Using Proper Orthogonal Decomposition
Authors: Astrid P. , Papaioannou G. V. , Vink J. C. , Jansen J. D. 
Published: February 2011

Abstract
We developed and implemented a new first-stage preconditioning method for large-scale reservoir simulation as an alternative to the popular Algebraic Multi Grid (AMG) method. We used Proper Orthogonal Decomposition (POD) to derive a reduced-order model for the linearized pressure equation in a proprietary reservoir simulator. A small set of pre-computed pressure solutions are used to transform the equation into a lower-order system that can be solved economically yet provides a relatively accurate estimation of the solution of the original high-order system. We present results for a two-phase (oil-water) reservoir model under water flooding conditions. Use of the POD-based preconditioner lead to a significantly faster convergence of the overall simulation compared to AMG, and reduced the linear solver times between approximately 70 % and 80%, i.e. we obtained accelerations with factors between three and five. The highest speed up was achieved for a case in which the flow rates in the eight injection wells were changed frequently during the entire production period, using ten POD basis functions obtained from short training simulations in which the injectors were operated only once each. These first results are very encouraging, especially because there is room to optimize the trade-off between the number of basis functions used in the POD method and the required number of linear iterations. However, further research is required to assess the applicability of the POD-based preconditioner to more complex cases including e.g. strongly compressible flow or compositional effects. The overhead required to pre-compute the POD solutions implies that the new method will be particularly attractive when many solutions of near-similar simulation models are required such as in computer-assisted history matching or flooding optimization.
An Ensemble Smoother for assisted History Matching
Authors: Skjervheim J.-A. , Evensen G. , Hove J. , Vabø J. G. 
Published: February 2011

Abstract
This paper compares two ensemble-based data-assimilation methods when solving the history-matching problem in reservoir-simulation models. The methods are the Ensemble Kalman Filter (EnKF) and the Ensemble Smoother (ES). Several publications have discussed the use of EnKF in petroleum applications while ES is now used for the first time for history matching. ES differs from EnKF by computing a global update in the space-time domain, rather than using recursive updates in time as in EnKF. Thus, the sequential updating of the realizations with associated restarts is avoided.
EnKF and ES provide identical solutions for state estimation with linear dynamical models. However, for nonlinear dynamical models, and in particular models with chaotic dynamics, EnKF is superior to ES, due to the fact that the recursive updates keep the model on track and close to the true solution. Thus, ES is not much used and EnKF has been the method of choice in most data assimilation studies where ensemble methods are used.
On the other hand, reservoir simulation models are rather diffusive systems when compared to the chaotic dynamical models that were previously used to test ES. If we can assume that the model solution is stable with respect to small perturbations in the initial conditions and the history-matching parameters, then ES should give similar results to EnKF, and ES will be a more efficient and much simpler method to implement and apply.
The technical advantages of using ES compared to EnKF are severe, especially when the methods are applied with complex real reservoir models. ES provides a significant reduction in simulation time. Furthermore, a more flexible parameterization is possible, which makes it easier to handle structural and geological model parameters in the history-matching process.
In this paper we compare EnKF and ES and show that ES indeed provide for an efficient ensemble-based method for history matching.
On the Coupling of Finite Volume and Discontinuous Galerkin for Reservoir Simulation Problems
Authors: Riviere Beatrice , Chidyagwai Prince , Mishev Ilya 
Published: February 2011

Abstract
Finite volume (FV) methods are still the most popular methods in practical reservoir simulation. They have good accuracy when used properly and are computationally very cheap. FV can be used on unstructured grids, but grid generation becomes a very difficult task. More advanced methods, discontinuous Galerkin (DG) for example, work on more general meshes, but are computationally more expensive and are still not accepted by the practitioners. We propose coupling of FV and DG methods that can improve the accuracy of the FV with reasonable increase of the computational cost.
We developed the algorithms for coupling DG and FV for model diffusion and convection diffusion problems on Voronoi /PEBI grids. We demonstrate with examples how DG can be used to alleviate the problems with grids in 2-D, gridding around pinch-outs and patch local refinement for areas with tensor coefficients.
A Loosely Coupled Hierarchical Fracture Model for the Iterative Multiscale Finite Volume Method
Authors: Hajibeygi H. , Karvounis D. , Jenny P. 
Published: February 2011

Abstract
The hierarchical fracture modeling approach is extended to a loosely coupled strategy in order to enhance its efficiency for highly conductive fractures and to easily integrate it into the iterative Multiscale Finite Volume (i-MSFV) framework. The fracture and matrix equations are decoupled and a sequential implicit approach is used to solve them. The i-MSFV method is then introduced for the efficient solution of the matrix equation which also involves fracture terms. Very important it that only one degree of freedom per connected fracture network appears in the coarse multiscale operator. This results in very efficient simulations for realistic problems, and also ensures total mass balance for each fracture network during the iterations. Local fracture functions are introduced in order to capture the fractures at the coarse scale accurately; similar to the complex well functions developed by Jenny and Lunati (2009). Numerical results show that the i-MSFV is an efficient conservative multiscale solver for simulations of flow in fractured porous media.
A New Method to Model Relative Permeability in Compositional Simulators to Avoid Discontinuous Changes Caused by Phase Identification Problems
Authors: Yuan Chengwu , Pope Gary A. 
Published: February 2011

Abstract
Simple methods such as using density during compositional simulations often fail to identify the phases correctly and this can cause discontinuities in the computed relative permeability values. The results are then physically incorrect. Furthermore, numerical simulators often slow down or even stop due to discontinuities. There are many important applications where the phase behavior can be single phase, gas-liquid, liquid-liquid, gas-liquid-liquid or gas-liquid-solid at different times in different grid blocks. Assigning physically correct phase identities during a compositional simulation turns out to be a very difficult problem that has resisted a general solution for decades. We know the intensive thermodynamic properties such as molar Gibbs free energy must be continuous assuming local equilibrium, but this condition is difficult to impose in numerical simulators because of the discrete nature of the calculations. An alternative approach is to develop a relative permeability model that is continuous and independent of the phase numbers assigned by the flash calculation. Relative permeability is a function of saturation, but also composition since composition affects the phase distribution in the pores i.e. the wettability. The equilibrium distribution of fluids in pores corresponds to the minimum in the Gibbs free energy for the entire fluid-rock system including interfaces. However, in general this relationship is very difficult to model from first principles. What we can easily do is calculate the molar Gibbs free energy (G) of each phase at reference compositions where the relative permeabilities are known or assumed to be known and then interpolate between these values using the G calculated during each time step of the simulation. Relative permeability values calculated this way are unconditionally continuous for all possible phase behavior changes including even critical points. We tested the new relative permeability model on a variety of extremely difficult simulation problems with up to four phases and it has not failed yet. We illustrate several of these applications.
CO2 Injection in Sub-salt Water Layers at 7000m Depth
Authors: Eftekhari Ali Akbar , Bruining J. , Wahanik Helmut , Marchesin Dan 
Published: February 2011

Abstract
Huge oil deposits (&gt; 8 billion barrels) were recently discovered off the coast of Brazil below 2000 m of seawater and 1000–2000 m of reservoir, below 2000 m of salt layers. The high pressures (~ 700 bars) make these reservoirs excellent candidates for high pressure miscible gas injection, e.g., CO2 injection. For the geothermal gradients encountered in this area the temperatures are around 473 K. At high pressures it is possible that the mass density of carbon dioxide is higher than of oil, which will allow gravity stabilized injection. We consider the CO2-brine system, firstly for its storage as a greenhouse gas. Secondly, it is an initial step to describe displacement in the CO2-oil-brine system.
In this paper we study the phase behavior of carbon dioxide in a brine bearing layer at a temperature of 473 K and pressures between 600 and 800 bars. The quantification of the displacement process requires the thermodynamic equilibrium relations in the oil-water-salt-CO2 system at the prevailing pressures. Here we limit our interest to the thermodynamic behavior of the carbon dioxide-brine system. We show that it is possible to use the PRSV equation of state to estimate the composition of the vapor and aqueous phases. The PRSV is a modification of the Peng-Robinson equation of state. We use an activity-coefficient based mixing rule for the thermodynamic calculations. A volume shift procedure is applied to improve liquid density.
Quasi-M-matrix Anisotropic Darcy-Flux Finite-volume Approximation for General Grids and MPFA Decoupling Analysis
Authors: Edwards Michael G. , Zheng Hongwen 
Published: February 2011

Abstract
This paper focuses on flux-continuous general tensor pressure equation approximation for strongly anisotropic media. Earlier methods have focused on single-parameter pointwise schemes which are effective for smaller anisotropy ratios. However they have conditional M-matrices depending on the strength of the tensor cross-terms. For strongly anisotropic full tensor cases, the methods can yield pressure solutions with severe spurious oscillations. The spurious oscillations are shown to be caused by decoupling on both structured and unstructured grids. Flow results and convergence tests confirm the decoupling theory.
Anisotropic flux-continuous full pressure continuity formulations are presented. The formulations lead to improved discretization schemes for the complete range of elliptic tensors. The new schemes are shown to be positive definite, have quasi-positive M-matrices for full-tensor fields with large anisotropy ratios, and compute well resolved solutions that are essentially free of spurious oscillations.
Anisotropic Darcy-flux schemes are presented on structured, unstructured and anisotropy favoring grids. The methods are applied to a range of geological test cases and grid-types including arbitrarily high anisotropy ratio in two and three dimensions.
Finding a Place for Environmental Studies: Tertiary Institutions as a Locus of Practice for Education for Sustainability
Authors: Buchan John , Griffin Janette 
Published: February 2011

Finding a Place for Environmental Studies: Tertiary Institutions as a Locus of Practice for Education for Sustainability
Education for sustainability (EfS) involves not only curriculum, but also demands responses in terms of management of resources and of grounds. It is asserted here that inclusion of education for sustainability in the curriculum, whether in a school or university context, is hollow and insincere in the absence of practical and social action on site and perhaps beyond. The present study focuses on students' views of opportunities and barriers with regard to issues, such as maintenance of grounds and management of resources in a tertiary institution context. A cohort of approximately 140 third year primary teacher education students were surveyed to ascertain their views on the value of, barriers to and opportunities for practical sustainability projects conducted by students in their tertiary context. Such projects are a precursor to similar endeavours that could be undertaken in the students' school contexts.
Students' Independent Professional Activity in Pedagogical Practice
Authors: Strode Aina 
Published: February 2011

Students' Independent Professional Activity in Pedagogical PracticeThe topicality of the research is determined by the need for changes in higher education concerned with implementing the principles of sustainable education. The article focuses on teacher training, highlighting the teacher's profession as an attractive choice of one's career that permits to ensure the development of general and professional skills and an opportunity for new specialists to align with the labour market. The empirical study of students' understanding of their professional activity and of the conditions for its formation is conducted by applying structured interviews (of practice supervisors, students, academic staff); students and experts' questionnaire. Comparative analysis of quantitative and qualitative data and triangulation were used in case studies. As a result, a framework of pedagogical practice organisation has been created in order to form students' independent professional activity. The criteria and indicators of independent professional activity have been formulated and suggestions for designers of study programmes and organisers of the study process have been provided.
The Factors of Well-Being in Schools as a Living Environment According to Students' Evaluation
Authors: Kuurme Tiiu , Carlsson Anu 
Published: February 2011

The Factors of Well-Being in Schools as a Living Environment According to Students' EvaluationHow do students describe their quality of school life, based on their views about what is pleasant and unpleasant about the school? How is student welfare related to education for sustainability? Theories of the quality of school life and authentic identity constitute the theoretical background of this study. The school experiences of 185 Estonian and 161 Finnish students of different school types were studied by a semi-structured open questionnaire. The answers were analysed by the qualitative phenomenological method. The quality of school life of the students is diminished by the routine hierarchical working system and bad relations. It is supported by a cooperation-orientated atmosphere and meaningful learning experiences. The meaningfulness of school is sensed to be the most important factor in the students' evaluation of their school experiences.
Obtained General Competence of Students in Language Immersion Classes
Authors: Talts Leida , Kukk Airi , Muldma Maia 
Published: February 2011

Obtained General Competence of Students in Language Immersion ClassesOne of the key ideas of the language immersion methodology is to guarantee sustainable development for children from non-Estonian families in respect to their linguistic and social development. The most important conditions are creating flexible possibilities to participate in language immersion groups and classes and applying the language immersion methodology in everyday teaching process systematically. The present research seeks to highlight teachers' views on the sustainability of general competences of children who have completed the language immersion group in the kindergarten. The domain of general competences includes 32 sub-skills related to learning to learn and social skills, which were assessed by the teachers on a 5-point scale. For the sake of clear data analysis, the general competences have been divided into general competences fostering learning and learner-centred general competences. The present study showed that language immersion methodology favours applying child-centred pedagogy in the actual teaching process and supports the sustainability of children's linguistic and social development.
Washback Effects of Handouts on the Teaching and Learning Process in Higher Education Institutions in Ethiopia: Adama University in Focus
Authors: Egne Robsan 
Published: February 2011

Washback Effects of Handouts on the Teaching and Learning Process in Higher Education Institutions in Ethiopia: Adama University in Focus
The present study investigates the washback effects of handouts on the teaching and learning process in the higher education institutions of Ethiopia, particularly in Adama University. A descriptive survey and analytical research methods were employed in the present study. The subjects of the study included instructors and some selected students of the university. A questionnaire, an interview, a document and content analysis were employed to collect data. The data were analysed using quantitative and qualitative approaches. The data collected through close-ended items of the questionnaire were analysed quantitatively using a chi-square. Whereas the data collected via some interview guides and some items of the questionnaire were analysed using percentage. Besides, the qualitative data gathered via open-ended items of the questionnaire, some items of the interview, content and document analysis were analysed qualitatively. The research results revealed that the way handouts are being prepared and used in higher education institutions of Ethiopia does not encourage active and independent learning. Some recommendations which are deemed crucial for alleviating the problem are suggested.
Possible Mesolithic activity in the west of Arran: evidence from peat deposits
Authors: Robinson D 
Published: February 2011

Summary Pollen and charcoal analyses of peat cores from Machrie Moor in the west of Arran are presented. These reveal evidence for disturbance and burning of the vegetation during the Mesolithic period at a level radiocarbon dated to 8665±155bp (GU–1427). The nature of the fires and their possible sources, either natural or anthropogenic, are discussed in the light of data from other Scottish sites of this period. The conclusion drawn is that the fires were most probably of human origin and that Arran supported a Mesolithic population for which the archaeological evidence remains to be discovered.
Archaeological Investigations at Mavis Grind, Shetland
Authors: Cracknell Stephen , Smith Beverley 
Published: February 2011

SummaryThe excavations revealed a stone house and showed that it was oval, 13 m × 10 m, with an interior about 7 m in diameter. In the first occupation phase the entrance was on the SE side. During the second phase this entrance was replaced with one to the NE and the interior was partitioned. The roof was supported on wooden posts. After the building was abandoned it was covered with peat-ash which was subsequently ploughed. There were numerous finds of steatite-tempered pottery and stone implements, which dated the site to late Bronze/early Iron Age.The second settlement, Site B, lay by the shore of the voe and consisted of two possible stone-built houses and a field system. Two trenches were dug across the structures and the results are reported in Appendix I. Although damaged in recent years it was in no further danger.
A Survey of Two Deserted Settlements at Braleckan and Brenchoillie, Argyll
Authors: Grant Shan , Macdonald J , Swanson Carol , Wood J S 
Published: February 2011

Summary The detailed survey of two deserted townships in mid Argyll is described, and the available historical evidence for their date and function recounted. The communities seem to have existed during the 16th, 17th and part of the 18th centuries. The excavation of a corn-drying kiln is also described.
The Excavation of a Settlement of the Later Bronze Age and Iron Age at Myrehead, Falkirk District
Authors: Barclay G J 
Published: February 2011

SUMMARY Myrehead has revealed the eroded remnants of activity from the Beaker period (Period A) onwards, with actual settlement evinced only from about the early first millennium be. The three houses and the cooking pits of Period B may have been constructed and used sequentially. This open settlement was probably replaced during the mid first millennium bc, possibly without a break, by a palisaded enclosure (Period C), which may have contained a ring-groove house and a four-post structure. Continued domestic activity (Period D) was suggested by a single pit outside the enclosure, dated to the late first millennium bc/early first millennium ad. The limited evidence of the economy of the settlements suggests a mixed farming system.