bluuebunny/crossref_metadata_2025_split huggingface dataset:
A Generalized Framework Model for Simulation of Gas Production in Unconventional Gas Reservoirs
Authors: Wu Yu-Shu , Li Jianfang , Ding Didier , Wang Cong , Di Yuan 
Published: March 2013

Abstract
Unconventional gas resources from tight sand and shale gas reservoirs have received great attention in the past decade around the world, because of their large reserves as well as technical advances in developing these resources. As a result of improved horizontal drilling and hydraulic fracturing technologies, the progresses are being made towards commercial gas production from such reservoirs, as demonstrated in the US. However, understandings and technologies needed for effective development of unconventional reservoirs are far behind the industry needs, e.g., gas recovery rates from those unconventional resources remain very low. There are some efforts in the literature on how to model gas flow in shale gas reservoirs using various approaches from modified commercial simulators to simplified analytical solutions, leading to limited success. Compared with conventional reservoirs, gas flow in ultra-low permeability unconventional reservoirs is subject to more nonlinear, coupled processes, including nonlinear adsorption/desorption, non-Darcy flow (at high flow rate and low flow rate), and strong rock-fluid interaction, and rock deformation within nano-pores or micro-fractures, coexisting with complex flow geometry and multi-scaled heterogeneity. Therefore, quantifying flow in unconventional gas reservoirs has been a significant challenge and traditional REV-based Darcy law, for example, may not be in general applicable.
In this paper, we will discuss a generalized mathematical model and numerical approach for unconventional gas reservoir simulation. We will present a unified framework model able to incorporate all known mechanisms and processes for two-phase gas flow and transport in shale gas or tight gas formations. The model and numerical scheme are based on generalized flow models using unstructured grids. We will discuss the numerical implementation of the mathematical model and show results of our model verification effort. Specifically, we discuss a multi-domain, multi-continuum concept for handling multi-scaled heterogeneity and fractures, i.e., using hybrid modeling approaches to describe different types and scales of fractures from explicitly modeling of hydraulic fractures and fracture network in simulated reservoir volume (SRV) to distributed naturally fractures, microfractures, and tight matrix. We will demonstrate model application to quantify hydraulic fractures and transient flow behavior in shale gas reservoirs.
An Optimization Algorithm Based on Combining Finite-Difference Approximations and Stochastic Gradients
Authors: Yan Xia , Reynolds Albert C. 
Published: March 2013

Abstract
Optimization algorithms that incorporate a stochastic gradient (SPSA, simplex, EnOpt) are easy to implement in conjunction with any reservoir simulator. However, for realistic problems, a stochastic gradient only provides a rough approximation of the true gradient, and, in particular, the angle between a stochastic gradient and the associated true gradient is typically far from zero even though a properly computed stochastic gradient usually represents an uphill direction.
This paper develops a more robust optimization procedure by replacing the largest components in magnitude of the stochastic gradient with a finite-difference approximation of the pertinent partial derivatives. Specifically, after ranking the magnitudes of the components of the stochastic gradient from largest to smallest in magnitude, we replace, in this same order, components of the stochastic gradient with finite difference approximations until we obtain a modified stochastic gradient which is in approximately the same direction as the unknown true gradient. The viability of the method relies on a theoretical result which allows us to approximate the angle between the modified stochastic gradient and the true gradient without knowledge of the true gradient. With this simple procedure, one can continue to replace components of the stochastic gradient with finite different approximations until one obtains a modified stochastic gradient where the angle between it and the unknown true gradient is sufficiently small. As the stochastic gradient is approximate, the n largest component magnitudes of the stochastic gradient do not exactly correspond to the n components of largest magnitude of the true gradient. However, another theoretical result allows us to approximately determine whether a dominant component of the stochastic gradient corresponds to a dominant component of the true gradient. If this correspondence does not hold, one option is to attempt to compute a more accurate stochastic gradient. In essence, the objective of the method is to determine which components of the unknown true gradient are most important and then replace the corresponding components of the stochastic gradient with more accurate finite- difference approximations. This modified gradient can then be used in a gradient-based optimization algorithm to find the minimum or maximum of a given cost function. In this work, we consider only a well known mathematical optimization test problem and published problems on estimating optimal well controls in closed-loop reservoir management although it is clear that the method could also be used for other applications including history matching.
Multiscale Formulations with CVD-MPFA Schemes on Structured and Unstructured Grids
Authors: Parramore Elliot , Edwards Michael G. , Lamine Sadok , Pal Mayur 
Published: March 2013

Abstract
This paper focuses on methods for fine scale modelling and presents development of multiscale methods with particular emphasis on the numerical flux approximation. Families of Darcy-flux approximations have been developed for consistent approximation of the general tensor pressure equation arising from Darcys law together with mass conservation. The schemes are control- volume distributed (CVD) with flow variables and rock properties sharing the same location in a given control-volume and are comprised of a multipoint flux family formulation (CVD-MPFA). The schemes are used to develop a CVD-MPFA based multiscale formulation applicable to both structured and unstructured grids in two-dimensions.
Adaptive-Implicit Strategy for Treating Velocity-Dependent Mobilities in Reservoir Simulation
Authors: de Loubens Romain , Patacchini Leonardo , Moncorgé Arthur 
Published: March 2013

Abstract
In petroleum reservoirs, it is common to assume that the superficial velocity of a fluid phase equals a mobility coefficient (permeability over viscosity) multiplying the opposite of the pressure gradient. Phase mobility is typically a function of phase saturations, compositions, temperature, and to a lesser extent pressure, but in specific circumstances it may also depend on the velocity itself. For example, in polymer flooding the aqueous phase viscosity is velocity-dependent due to non-Newtonian effects.
The numerical treatment of velocity-dependent mobilities in reservoir simulators based on the finite-volumes method is still an unsettled topic. For a fully implicit discretization, the main difficulty lies in the fact that velocities are not necessarily aligned with the grid; therefore the velocity-dependent mobility governing the flux across a cell face is not merely a function of the normal pressure drop. In addition, the implicit relationship between velocity and pressure gradient needs to be inverted for each flux, at each nonlinear iteration. To get around the above difficulties, several commercial or academic simulators implement a semi-implicit scheme where the pressure gradient driving the flow is evaluated implicitly, while the velocity used in mobility calculations is evaluated explicitly based on the previously converged time step. However, a semi-implicit formulation may be subject to stability restrictions.
In this work, we first review the derivation of a linear stability criterion for the two-level semi-implicit discretization of simplified monophasic, non-Newtonian or non-Darcy flow equations. Based on this criterion, we propose an adaptive-implicit strategy where for each individual flux the velocity argument in the mobility function is evaluated either explicitly or implicitly. We discuss the numerical accuracy of this scheme and its benefits in terms of computational cost. Finally, using a specifically designed MATLAB code, we validate our adaptive-implicit strategy on representative 1D and 2D nonlinear test cases.
Upscaling of Geomechanics in Heterogeneous Compacting Reservoirs
Authors: Settari A. (Tony) , Al-Ruwaili K. , Sen V. 
Published: March 2013

Abstract
Upscaling of properties for reservoir simulation has reached a stage of maturity and uses sophisticated techniques. In contrast, little work has been done on upscaling of mechanical properties for coupled modeling, and the geomechanical model is usually assumed to be representative (upscaled) without actually being subjected to the same rigor of process or scrutiny. Compacting reservoirs often contain fine sand-shale sequences on sub-grid scale (compared to flow modeling grid) and are typically represented in simulators by the net-to-gross (NTG) concept, while being ignored in geomechanical modeling.
In this work, we present a method to upscale shales in the geomechanical component of a coupled simulation by computing dynamically changing equivalent moduli. The method is based on estimating the depletion of interbedded shales, coupled with analytical solutions for equivalent moduli under the assumption of uniaxial deformation. The technique has been verified by sub-grid scale simulations and requires geometric characterization of the shales but is relatively simple and can be easily implemented in coupled simulators. The analysis shows that the inclusion of the shales generally reduces computed compaction, with the controlling variables being NTG, dimensionless pressure depletion of the shales (which in turn depends on their flow properties) and mechanical properties of the shales.
The approach developed was incorporated in the subsidence and compaction analysis over a complex offshore oil reservoir. The reservoir zone consisted of a number of intervals that included relatively undeformed as well as highly deformed layers, and on reservoir model scale had significant NTG ratios. A comprehensive study (based on coupled flow and geomechanics simulation) was conducted to evaluate well integrity, fault slip (reactivation) and compaction drive. A large full field model was built using data from a multitude of sources and production data was modeled and history matched, allowing us to estimate the magnitude of reservoir subsidence and the contribution of the NTG effect on predictions. The study presented here showed that in the absence of any consideration of the NTG effect, predictions (for compaction and subsidence) could be significantly overestimated. In addition, the method can be used to study the phenomena of "time-lag" of subsidence which has been observed in some reservoirs.
An Adaptive Hierarchical Algorithm for Estimation of Optimal Well Controls
Authors: Oliveira D. F. , Reynolds A. 
Published: March 2013

Abstract
In determining the optimal well controls by maximizing net-present-value (NPV) for the remaining life of a reservoir, one typically defines the length of the control steps a priori. Moreover, these control steps are often the same for all wells. We provide a scale splitting/merging method for adaptively selecting the lengths of control steps as the overall optimization proceeds. We start with reasonably small number of control steps and find the associated optimal controls by maximizing NPV. Then, using the control values of the current best estimate for the optimum solution and at most two gradient calculations, we decide which set of control steps will potentially yield the greatest improvement in NPV without unnecessarily increasing the number of control variables. Once new control steps have been defined by refining and/or coarsening the control steps at the previous stage, we again maximize NPV. This process of adaptively selecting control steps and optimizing is continued until no further improvement in NPV is achieved. Both adjoint-gradient-based and derivative- free optimization algorithms are considered. The adaptive approach is applied to two example problems and the results are compared with those obtained using a pre-determined number of control steps.
Parallel Reservoir Simulation Using a Specific Software Framework
Authors: Maliassov S. , Beckner B. , Dyadechko V. 
Published: March 2013

Abstract
Numerical simulation of reservoirs is an integral part of geoscientific studies to optimize petroleum recovery. Modern petroleum reservoir simulation requires simulating detailed and computationally expensive geological and physical models. Parallel reservoir simulators have the potential to solve larger, more realistic problems than previously possible.
To make the solution of these large problems feasible, an efficient parallel implementation of the algorithm is necessary. Such a parallelization of the algorithm requires proper data structures and data layout, some parallel direct and iterative solvers, and some parallel preconditioners. For the development of parallel reservoir simulator, we found the Trilinos Project to be an efficient environment to develop such a complex parallel application.
In this paper, we study the development of a parallel 3-phase black oil reservoir simulator on unstructured meshes. Our work is based on the Trilinos project which provides an object-oriented software framework of integrated algorithms for the solution of large-scale complex multi-physics engineering and scientific problems on massively parallel computing platforms. Trilinos addresses two fundamental issues of developing software for these problems: (i) providing a streamlined process and set of tools for development of new algorithmic implementations and (ii) promoting interoperability of independently developed software packages.
The ability to rapidly incorporate different packages and methods in the Trilinos framework allowed leveraging implementation of computationally intensive modules and development of reservoir simulator in remarkably short timeframe, reducing the development cycle from months to weeks. Our study illustrates how the combination of very efficient state-of-the-art nonlinear solvers with robust partitioning methods, sparse linear solvers, and preconditioning techniques implemented in the Trilinos framework can be used to prototype new computational technologies in a very time efficient manner. Computational tests demonstrate that leveraging parallel computational modules form Trilinos produced computationally efficient and scalable parallel reservoir simulation.
Efficient Coupling for Nonlinear Multiphase Flow with Strong Gravity
Authors: Shahvali M. , Tchelepi H. A. 
Published: March 2013

Abstract
The Fully Implicit Method (FIM) is widely used to discretize the nonlinear conservation equations that govern multiphase flow in porous media. Although FIM is unconditionally stable, the nonlinear solver, which is typically based on Newton’s method, often fails to converge for large time steps. As the time step size increases, the complexity of the nonlinear coupling increases. In the presence of strong buoyancy in a heterogeneous domain, for example, counter-current flow may be present, and phase-flow reversal - as a function of time or Newton iteration - becomes an issue. It does not take too many interfaces with flow reversal across them to make the nonlinear solver stall. Even when the Newton-based iterative procedure converges, the cost associated with updating all the unknowns simultaneously - everywhere and all the time - can be quite expensive. Conventional sequential-implicit strategies can be used to reduce the cost, but they suffer from severe restrictions on the allowable timestep size. We propose a new nonlinear solution strategy to improve the robustness and efficiency of FIM. Specifically, for each FIM Newton iteration, we update pressure by solving a linear system obtained from the full Jacobian. Then, we construct a directed graph with grid-cells as nodes and phase fluxes at grid-cell interfaces as edges, to identify the counter-current flow regions. After that, we employ a topological ordering that makes it possible to visit the components once, from upstream to downstream. The graph components vary in size from a single cell to multiple ‘strongly connected’ cells. For each component made up of a single cell, we update the saturation nonlinearly. For each component made up of multiple cells (strongly connected region), we perform a simultaneous linear update of both saturation and pressure. The saturation iterates are safeguarded based on ‘trust regions’ of the flux (fractional flow) functions. We compare the new solver with the current state-of-the-art for a wide range of heterogeneous problems with special focus on counter-current flow due to buoyancy. Our results show that the proposed FIM nonlinear solver converges for large time steps, with excellent convergence rates and lower computational cost per iteration compared with standard or safeguarded Newton.
Formulation and Solution of Compositional Displacements in Tie-Simplex Space
Authors: Zaydullin R. , Voskov D. V. , Tchelepi H. A. 
Published: March 2013

Abstract
We describe an Adaptive Compositional Space Parametrization (ACSP) method for compositional flow simulation. The method is based on casting the nonlinear governing equations, including those that describe the thermodynamic phase equilibrium, in terms of the tie-simplex (tie-lines for two-phase flow) space. The tie-simplex compositional space, which is a function of composition, pressure, and temperature, is then discretized using a limited number of tie-simplexes. The coefficients in the governing system of equations, including the composition, density, and mobility of the phases, are computed using multilinear interpolation in the discretized space.
ACSP is different from the CSAT (Compositional Space Adaptive Tabulation) approach, which we have evolved over the last few years. In CSAT, the tie-line (tie-simplex) information, which is stored and updated adaptively, is used to accelerate the equation-of-state (EOS) computations in standard (e.g., natural variables) compositional reservoir simulators. This is why CSAT was used primarily to essentially replace the phase-stability test in the natural-variables formulation, but not the flash procedure. In ACSP, on the other hand, the full mathematical statement is cast in tie-simplex space and consistent discretization is applied directly. The ACSP framework is supplemented with a new tie-line based flash procedure that avoids the use of the Rachford-Rice equation. Thus, ACSP replaces all the standard EOS computations (phase-stability and flash). A grid refinement study of the ACSP method shows that the discrete formulation is convergent; more importantly, even for highly nonlinear near-miscible displacements, the number of tie-simplex needed is small.
Monoclonal Anti-TNF-α Antibodies for Severe Steroid-Dependent Asthma: A Case Series
Authors: Taillé Camille , Poulet Claire , Marchand-Adam Sylvain , Borie Raphaël , Dombret Marie-Christine , Crestani Bruno , Aubier Michel 
Published: March 2013

Background:
Refractory asthma represents an important condition, with considerable morbidity and mortality. Tumor necrosis factor α (TNF-α) is a potential target for treatment of severe asthma. However, controlled studies have shown controversial results and the risk-benefit profile of TNF-blocking agents is still debated.

Objectives:
To describe the effect of infliximab on asthma control in patients with severe, uncontrolled, steroid-dependent asthma.

Methods:
From 2007 to 2010, 7 patients received infliximab in our center. All had severe refractory asthma, with frequent severe exacerbations and hospitalizations in the intensive care unit despite maximal inhaled treatment, daily oral steroids and omalizumab treatment.

Results:
Asthma control improved in the 6 patients who received infliximab for at least 3 months. Oral steroids could be stopped in 4 and the frequency of exacerbations and hospitalizations was greatly reduced, especially for the 3 patients with brittle asthma. Two patients showed severe adverse effects (bacterial pneumonia and extension of spreading melanoma). Three patients have received infliximab for more than 2 years, with good tolerance.

Conclusion:
This case series suggests that anti-TNF-α drugs may improve the condition of a subgroup of patients with severe steroid-refractory asthma, with a favourable risk-benefit profile for most, considering asthma severity, occurrence of life-threatening exacerbations and complications of long-term oral steroids. Specific controlled trials of this subgroup are warranted.
Isometries of some F-algebras of holomorphic functions on the upper half plane
Authors: Iida Yasuo , Takahashi Kei 
Published: March 2013

Abstract
Linear isometries of N
p(D) onto N
p(D) are described, where N
p(D), p > 1, is the set of all holomorphic functions f on the upper half plane D = {z ∈ ℂ: Im z > 0} such that supy>0 ∫ℝ lnp (1 + |(x + iy)|) dx < +∞. Our result is an improvement of the results by D.A. Efimov.
Solvability of a mathematical model of dissociative adsorption and associative desorption type
Authors: Ambrazevičius Algirdas , Eismontaitė Alicija 
Published: March 2013

Abstract
A mathematical model of dissociative adsorption and associative desorption for diatomic molecules is generalized. The model is described by a coupled system of parabolic and ordinary differential equations. The existence and uniqueness theorem of the classical solution is proved.
First (fuzzy) Hopf map from irreps of SU(2)
Authors: Fakhri Hossein , Lotfizadeh Mehdi 
Published: March 2013

Abstract
Using the spherical basis of the spin-ν operator, together with an appropriate normalized complex (2ν +1)-spinor on S
3 we obtain spin-ν representation of the U(1) Hopf fibration S
3 → S
2 as well as its associated fuzzy version. Also, to realize the first Hopf map via the spherical basis of the spin-1 operator with even winding numbers, we present an appropriate normalized complex three-spinor. We put the winding numbers in one-to-one correspondence with the monopole charges corresponding to different associated complex vector bundles.
Analysing global food waste problem: pinpointing the facts and estimating the energy content
Authors: Melikoglu Mehmet , Lin Carol , Webb Colin 
Published: March 2013

Abstract
Food waste is a global problem. Each year food worth billions of dollars is wasted by the developed economies of the world. When food is wasted, the problem does not end at that point. More than 95% of the food waste ends at landfill sites, where converted into methane, carbon dioxide and other greenhouse gasses by anaerobic digestion. The impact of food waste to climate change is catastrophic. Food waste problem tends to increase in next 25 years due to economic and population growth mainly in Asian countries. In addition, when food wastes buried at landfill sites their energy content is lost. Although food waste is a huge problem, its global size and extent has recently become a hot topic in the academic community. This paper summarises the size of the global food waste problem together with the estimation of the amount of energy lost when food wastes dumped at landfill sites. Calculations in this study also revealed that energy lost at landfill sites equals to 43% of the delivered energy used for the preparation of foods in the US, 37% of the hydroelectric power generation of Japan, and more than 100% of the current annual renewable energy demand of UK industries.