391 research outputs found
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Capacity investigation of brine-bearing sands of the Frio Formation for geologic sequestration of CO2
The capacity of fluvial brine-bearing formations to sequester CO2 is investigated using numerical simulations of CO2 injection and storage. Capacity is defined as the volume fraction of the subsurface available for CO2 storage and is conceptualized as a product of factors that account for two-phase flow and transport processes, formation geometry, formation heterogeneity, and formation porosity. The space and time domains used to define capacity must be chosen with care to obtain meaningful results, especially when comparing different authors’ work. Physical factors that impact capacity include permeability anisotropy and relative permeability to CO2, brine/CO2 density and viscosity ratios, the shape of the trapping structure, formation porosity and the presence of low permeability layering.National Energy Technology LaboratoryBureau of Economic Geolog
Efficient computation of high index Sturm-Liouville eigenvalues for problems in physics
Finding the eigenvalues of a Sturm-Liouville problem can be a computationally
challenging task, especially when a large set of eigenvalues is computed, or
just when particularly large eigenvalues are sought. This is a consequence of
the highly oscillatory behaviour of the solutions corresponding to high
eigenvalues, which forces a naive integrator to take increasingly smaller
steps. We will discuss some techniques that yield uniform approximation over
the whole eigenvalue spectrum and can take large steps even for high
eigenvalues. In particular, we will focus on methods based on coefficient
approximation which replace the coefficient functions of the Sturm-Liouville
problem by simpler approximations and then solve the approximating problem. The
use of (modified) Magnus or Neumann integrators allows to extend the
coefficient approximation idea to higher order methods
An adaptive algorithm for n-body field expansions
An expansion of a density field or particle distribution in basis functions
which solve the Poisson equation both provides an easily parallelized n-body
force algorithm and simplifies perturbation theories. The expansion converges
quickly and provides the highest computational advantage if the lowest-order
potential-density pair in the basis looks like the unperturbed galaxy or
stellar system. Unfortunately, there are only a handful of such basis in the
literature which limits this advantage. This paper presents an algorithm for
deriving these bases to match a wide variety of galaxy models. The method is
based on efficient numerical solution of the Sturm-Liouville equation and can
be used for any geometry with a separable Laplacian. Two cases are described in
detail. First for the spherical case, the lowest order basis function pair may
be chosen to be exactly that of the underlying model. The profile may be cuspy
or have a core and truncated or of infinite extent. Secondly, the method yields
a three-dimensional cylindrical basis appropriate for studying galaxian disks.
In this case, the vertical and radial bases are coupled; the lowest order
radial part of the basis function can be chosen to match the underlying profile
only in the disk plane. Practically, this basis is still a very good match to
the overall disk profile and converges in a small number of terms.Comment: 16 pages, 5 figures, submitted to A
The precautions of clinical waste: disposable medical sharps in the United Kingdom
This article deals with recent changes in UK guidance on clinical waste, in particular a shift to disposable, single-use instruments and sharps. I use interviews conducted with nurses from a GP practice and two clinical waste managers at alternative treatment and incineration sites as a springboard for reflection on the relationship between the legislation on clinical waste management and its implementation. Scrutinizing the UK guidance, European legislation and World Health Organization principles, I draw out interviewees’ concerns that the changed practices lead to an expansion of the hazardous waste category, with an increased volume going to incineration. This raises questions regarding the regulations’ environmental and health effects, and regarding the precautionary approach embedded in the regulations. Tracing the diverse reverberations of the term ‘waste’ in different points along the journeys made by sharps in particular, and locating these questions in relation to existing literature on waste, I emphasize that public health rationales for the new practices are not made clear in the guidance. I suggest that this relative silence on the subject conceals both the uncertainties regarding the necessity for these means of managing the risks of infectious waste, and the tensions between policies of precautionary public health and environmental sustainability
Interaction between Injection Points during Hydraulic Fracturing
We present a model of the hydraulic fracturing of heterogeneous poroelastic
media. The formalism is an effective continuum model that captures the coupled
dynamics of the fluid pressure and the fractured rock matrix and models both
the tensile and shear failure of the rock. As an application of the formalism,
we study the geomechanical stress interaction between two injection points
during hydraulic fracturing (hydrofracking) and how this interaction influences
the fracturing process. For injection points that are separated by less than a
critical correlation length, we find that the fracturing process around each
point is strongly correlated with the position of the neighboring point. The
magnitude of the correlation length depends on the degree of heterogeneity of
the rock and is on the order of 30-45 m for rocks with low permeabilities. In
the strongly correlated regime, we predict a novel effective fracture-force
that attracts the fractures toward the neighboring injection point.Comment: Submitte
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On the development of MP-TOUGH2
The authors are developing MP-TOUGH2 for exploiting massively parallel computers. The goals of this effort are to (1) create a data-parallel subsurface transport code for solving larger problems than currently practical on workstations, (2) write portable code that can take advantage of scalability to run on machines with more processors, and (3) minimize the necessity for additional validation and verification of the resulting code. The initial strategy they have followed is to focus on optimizing the generic and time-consuming task of linear equation solution while leaving the bulk of TOUGH2 unmodified. In so doing, they have implemented a massively parallel direct solver (MPDS) that takes advantage of the banded structure of TOUGH2 Jacobian matrices. The authors have compared timings of the iterative conjugate gradient solvers DSLUBC, DSLUCS, and DSLUGM written in Fortran77 for the front end with the MPDS which uses the data parallel unit. The MPDS shows good performance relative to the iterative conjugate gradient solvers on the free-convection test problem. The robust direct solution provided by MPDS can be used to (1) check on the veracity of a given iterative conjugate gradient solution, or (2) be used on certain problems where iterative solvers fail to converge. The test problem used in this study is the Elder pure thermal convection problem. For generality, the authors use the TOUGH2 equation of state module EOS3 for water, air, and heat, although this particular problem is a single-phase flow problem
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Mathematical modeling of permeation grouting and subsurface barrier performance
The injection of solution grouts into the subsurface can be used to form underground barriers for the containment of contaminants. The technology requires identifying suitable grout materials, specifically fluids which exhibit a large increase in viscosity after injection and eventually solidify after a controllable period, thus sealing permeable zones. The authors have developed a new fluid property module for the reservoir simulator TOUGH2 to model grout injection, taking into account the increase of liquid viscosity as a function of time and gel concentration. They have also incorporated into the simulator a model which calculates soil hydraulic properties after solidification of the gel within the pore space. The new fluid property module has been used to design and analyze laboratory experiments and field pilot tests in saturated and unsaturated formations under a variety of subsurface conditions. These applications include modeling barrier emplacement in highly heterogeneous soils in the vadose zone, grout injection into the saturated zone in combination with extraction wells for flow control, the design of verification strategies, and the analysis of barrier performance. In this paper the authors discuss the modeling approach and present simulation results of multiple grout injections into a heterogeneous, unsaturated formation
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Capacity investigation of brine-bearing sands of the Fwwm formation for geologic sequestration of CO{sub 2}
The capacity of fluvial brine-bearing formations to sequester CO{sub 2} is investigated using numerical simulations of CO{sub 2} injection and storage. Capacity is defined as the volume fraction of the subsurface available for CO{sub 2} storage and is conceptualized as a product of factors that account for two-phase flow and transport processes, formation geometry, formation heterogeneity, and formation porosity. The space and time domains used to define capacity must be chosen with care to obtain meaningful results, especially when comparing different authors' work. Physical factors that impact capacity include permeability anisotropy and relative permeability to CO{sub 2}, brine/CO{sub 2} density and viscosity ratios, the shape of the trapping structure, formation porosity and the presence of low-permeability layering
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