Mineral precipitation and dissolution in aqueous solutions has a significant effect on solute transport and structural properties of porous media. The understanding of the involved physical ...mechanisms, which cover a large range of spatial and temporal scales, plays a key role in several geochemical and industrial processes. Here, by coupling pore scale reactive transport simulations with classical nucleation theory, we demonstrate how the interplay between homogeneous and heterogeneous precipitation kinetics along with the non-linear dependence on solute concentration affects the evolution of the system. Such phenomena are usually neglected in pure macroscopic modelling. Comprehensive parametric analysis and comparison with laboratory experiments confirm that incorporation of detailed microscale physical processes in the models is compulsory. This sheds light on the inherent coupling mechanisms and bridges the gap between atomistic processes and macroscopic observations.
Diffusion of cations and other contaminants through clays is of central interest, because clays and clay rocks are widely considered as barrier materials for waste disposal sites. An intriguing ...experimental observation has been made in this context: Often, the diffusive flux of cations at trace concentrations is much larger and the retardation smaller than expected based on their sorption coefficients. So-called surface diffusion of sorbed cations has been invoked to explain the observations but remains a controversial issue. Moreover, the corresponding surface diffusion coefficients are largely unknown. Here we show that, by an appropriate scaling, published diffusion data covering a broad range of cations, clays, and chemical conditions can all be modeled satisfactorily by a surface diffusion model. The average mobility of sorbed cations seems to be primarily an intrinsic property of each cation that follows inversely its sorption affinity. With these surface mobilities, cation diffusion coefficients can now be estimated from those of water tracers. In pure clays at low salinities, surface diffusion can reduce the cation retardation by a factor of more than 1000.
In this paper we describe the OpenGeoSys (OGS) project, which is a scientific open-source initiative for numerical simulation of thermo-hydro-mechanical-chemical processes in porous media. The basic ...concept is to provide a flexible numerical framework (using primarily the Finite Element Method (FEM)) for solving multifield problems in porous and fractured media for applications in geoscience and hydrology. To this purpose OGS is based on an object-oriented FEM concept including a broad spectrum of interfaces for pre- and postprocessing. The OGS idea has been in development since the mid-eighties. We provide a short historical note about the continuous process of concept and software development having evolved through Fortran, C, and C++ implementations. The idea behind OGS is to provide an open platform to the community, outfitted with professional software-engineering tools such as platform-independent compiling and automated benchmarking. A comprehensive benchmarking book has been prepared for publication. Benchmarking has been proven to be a valuable tool for cooperation between different developer teams, for example, for code comparison and validation purposes (DEVOVALEX and CO
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BENCH projects). On one hand, object-orientation (OO) provides a suitable framework for distributed code development; however, the parallelization of OO codes still lacks efficiency. High-performance-computing efficiency of OO codes is subject to future research.
Porosity changes due to mineral dissolution–precipitation reactions in porous media and the resulting impact on transport parameters influence the evolution of natural geological environments or ...engineered underground barrier systems. In the absence of long-term experimental studies, reactive transport codes are used to evaluate the long-term evolution of engineered barrier systems and waste disposal in the deep underground. Examples for such problems are the long-term fate of CO
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in saline aquifers and mineral transformations that cause porosity changes at clay–concrete interfaces. For porosity clogging under a diffusive transport regime and for simple reaction networks, the accuracy of numerical codes can be verified against analytical solutions. For clogging problems with more complex chemical interactions and transport processes, numerical benchmarks are more suitable to assess model performance, the influence of thermodynamic data, and sensitivity to the reacting mineral phases. Such studies increase confidence in numerical model descriptions of more complex, engineered barrier systems. We propose a reactive transport benchmark, considering the advective–diffusive transport of solutes; the effect of liquid-phase density on liquid flow and advective transport; kinetically controlled dissolution–precipitation reactions causing porosity, permeability, and diffusivity changes; and the formation of a solid solution. We present and analyze the results of five participating reactive transport codes (i.e., CORE
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, MIN3P-THCm, OpenGeoSys-GEM, PFLOTRAN, and TOUGHREACT). In all cases, good agreement of the results was obtained.
Many applied problems in geoscience require knowledge about complex interactions between multiple physical and chemical processes in the sub-surface. As a direct experimental investigation is often ...not possible, numerical simulation is a common approach. The numerical analysis of coupled thermo-hydro-mechanical (THM) problems is computationally very expensive, and therefore the applicability of existing codes is still limited to simplified problems. In this paper we present a novel implementation of a parallel finite element method (FEM) for the numerical analysis of coupled THM problems in porous media. The computational task of the FEM is partitioned into sub-tasks by a priori domain decomposition. The sub-tasks are assigned to the CPU nodes concurrently. Parallelization is achieved by simultaneously establishing the sub-domain mesh topology, synchronously assembling linear equation systems in sub-domains and obtaining the overall solution with a sub-domain linear solver (parallel BiCGStab method with Jacobi pre-conditioner). The present parallelization method is implemented in an object-oriented way using MPI for inter-processor communication. The parallel code was successfully tested with a 2-D example from the international DECOVALEX benchmarking project. The achieved speed-up for a 3-D extension of the test example on different computers demonstrates the advantage of the present parallel scheme.
•SrSO4 replacement by BaSO4 was studied at the pore scale in a porous medium.•Spatially resolved μ-XRD data showed formation of two distinct BaSO4 generations.•BaSO4 was found to grow as nanocrystals ...in the pore and as an epitaxial rim on SrSO4.•The results could be partially explained using classical nucleation theory.
A reaction cell experiment was designed to examine mineral dissolution/precipitation processes both at the macroscopic and pore scale. A rectangular flow cell was filled with a reactive porous layer lying between two porous layers composed of quartz sand (SiO2). The reactive layer consisted of celestite (or celestine, SrSO4) with a bimodal grain size distribution (<63μm and 125–400μm). A barium chloride solution was then injected into the flow cell, leading to fast dissolution and replacement of celestite by barite (or baryte, BaSO4). Due to the higher molar volume of barite compared to celestite, the porosity decreased in the reactive layer. We concentrated on the refinement of post-mortem analysis and the investigation of the dissolution/precipitation mechanisms at the pore scale (10–100μm). The sequential evolution of mineral transformations occurring in the reactive layer was determined. Our analytical techniques, combining scanning electron microscopy and synchrotron X-ray microdiffraction/microfluorescence, showed that the small celestite grain fraction dissolved rapidly to form nano-crystalline barite filling the pore space, while large celestite grains were covered with a thin rim of epitaxial micro-crystalline barite. Two distinct nucleation mechanisms for barite precipitation were involved: homogeneous nucleation (nucleation of barite in the pore space) and heterogeneous nucleation (nucleation on the surface of a solid substrate). Classical nucleation theory, using well-established and estimated parameters (e.g. effective interfacial tension) describing barite nucleation, was applied to explain the mineralogical changes occurring in our system.
Transport experiments with colloids and radionuclides in a shear zone were conducted during the Colloid and Radionuclide Retardation experiment (CRR) at Nagra's Grimsel Test Site. Breakthrough curves ...of bentonite colloids and uranine, a non-sorbing solute, were measured in an asymmetric dipole flow field. The colloid breakthrough is earlier than that of uranine. Both breakthrough curves show anomalously long late time tails and the slope of the late time tails for the colloids is slightly higher. Anomalous late time tails are commonly associated with matrix diffusion processes; the diffusive interaction of solutes transported in open channels with the adjacent porous rock matrix or zones of stagnant water. The breakthrough curves for different colloid size classes are very similar and show no signs of fractionation due to their (size-dependent) diffusivity. It is proposed that tailing of the colloids is mainly caused by the structure of the flow field and that for the colloid transport, matrix diffusion is of minor importance. This has consequences for the interpretation of the uranine breakthrough. Comparisons of experimental results with numerical studies and with the evaluation of the colloid breakthrough with continuous time random theory imply that the tailing in the conservative solute breakthrough in this shear zone is not only caused by matrix diffusion. Part of the tailing can be attributed to advective transport in fracture networks and advection in low velocity regions. Models based on the advection–dispersion equation and matrix diffusion do not properly describe the temporal and spatial evolution of colloid and solute transport in such systems with a consistent set of parameters.
We examine a set of analytical solutions based on the continuous time random walk (CTRW) approach, which can be evaluated numerically and used to analyze breakthrough data from tracer tests. ...Practical application of these solutions, with discussion of the physical meaning of the relevant model parameters, is emphasized. The CTRW theory accounts for the often observed non‐Fickian (or scale‐dependent) dispersion behavior that cannot be properly quantified by using the advection‐dispersion equation. The solutions given here, valid for a wide range of dispersive behaviors of conservative tracers, and useful for both characterization and prediction, have been integrated into a library of external functions for use with the GRACE graphical display and analysis package. Example applications of these solutions are presented. The library and graphics software are freely accessible from a Web site.
We use numerical simulations to examine the variability of flow patterns in representative fracture intersection geometries. In contrast to existing studies of perfectly orthogonal intersections, we ...demonstrate that more realistic geometries lead to a rich spectrum of flow patterns. Moreover, numerical solutions of the Navier‐Stokes equations in these fracture intersections indicate that non‐linear inertial effects become important for Reynolds numbers as low as 1–100. Such Reynolds numbers often exist in naturally fractured formations, particularly in karst systems and in the vicinity of wells during pump tests.
We analyze a set of observations from a recently published, field-scale tracer test in a fractured till. These observations demonstrate a dominant, underlying non-Fickian behavior, which cannot be ...quantified using traditional modeling approaches. We use a continuous time random walk (CTRW) approach which thoroughly accounts for the measurements, and which is based on a physical picture of contaminant motion that is consistent with the geometric and hydraulic characterization of the fractured formation. We also incorporate convolution techniques in the CTRW theory, to consider transport between different regions containing distinct heterogeneity patterns. These results enhance the possibility that limitations in predicting non-Fickian modes of contaminant migration can be overcome.
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