Scott L. Painter

dfnWorks: A discrete fracture network framework for modeling subsurface flow and transport

Abstract dfn W orks is a parallelized computational suite to generate three-dimensional discrete fracture networks (DFN) and simulate flow and transport. Developed at Los Alamos National Laboratory over the past five years, it has been used to study flow and transport in fractured media at scales ranging from millimeters to kilometers. The networks are created and meshed using dfn G en , which combines fram (the feature rejection algorithm for meshing) methodology to stochastically generate three-dimensional DFNs with the L a G ri T meshing toolbox to create a high-quality computational mesh representation. The representation produces a conforming Delaunay triangulation suitable for high performance computing finite volume solvers in an intrinsically parallel fashion. Flow through the network is simulated in dfn F low , which utilizes the massively parallel subsurface flow and reactive transport finite volume code pflotran . A Lagrangian approach to simulating transport through the DFN is adopted within dfn T rans to determine pathlines and solute transport through the DFN. Example applications of this suite in the areas of nuclear waste repository science, hydraulic fracturing and CO2 sequestration are also included.

Conforming Delaunay Triangulation of Stochastically Generated Three Dimensional Discrete Fracture Networks: A Feature Rejection Algorithm for Meshing Strategy

We introduce the feature rejection algorithm for meshing (FRAM) to generate a high quality conforming Delaunay triangulation of a three-dimensional discrete fracture network (DFN). The geometric features (fractures, fracture intersections, spaces between fracture intersections, etc.) that must be resolved in a stochastically generated DFN typically span a wide range of spatial scales and make the efficient automated generation of high-quality meshes a challenge. To deal with these challenges, many previous approaches often deformed the DFN to align its features with a mesh through various techniques including redefining lines of intersection as stair step functions and distorting the fracture edges. In contrast, FRAM generates networks on which high-quality meshes occur automatically by constraining the generation of the network. The cornerstone of FRAM is prescribing a minimum length scale and then restricting the generation of the network to only create features of that size and larger. The process is f...

Permafrost degradation and subsurface-flow changes caused by surface warming trends

Change dynamics of permafrost thaw, and associated changes in subsurface flow and seepage into surface water, are analysed for different warming trends in soil temperature at the ground surface with a three-phase two-component flow system coupled to heat transport. Changes in annual, seasonal and extreme flows are analysed for three warming-temperature trends, representing simplified climate-change scenarios. The results support previous studies of reduced temporal variability of groundwater flow across all investigated trends. Decreased intra-annual flow variability may thus serve as an early indicator of permafrost degradation before longer-term changes in mean flows are notable. This is advantageous since hydrological data are considerably easier to obtain, may be available in longer time series, and generally reflect larger-scale conditions than direct permafrost observations. The results further show that permafrost degradation first leads to increasing water discharge, which then decreases as the permafrost degradation progresses further to total thaw. The most pronounced changes occur for minimum annual flows. The configuration considered represents subsurface discharge from a generic heterogeneous soil-type domain.RésuméLe changement de dynamiques du dégel du permafrost et les changements associés des écoulements souterrains et des exfiltration vers les eaux de surface sont analysés pour différentes tendances de réchauffement de la température du sol à la surface selon un système d’écoulement triphasé à deux composantes couplé au transport de la chaleur. Les changements des écoulements annuels, saisonniers et extrêmes sont analysés pour trois tendances d’élévation de la température représentant des scénarios simplifiés de changement climatique. Les résultats sont cohérents avec des études antérieures concernant la réduction de la variabilité temporelle des écoulements souterrains pour toutes les tendances simulées. La diminution de la variabilité des écoulements inter annuels peut ainsi servir d’indicateur précoce de la dégradation du permafrost avant que l’effet des changements sur le long terme soit notable sur les écoulements moyens. Ceci représente un avantage du fait que les données hydrologiques sont beaucoup plus faciles à obtenir, peuvent être disponibles pour de longues séries temporelles et reflètent généralement des évolutions à plus grande échelle que des observations directes sur le permafrost. Les résultats de plus montrent que la dégradation du permafrost conduit d’abord à une augmentation des débits dans les zones de décharge, puis à leur diminution lors de la progression de la dégradation du permafrost jusqu’au dégel complet. Les modifications les plus marquées se produisent pour les écoulements annuels minimums. La configuration considérée concerne le débit d’écoulement souterrain dans les zones de décharge pour un sol type générique hétérogène.ResumenSe analiza la dinámica del cambio en el descongelamiento del permafrost, y los cambios asociados al flujo subsuperficial y a la infiltración del agua superficial, para diferentes tendencias de calentamiento en la temperatura de los suelos en la superficie del terreno con tres fases, dos componentes del sistema de flujo acoplados con el transporte de calor. Los cambios en los flujos anual, estacional y extremos son analizados para tres tendencias de las temperaturas de calentamiento representando escenarios simplificados de cambios climáticos. Los resultados se apoyan en estudios previos de la reducida variabilidad temporal del flujo de agua subterránea a través de todas las tendencias investigadas. La disminución de la variabilidad del flujo interanual puede así servir como un indicador temprano de la degradación del permafrost antes que los cambios a largo plazo en los flujos medios sean notables. Esto es ventajoso ya que los datos hidrológicos son considerablemente más fáciles de obtener, pueden estar disponibles en series de tiempo más largas, y generalmente reflejan condiciones de una escala mayor que las observaciones directas del permafrost. Los resultados además muestran que la degradación del permafrost primero lleva a un aumento en la descarga del agua, que luego disminuye cuando la degradación del permafrost progresa más allá del descongelamiento total. Los cambios más pronunciados ocurren para los flujos mínimos anuales. La configuración considerada representa la descarga subsuperficial a partir de un dominio genérico de un suelo de tipo heterogéneo.摘要本文通过耦合热交换三相、二组份流动系统探讨地表面土壤温度不同增温条件下永久冻土消融,以及与之相关的地下径流改变及地表水渗漏的变化动力学。将气候变化场景简化为三种不同的增温趋势,分析多年、季节性以及极端的径流变化。所有研究趋势下的结果均支持地下水径流时间变化减少这一先前研究。因此在年均径流长期变化量显著前,年内径流变化量减少可作为永久冻土退化的一个早期信号。因为水文数据相对容易获得,可能存在长时间序列,比起直接的冻土测量,通常更能反映大规模的条件,因此更为有利。结果进一步表明,永久冻土退化首先导致排泄量增加,然后随着永久冻土进一步解冻,排泄量减少。最显著的变化发生在年均径流量最小的时候。考虑的结构代表了来自各向异性土壤类型主导的地下排泄区。ResumoA mudança da dinâmica do degelo no permafrost e as correspondentes alterações no escoamento subsuperficial e na drenagem para as águas superficiais são analisadas para diferentes tendências de aquecimento da temperatura do solo à superfície do terreno com um sistema de fluxo, acoplado a transporte de calor, para três fases e duas componentes. São analisadas as mudanças dos escoamentos anuais, sazonais e extremos para três tendências de aumento de temperaturas, representando cenários simplificados de alterações climáticas. Os resultados apoiam estudos anteriores que mostram a reduzida variabilidade temporal do escoamento de água, atravessando todas as tendências investigadas. A diminuição da variabilidade do escoamento intra-anual pode, portanto, servir como um indicador precoce da degradação do permafrost, antes de se notarem mudanças nos escoamentos médios a prazos mais longos. Isto é vantajoso uma vez que os dados hidrológicos são consideravelmente mais fáceis de obter, podendo estar disponíveis séries temporais mais longas e, em geral, refletindo uma maior escala temporal que as condições de observação direta do permafrost. Os resultados mostram ainda que a primeira degradação do permafrost leva ao aumento da descarga de água, a qual, em seguida, diminui à medida que a degradação do permafrost progride mais para o descongelamento total. As mudanças mais acentuadas ocorrem para os escoamentos anuais mínimos. A configuração considerada representa a descarga subsuperficial proveniente de um domínio genérico de solo heterogéneo.

Modeling challenges for predicting hydrologic response to degrading permafrost

The fate of the approximately 1,700 billion metric tons of carbon (Tarnocai et al. 2009) currently frozen in permafrost affected regions of the Arctic and subarctic is highly uncertain (IPCC 2007), primarily because of the potential for topographic evolution and resulting drainage network reorganization as permafrost degrades and massive ground ice contained in ice-rich permafrost soils melts. Computer modeling is a key tool in untangling these complex feedbacks to understand the evolution of the Arctic and subarctic landscapes and the potential feedbacks with the global climate system. Some of the challenges associated with modeling the hydrologic system in and around degrading permafrost are discussed in this essay. Modeling requirements depend very strongly on the spatial resolution of the model. Two different classes can be identified, depending on whether microtopography is explicitly resolved or incorporated into the model through a subgrid parameterization. The focus here is on the computational challenges associated with microtopography-resolving models using hydrologic response of polygon mires as an example. In such microtopography-resolving models, horizontal grid spacing on the order of 0.25mwould typically be required. Although highand low-centered ice wedge polygons have been identified as important controls on Arctic surface hydrology (e.g. Liljedahl et al. 2012) and evolution from lowto highcentered polygon landscapes is expected as Arctic temperatures increase (Jorgenson et al. 2006), as far as we are aware, there is no existing computer code that represents the full range of processes required to model the co-evolution of surface topography, active layer, and permafrost at the microtopography-resolving scale. For microtopography-resolving models of hydrology in permafrost landscapes, it is convenient to partition the large number of coupled processes into four critical sets: subsurface thermal/hydrology, surface thermal processes, mechanical deformation, and overland flow processes. However, it is important to recognize that the partitioning is somewhat arbitrary and that multiple tightly coupled processes exist within each set.

Particle tracking approach for transport in three-dimensional discrete fracture networks

The discrete fracture network (DFN) model is a method to mimic discrete pathways for fluid flow through a fractured low-permeable rock mass, and may be combined with particle tracking simulations to address solute transport. However, experience has shown that it is challenging to obtain accurate transport results in three-dimensional DFNs because of the high computational burden and difficulty in constructing a high-quality unstructured computational mesh on simulated fractures. We present a new particle tracking capability, which is adapted to control volume (Voronoi polygons) flow solutions on unstructured grids (Delaunay triangulations) on three-dimensional DFNs. The locally mass-conserving finite-volume approach eliminates mass balance-related problems during particle tracking. The scalar fluxes calculated for each control volume face by the flow solver are used to reconstruct a Darcy velocity at each control volume centroid. The groundwater velocities can then be continuously interpolated to any point in the domain of interest. The control volumes at fracture intersections are split into four pieces, and the velocity is reconstructed independently on each piece, which results in multiple groundwater velocities at the intersection, one for each fracture on each side of the intersection line. This technique enables detailed particle transport representation through a complex DFN structure. Verified for small DFNs, the new simulation capability enables numerical experiments on advective transport in large DFNs to be performed. We demonstrate this particle transport approach on a DFN model using parameters similar to those of crystalline rock at a proposed geologic repository for spent nuclear fuel in Forsmark, Sweden.

Pathline tracing on fully unstructured control-volume grids

The trend toward unstructured grids in subsurface flow modeling has prompted interest in the issue of streamline or pathline tracing on unstructured grids. Streamline tracing on unstructured grids is problematic because a continuous velocity field is required for the calculation, while numerical solutions to the groundwater flow equations provide velocity in discretized form only. A method for calculating flow streamlines or pathlines from a finite-volume flow solution is presented. The method uses an unconstrained least squares method on interior cells and a constrained least squares method on boundary cells to approximate cell-centered velocities, which can then be continuously interpolated to any point in the domain of interest. Two-dimensional tests demonstrate that the method correctly reproduces uniform and corner-to-corner flow on fully unstructured grids. In three dimensions using regular hexahedral grids, the method agrees well with established semianalytical methods. Tests also demonstrate that the method produces physically realistic results on fully unstructured three-dimensional grids.

Effect of advective flow in fractures and matrix diffusion on natural gas production

Although hydraulic fracturing has been used for natural gas production for the past couple of decades, there are significant uncertainties about the underlying mechanisms behind the production curves that are seen in the field. A discrete fracture network based reservoir-scale work flow is used to identify the relative effect of flow of gas in fractures and matrix diffusion on the production curve. With realistic three dimensional representations of fracture network geometry and aperture variability, simulated production decline curves qualitatively resemble observed production decline curves. The high initial peak of the production curve is controlled by advective fracture flow of free gas within the network and is sensitive to the fracture aperture variability. Matrix diffusion does not significantly affect the production decline curve in the first few years, but contributes to production after approximately 10 years. These results suggest that the initial flushing of gas-filled background fractures combined with highly heterogeneous flow paths to the production well are sufficient to explain observed initial production decline. Lastly, these results also suggest that matrix diffusion may support reduced production over longer time frames.

Managing complexity in simulations of land surface and near-surface processes

Increasing computing power and the growing role of simulation in Earth systems science have led to an increase in the number and complexity of processes in modern simulators. We present a multiphysics framework that specifies interfaces for coupled processes and automates weak and strong coupling strategies to manage this complexity. Process management is enabled by viewing the system of equations as a tree, where individual equations are associated with leaf nodes and coupling strategies with internal nodes. A dynamically generated dependency graph connects a variable to its dependencies, streamlining and automating model evaluation, easing model development, and ensuring models are modular and flexible. Additionally, the dependency graph is used to ensure that data requirements are consistent between all processes in a given simulation. Here we discuss the design and implementation of these concepts within the Arcos framework, and demonstrate their use for verification testing and hypothesis evaluation in numerical experiments. We describe a conceptual model for managing complexity in multiphysics software.A process tree describes how individual equations are coupled.A dependency graph describes how variables are dependent upon each other.The model is implemented within the Arcos software framework.Examples of code and model runs are shown to demonstrate the idea.

Influence of injection mode on transport properties in kilometer-scale three-dimensional discrete fracture networks

We investigate how the choice of injection mode impacts transport properties in kilometer-scale three-dimensional discrete fracture networks (DFN). The choice of injection mode, resident or flux-weighted, is designed to mimic different physical phenomena. It has been hypothesized that solute plumes injected under resident conditions evolve to behave similarly to solutes injected under flux-weighted conditions. Previously, computational limitations have prohibited the large scale simulations required to investigate this hypothesis. We investigate this hypothesis by using a high performance DFN suite, dfnWorks, to simulate flow in kilometer-scale three-dimensional DFNs based on fractured granite at the Forsmark site in Sweden, and adopt a Lagrangian approach to simulate transport therein. Results show that after traveling through a pre-equilibrium region both injection methods exhibit linear scaling of the first moment of travel time and power law scaling of the breakthrough curve with similar exponents, slightly larger than two. The physical mechanisms behind this evolution appear to be the combination of in-network channeling of mass into larger fractures, which offer reduced resistance to flow, and in-fracture channeling, which results from the topology of the DFN. This article is protected by copyright. All rights reserved.

Thermal effects of groundwater flow through subarctic fens: A case study based on field observations and numerical modeling

Modeling and observation of ground temperature dynamics are the main tools for understanding current permafrost thermal regimes and projecting future thaw. Until recently, most studies on permafrost have focused on vertical ground heat fluxes. Groundwater can transport heat in both lateral and vertical directions but its influence on ground temperatures at local scales in permafrost environments is not well understood. In this study we combine field observations from a subarctic fen in the sporadic permafrost zone with numerical simulations of coupled water and thermal fluxes. At the Tavvavuoma study site in northern Sweden, ground temperature profiles and groundwater levels were observed in boreholes. These observations were used to set up one- and two-dimensional simulations down to 2 m depth across a gradient of permafrost conditions within and surrounding the fen. Two-dimensional scenarios representing the fen under various hydraulic gradients were developed to quantify the influence of groundwater flow on ground temperature. Our observations suggest that lateral groundwater flow significantly affects ground temperatures. This is corroborated by modeling results that show seasonal ground ice melts 1 month earlier when a lateral groundwater flux is present. Further, although the thermal regime may be dominated by vertically conducted heat fluxes during most of the year, isolated high groundwater flow rate events such as the spring freshet are potentially important for ground temperatures. As sporadic permafrost environments often contain substantial portions of unfrozen ground with active groundwater flow paths, knowledge of this heat transport mechanism is important for understanding permafrost dynamics in these environments.