Browsing by Author "Helmig, Rainer (Prof. Dr.)"

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    From discrete to continuum concepts of flow in fractured porous media
    (2012) Tatomir, Alexandru-Bogdan; Helmig, Rainer (Prof. Dr.)
    As more and more engineering applications require the correct simulation of flow and transport processes in porous media, and while many of these media present a certain degree of fracturing, this work deals with the development of numerical models that can simulate two-phase flow in large-scale fractured reservoirs. Among the applications which these models are addressing to, there are the estimation of contaminant spreading and removal, the reservoir exploitation, or more recently the CO2 sequestration, the geothermal reservoir exploitation, and the nuclear waste repositories. Fractured systems are ubiquitous around the world and can occur on avariety of lengths and scales which makes difficult the development of a general model that can handle easily all of them. The overall purpose of this work is to improve the understanding over the concepts of the multiphase flow and processes in the fractured porous media and develop a conceptual model that allows the study of two-phase flow in fractures of arbitrary size, orientation and shape. The flow models have been roughly classified in discrete fracturemodels (DFM), continuum fracturemodels (CFM) and hybrid. The DFMs consider the fractures explicitly and thereforerequirehuge computation power,whereas CFMs require the determination of arepresentative elementary volume (REV), the appropriate effective parameters and transfer functions between continua. For alarge scale problem, like aCO2 storage reservoir,there can be millions of fractures which might have to be considered and could be a formidable task for a DFM simulator.In this sense a continuum model, which is in this case in the form of a generalized dual-porosity representation, does not require the fine discretization of a DFM and the detailed fracturecharacterization during simulations. Thus, another goal of this work is to build areliable large scale multiphase flow simulator based on the continuum approach. Two flow simulators, 2pDFM and 2pMINC,have been developed and tested based on the two different fracture model concepts. Both simulators are implemented in the numerical toolbox DuMux.The 2pDFM model simulates the two-phase flow in fractured porous media using aDFM approach, with a lower dimensional representation for the fracture network (DFM-L). The model is capable to account for storage in the fractures. The 2pMINC model simulates two-phase flow in fractured porous media using the multiple interacting continua (MINC) method with an improved upscaling technique. The complex transient behavior of the flow processes in fractured porous media is captured by subgridding the coarse blocks in nested volume elements which have effective properties calculated from the detailed representation of the fracturesystem. In this way,the physically based approach is kept, the accuracy of the model is preserved ,the common use of empirically derived transfer functions is avoided and the complexity of the problem is considerably reduced which is reflected in the speedup factors up to 1000. This research extends the applicability range of the upscaling procedure to include entry pressure effects. Moreover,a general workflow has been developed for the numerical simulation of the two-phase flow in large-scale fractured porous media. The results are verified, validated and tested in a fully comprehensive way for both models. To test the behaviour of the simulator for field scale problems they are applied to an idealized medium with periodic fracture pattern and to areal, naturally fractured reservoir from the Bristol Channel. The evaluation shows that the extended MINC model is able to reproduce both, the large-scale permeability and the dynamics of the fracture-matrix mass transfer,correctly. The extended MINC method and the simulation procedure is flexible as it allows choosing the accuracy of the solution, the computation speed, and allows working with spatial information about the fracture system of various complexity and detail.
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    Scale dependence of flow and transport parameters in porous media
    (2006) Neuweiler, Insa; Helmig, Rainer (Prof. Dr.)
    The study discusses the problem of the influence of spatial scales on modeling of flow and transport processes in porous media. As soil and rock formations are usually heterogeneous, this problem is strongly coupled to the question as to how information about structure is transported over length scales. On different length scales different driving forces determine flow and transport processes. In order to derive an appropriate model for a certain scale of interest it is crucial to include information about the impact of structure on smaller scales into the model. The transfer from a detailed heterogeneous model to an equivalent model on a larger scale, where averaged properties are described, is called upscaling. To derive an upscaled model the following questions have to be addressed: • What are the relevant time scales and length scales? • What are the relevant processes on the scale, where heterogeneities are resolved? • How does the upscaled model look like? • What are the effective parameters for the upscaled model? As detailed information about the parameter distribution in the heterogeneous model is in most cases not known or it is not possible to deal with the exact distribution, further questions have to be addressed: • How can the effective parameters be approximated based on incomplete knowledge about the parameter distribution? • What are the relevant quantities to characterize the heterogeneous distribution and how can they be taken into account? Upscaling for different two-phase flow problems in porous media is discussed in the study.
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    Simulation von Mehrphasenvorgängen in porösen Medien unter Einbeziehung von Hysterese-Effekten
    (1999) Sheta, Hussam; Helmig, Rainer (Prof. Dr.)
    Die vorliegende Arbeit entstand im Rahmen des Forschungsprogramms der Versuchseinrichtung zur Grundwasser- und Altlastensanierung VEGAS. Sie beschäftigt sich mit einem zentralen Thema zur Beschreibung von Transport und Schadstoffausbreitung im natürtlichen Untergrund.
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    Theorie und numerische Modellierung nichtisothermer Mehrphasenprozesse in NAPL-kontaminierten porösen Medien
    (2001) Class, Holger; Helmig, Rainer (Prof. Dr.)
    In dieser Arbeit werden die physikalischen Fragestellungen als auch die mathematisch-numerische Modellbildung der nichtisothermen Mehrphasen- und Mehrkomponentenprozesse in porösen Medien aufbereitet.