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Institute: search.filters.UBSInstitut.Institut für Wasser- und UmweltsystemmodellierungSubject: search.filters.subject.550Start date: 2002End date: 2009

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    Modelling of biofilm growth and its influence on CO2 and water (two-phase) flow in porous media
    (2009) Ebigbo, Anozie; Helmig, Rainer (Prof. Dr.-Ing.)
    Bacterial biofilms are groups of microbial cells attached to surfaces and to each other. Cells in a biofilm are protected from adverse external conditions. In natural environments, this attached mode of growth is more successful than the suspended mode, and a major portion of microbial activity takes place at surfaces. In porous media, biofilms are used as bioreactors (e.g, in wastewater treatment) and as biobarriers (e.g., in enhanced oil recovery). They are also used in the containment and degradation of contaminants in groundwater aquifers. It has been proposed that biofilms be used as biobarriers for the mitigation of carbon dioxide (CO2) leakage from a geological storage reservoir. The concentration of greenhouse gases -- particularly carbon dioxide (CO2) -- in the atmosphere has been on the rise in the past decades. One of the methods which have been proposed to help reduce anthropogenic CO2 emissions is the capture of CO2 from large, stationary point sources and storage in deep geological formations. The caprock is an impermeable geological layer which prevents the leakage of stored CO2, and its integrity is of utmost importance for storage security. As mentioned above, biofilms could be used as biobarriers which help prevent the leakage of CO2 through the caprock in injection well vicinity. Due to the high pressure build-up during injection, the caprock in the vicinity of the well is particularly at risk of fracturing. The biofilm could also protect well cement from corrosion by CO2-rich brine. The goal of this work is to develop and test a numerical model which is capable of simulating the development of a biofilm in a CO2 storage reservoir. This involves the description of the growth of the biofilm, flow and transport in the geological formation, and the interaction between the biofilm and the flow processes. Important processes which are accounted for in the model include the effect of biofilm growth on the permeability of the formation, the hazardous effect of supercritical CO2 on suspended and attached bacteria, attachment and detachment of biomass, and two-phase fluid flow processes. The partial differential equations which describe the system are discretised in space with a vertex-centered finite volume method, and an implicit Euler scheme is used for time discretisation. The model is tested by comparing simulation results to experimental data. In a test case simulation, the model predicts the extent of biomass accumulation near an injection well and its effect on the permeability of the formation. The simulations show that the biobarrier is only effective for a limited amount of time. Regular injection of nutrients would be necessary to sustain the biofilm. In future work, the model could be extended to account for the active precipitation of minerals by the biofilm which would lead to a more enduring barrier. The model also needs to be extended to account for more than one growth-limiting factor. This would allow for the simulation of injection strategies which aim at growing a biofilm at some distance from the injection well.
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    ItemOpen Access
    Numerical Modelling of Stratification in Lake Constance with the 1-D hydrodynamic model DYRESM
    (2002) Hornung, Ralf
    Numerical models assist in the understanding of the complex interactions of physical, chemical and biological processes taking place in a lake. A validated model may be employed as a tool in the management of a lake. For example, the model may be capable to predict the consequences of a climate change, a reduced nutrient load or the fate of a contaminant spilled into the lake. While a three-dimensional model is commonly used for detailed but short simulations of single events, the one-dimensional model DYRESM is used for seasonal or long-term simulations of the vertical salinity and temperature distribution. The accurate simulation of salinity and temperature is a crucial prerequisite to couple a water-quality model to DYRESM that models nutrients, plankton and suspended solids. DYRESM is virtually calibration free and has a very short run time. Lake Constance (in German: Bodensee) is an important water resource as it supplies high quality drinking water for several million people. The goal of this Master's thesis was to test DYRESM for its applicability for long-term simulations at Lake Constance. Almost all available data required to run and validate DYRESM since 1960 were collected and carefully interpreted. The standard version of DYRESM, however, was not capable to model stratification in Lake Constance with sufficient accuracy. It was observed that the simulated thermocline had much stronger gradients than in reality. A DYRESM code extended with an algorithm that parameterizes internal and benthic boundary layer mixing by two non-dimensional numbers was then applied. The fundamental relationships of the algorithm state that the degree of vertical turbulent diffusion is inversely proportional to the Lake number and that the distribution of internal mixing to benthic boundary layer mixing is dependent on the Burger number. It was then shown that the smooth thermocline frequently observed at Lake Constance can be simulated successfully with the implemented mixing algorithm. Hence, the one-dimensional model DYRESM confirmed the enhanced metalimnetic mixing of Lake Constance that has been observed by other researchers. The sound data base and the successful application of DYRESM enables the continuation of this work by running the water-quality model CAEDYM for Lake Constance.