Browsing by Author "Ebigbo, Anozie"

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Open Access
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.