Biofilm-induced carbonate precipitation at the pore-scale
Connolly, James Martin
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There are many methods available to decrease permeability in the subsurface but one that has been the subject of much research over the last decade is microbially-induced carbonate precipitation (MICP). In this process, microbial activity is promoted that increases pore water alkalinity. When calcium or other divalent cations are supplied to the system, solid carbonate minerals can form which occupy pore space and can decrease permeability. Permeability reduction can also come from microbial biofilms forming in the pore space. The goal of the work presented in this dissertation is to understand how pore space is affected, both physically and chemically, by biofilms and the precipitates that they can form. Fundamental research presented here is intended to inform ongoing application-based research and development. Previously it has been a challenge to image MICP at high resolution without the use of destructive techniques. To overcome that obstacle, a fluorescently-tagged bacterium capable of urea hydrolysis-driven MICP was constructed. Biofilms were grown in two-dimensional microscale porous media reactors and allowed to precipitate calcium carbonate under varied conditions. These reactors were imaged noninvasively using confocal microscopy so that both biofilms and carbonate minerals could be resolved at micrometer resolution. Image analysis was utilized to quantify how much pore space was occupied by the biofilm and minerals in order to estimate porosity reduction. Finally, pore-scale reactive transport modeling was utilized in order to estimate local concentrations within the reactors. The results show that the extent to which the porosity and permeability of the porous medium was decreased depended on when the calcium was added to the system. Also, periods of low flow were found to decrease porosity and permeability to a greater extent. This result adds to the evidence that a pulsed flow injection strategy may be most effective for permeability reduction via MICP in the subsurface. Additionally, reactive transport modeling predicts a heterogeneous mineral saturation environment at the pore-scale which highlights the challenge of predicting precipitation behavior in Darcy-scale reactive transport models.