Biological sequestration of carbon dioxide (CO2) in geological formations is one of the proposed methods to reduce the carbon dioxide released into the atmosphere. In this method, an enzyme is used to enhance the hydration and subsequent precipitation of CO2. In the present work, the effect of bovine carbonic anhydrase on the hydration of CO2, and its precipitation in the form of calcium carbonate, was studied. The enzyme enhanced the hydration reaction. The rate of hydration reaction increased with both the enzyme concentration and temperature. The precipitation of calcium carbonate was promoted in the presence of the enzyme. The concentration of the enzyme did not affect the precipitation; however, temperature impacted the precipitation of calcium carbonate. At higher temperatures, less calcium carbonate was formed. Also, in the presence of the enzyme, calcium carbonate settled more quickly. The enzyme activity was not influenced by the pH of the reaction mixture. In contrast, the formation of calcium carbonate was affected by the pH of the solution. A kinetic analysis was performed for the bovine carbonic anhydrase. Based on the experimental results, the activation energy and catalytic rate constant are estimated as 700.91 cal/mol and 0.65 s-1, respectively.

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Successful operation of sulphate-reducing bioreactors using complex organic materials depends on providing a balance between more easily degrading material that achieves reasonable kinetics and low hydraulic retention times, and more slowly decomposing material that sustains performance in the long term. In this study, two organic mixtures containing the same ingredients typical of bioreactors used at mine sites (woodchips, hay and cow manure) but with different ratios of wood (recalcitrant) to hay (labile) were tested in six continuous flow bioreactors treating synthetic mine-affected water containing 600 mg/L of sulphate and 15 μg/L of selenium. The reactors were operated for short (5-6 months) and long (435-450 days) periods of time at the same hydraulic retention time of 15 days. There were no differences in the performance of the bioreactors in terms of sulphate-reduction over the short term, but the wood-rich bioreactors experienced variable and sometimes unreliable sulphate-reduction over the long term. Presence of more hay in the organic mixture was able to better sustain reliable performance. Production of dissolved organic compounds due to biodegradation within the bioreactors was detected for the first 175-230 days, after which their depletion coincided with a crash phase observed in the wood-rich bioreactors only.

Successful long-term bioremediation of mining-influenced water using complex organic matter and naturally-occurring microorganisms in sub-surface flow constructed wetlands requires a balance between easily and more slowly degrading material. This can be achieved by combining different types of organic materials. To provide guidance on what mixture combinations to use, information is needed on how the ratio of labile to recalcitrant components affects the degradation rate and the types of microbial populations supported. To investigate this, different ratios of wood and hay were used in up-flow column bioreactors treating selenium-and sulphate-containing synthetic mine-influenced water. The degradation rates of crude fibre components appeared to be similar regardless of the relative amounts of wood and hay. However, the nature of the degradation products might have differed in that those produced in the hay-rich bioreactors were more biodegradable and supported high sulphate-reduction rates. Microorganisms in the sulphate-reducing and cellulose-degrading inocula persisted in the bioreactors indicating that bio-augmentation was effective. There was a shift in microbial community composition over time suggesting that different microbial groups were involved in decomposition of more recalcitrant material. When dissolved organic carbon (DOC) was over-supplied, the relative abundance of sulphate-reducers was low even through high sulphate-reduction rates were achieved. As DOC diminished, sulphate-reducers become more prevalent and their relative abundance correlated with sulphate concentrations rather than sulphate-reduction rate.

This paper presents the results of an investigation on the application of the enzyme bovine carbonic anhydrase for reducing the permeability of a sandstone porous medium during CO2 flooding. This process provides a method for improving conformance of injected water and CO2 in an absolutely environmentally friendly manner. Carbonic anhydrase enzyme accelerates the hydration reaction of CO2 with water. Hence, in presence of carbon dioxide and divalent ions, such as calcium, this enzyme leads to rapid precipitation of calcium carbonate. The precipitation reaction causes reduction in the permeability of the flooded regions of the reservoir. Therefore, upon subsequent injection of CO2 and water, injected fluids would flow through the unswept parts of reservoir, improving the conformance of the injected CO2 and water. Experiments were carried out to investigate the effect of various enzyme concentrations, temperature and pH on the precipitation reaction. Enzyme concentrations at 2, 4 and 8 micro mole per litter were tested. Also, effect of temperature was studied by performing experiments at room temperature, 30 ºC and 55 ºC. A mathematical model was developed to predict the extent of precipitation of calcium carbonate. Subsequently, coreflood experiments, using Berea sandstone cores, were conducted and the degree of reduction in the permeability of the cores was measured. Also, in order to investigate the effect of injection scheme on permeability reduction, two different methods of injection were tested. For all sets of flow experiments, the permeability of the core was reduced to less than half of its initial value. Introduction Carbon dioxide miscible flooding is one of the most promising enhanced oil recovery (EOR) methods for light or medium oil reservoirs[1]. Recently, the interest in this method has been boosted because of the higher oil prices. If the reservoir pressure is at or beyond the minimum miscibility pressure (MMP) of the injected stream of CO2, multiple contact miscibility will be achieved in the reservoir. The ability to achieve dynamic miscibility at attainable pressures in a wide range of reservoirs is a major advantage of the CO2 miscible process. This makes CO2 an ideal displacement fluid for many crude oils. However, even when pressure conditions for miscibility are met, this high microscopic sweep efficiency is not often approached in reservoir operations, due principally to the non-uniformity of the flow patterns and unfavorable mobility ratio between injected CO2 and oil. Large-scale reservoir heterogeneities, such as fractures or high-permeability streaks can intensify viscous fingering of CO2 and cause early breakthrough of injected carbon dioxide, which will reduce oil recovery efficiency [2]. In order to improve the sweep efficiency of CO2 flooding projects, various methods have been proposed and tested via laboratory and field tests. In some of these studies, attempts have been made to achieve a more favorable CO2 mobility by changing the CO2 relative permeability. WAG (Water Alternating Gas) process is an example of these methods. In this process, CO2 mobility is reduced by reducing the relative permeability to CO2 via increased water saturation [3]. Other methods, such as CO2-foam and viscosified carbon dioxide process aim at increasing the viscosity of the injected gas phase, e.g. carbon dioxide, by using a surfactants or chemical thickening agents [4, 5]. A more common alternative to improve the problem of poor sweep efficiency of CO2 in oil reservoirs is by blocking the high permeability streaks, and/or fractures, in reservoirs. This can be achieved through in-depth placement of polymer gels. In in-depth gel placement technique, a gelling solution is injected into the reservoir, where the gelation takes place and the liquid solution changes form to a gel which blocks the high permeability regions of reservoir. In-depth gel placement has been used for water shut-off, as well as improving CO2 conformance, purposes extensively.