Geophysical monitoring and reactive transport modeling of ureolytically-driven calcium carbonate precipitation

Ureolytically-driven calcium carbonate precipitation is the basis for a promising in-situ remediation method for sequestration of divalent radionuclide and trace metal ions. It has also been proposed for use in geotechnical engineering for soil strengthening applications. Monitoring the occurrence, spatial distribution, and temporal evolution of calcium carbonate precipitation in the subsurface is critical for evaluating the performance of this technology and for developing the predictive models needed for engineering application. In this study, we conducted laboratory column experiments using natural sediment and groundwater to evaluate the utility of geophysical (complex resistivity and seismic) sensing methods, dynamic synchrotron x-ray computed tomography (micro-CT), and reactive transport modeling for tracking ureolytically-driven calcium carbonate precipitation processes under site relevant conditions. Reactive transport modeling with TOUGHREACT successfully simulated the changes of the major chemical components during urea hydrolysis. Even at the relatively low level of urea hydrolysis observed in the experiments, the simulations predicted an enhanced calcium carbonate precipitation rate that was 3-4 times greater than the baseline level. Reactive transport modeling results, geophysical monitoring data and micro-CT imaging correlated well with reaction processes validated by geochemical data. In particular, increases in ionic strength of the pore fluid during urea hydrolysis predicted by geochemical modeling were successfully captured by electrical conductivity measurements and confirmed by geochemical data. The low level of urea hydrolysis and calcium carbonate precipitation suggested by the model and geochemical data was corroborated by minor changes in seismic P-wave velocity measurements and micro-CT imaging; the latter provided direct evidence of sparsely distributed calcium carbonate precipitation. Ion exchange processes promoted through NH4+ production during urea hydrolysis were incorporated in the model and captured critical changes in the major metal species. The electrical phase increases were potentially due to ion exchange processes that modified charge structure at mineral/water interfaces. Our study revealed the potential of geophysical monitoring for geochemical changes during urea hydrolysis and the advantages of combining multiple approaches to understand complex biogeochemical processes in the subsurface

Kinetics of urease mediated calcite precipitation and permeability reduction of porous media evidenced by magnetic resonance imaging

The enzyme urease drives the hydrolysis of urea leading to the release of ammonium ions and bicarbonate; in the presence of calcium, the rise in pH leads to increased calcium carbonate saturation and the subsequent precipitation of calcite. Although such alkalinizing ureolysis is widespread in nature, most studies have focussed on bacteria (i.e. indigenous communities or urease-active Sporosarcina pasteurii) for calcite precipitation technologies. In this study, urease-active jack bean meal (from the legume Canavalia ensiformis) was used to drive calcite precipitation. The rates of ureolysis (k urea), determined from measured NH4 +, enabled a direct comparison to microbial ureolysis rates reported in literature. It is also demonstrated that a simple single reaction model approach can simulate calcite precipitation very effectively (3-6 % normalised root-mean-square deviation). To investigate the reduction of permeability in porous media, jack bean meal (0.5 g L-1) and solutions (400 mM urea and CaCl2) were simultaneously pumped into a borosilicate bead column. One-dimensional magnetic resonance profiling techniques were used, non-invasively, for the first time to quantify the porosity changes following calcite precipitation. In addition, two-dimensional slice selective magnetic resonance images (resolution of ~0.5 × 1.0 mm) revealed that the exact location of calcite deposition was within the first 10 mm of the column. Column sacrifice and acid digestion also confirmed that 91.5 % of calcite was located within the first 14 mm of the column. These results have important implications for the design of future calcite precipitation technologies and present a possible alternative to the well known bacterial approaches

Geo-electrical Characterisation for CO2 Sequestration in Porous Media

This is a post-peer-review, pre-copyedit version of an article published in Environmental Processes. The final authenticated version is available online at: http://dx.doi.org/10.1007/s40710-017-0222-2.Developing monitoring strategies for the detection and monitoring of possible CO2 leakage or migration from existing and anticipated storage media are important because they can provide an early warning of unplanned CO2 leakage from a storage site. While previous works have concentrated on silicate and carbonate porous media, this work explores geoelectrical techniques in basalt medium in a series of well-defined laboratory experiments. These were carried out to identify the key factors which affect geoelectrical monitoring technique of CO2 in porous media using low cost and efficient time domain reflectometry (TDR). The system has been set up for simultaneous measurement of the bulk electrical conductivity and bulk dielectric permittivity of CO2-water-porous media system in silica sand, basalt and limestone. Factors investigated include pH, pressure, temperature, salinity, salt type and the materials of the porous media. Results show that the bulk electrical conductivity and dielectric permittivity decrease as water saturation decreases. Noticeably, electrical conductivity and permittivity decrease due to the changes in water saturation and the relationship remains the highest in limestone except at the start of the experiment. Also, an increase in temperature, pressure and salinity tend to increase the bulk electrical conductivity (σb) and permittivity (εb) of the CO2-water-porous media system during the drainage experiment. On the other hand, pH and concentrations of different types of salt do not seem to have significant effect on the geoelectrical characteristics of the system. It was evident that Archie’s equation fit the experimental results well and the parameters obtained were in good agreement with those in the literatures. The regression shows a good reliability in the prediction of electrical properties during the monitoring process of CO2 sequestration