Effect of abiotic carbonate cementation on hydraulic conductivity of sand - Grain-scale analysis using X-ray microtomography imaging

Abstract

Calcium carbonate ($CaCO_3$) cementation is a frequently occurring natural phenomenon in the subsurface. $CaCO_3$ precipitates in coarse-grained soils via natural chemical depositional processes, and forms cemented and consolidated earth materials. $CaCO_3$ cementation can also be intentionally induced for soil improvement. In addition, storing of carbon dioxide ($CO_2$) in carbonate-rich formations as a means of geologic carbon storage is inevitably accompanied by $CaCO_3$ mineral precipitation in porous media. Such $CaCO_3$ cementation can have a profound effect on the modification of engineering properties of soils, including porosity, permeability, stiffness, and strength. However, grain-scale patterns of $CaCO_3$ cementation have not been fully understood

hence, their effect on macroscale parameters, in particular, hydraulic conductivity, remains poorly understood. Therefore, the goal of this thesis is to understand $CaCO_3$ cementation patterns at the grain scale, and identify their effect on hydraulic conductivity. To achieve this goal, (a) grain-scale patterns of carbonate cementation were visualized using X-ray computed micro-tomography (X-ray CMT) imaging

(b) micro-scale characteristics, such as internal structures and thickness of cemented carbonate minerals and specific surface areas of grains, were examined

and (c) correlations between reductions in hydraulic conductivity and quantity of cemented carbonate were identified. Herein, a column experiment was conducted, in which carbonate minerals were precipitated in coarse sand from a solution supersaturated with $CaCO_3$. Simultaneously, index properties, such as porosity and $CaCO_3$ pore saturation, were calculated by simple bulk mass measurement. Reduction in the hydraulic conductivity of the cemented sand column was monitored during $CaCO_3$ cementation and dissolution. When a significant amount of carbonate was precipitated, the cemented sand was periodically imaged using X-ray CMT. This cementation process continued until the flow path was completely blocked. Then, a dissolution experiment was conducted using 2 M acetate solution, followed by mass and hydraulic conductivity measurement and X-ray CMT imaging. Decreases in porosity and hydraulic conductivity and an increase in $CaCO_3$ pore saturation were found, while dissolution showed the opposite trend. During cementation, it was found that the porosity of the sand column decreased from 32.7% to 23.4%, and the hydraulic conductivity decreased from 0.123 cm/s to 7.54×$10^-5$ cm/s after two-million pore volume of supersaturated solution had passed. Acquired images during cementation and dissolution were analyzed to identify grain-scale cementation patterns. The acquired images during cementation and dissolution were analyzed to identify the grain-scale cementation patterns. While the bulk porosity and $CaCO_3$ saturation calculated from the acquired images appeared to be similar to the mass measurement, spatial variations in porosity and $CaCO_3$ saturation were observed across the column in terms of less cementation near the outlet and more cementation in the middle and near the inlet. Hydraulic conductivity was significantly reduced by local clogging, where the minimum porosity was found to be as low as approximately 7%. $CaCO_3$ layer thickness ranged from 0-0.05 mm while the mean grain radius ranged from 0.45-0.50 mm. Based on X-ray CMT image analyses, the specific surface area was found to increase from 5320 $m^-1$ to 8615 $m^-1$ until the bulk porosity decreased from 33% to 23%, a result of the microscale carbonate minerals precipitating on smooth grain surfaces, increasing the roughness. Thereafter, the specific surface area began decreasing when the porosity reached less than 23% as the sand grains that were not initially contacted bonded and were contacted by cemented carbonate minerals, creating a clumping effect. The experimental results was also compared with analytical models. The Kozeny-Carman model was chosen for the comparison, indicating that hydraulic conductivity was significantly affected by surface area and local porosity. This indicates the existence of micro-scale internal porosity in the precipitated carbonate minerals. This has also been confirmed by scanning electron microscopy (SEM) and focused ion beam-scanning electron microscopy (FIB-SEM) images. The rough surface of the cemented carbonate layer and the internal porosity can dramatically reduce the hydraulic conductivity by the dragging force while not significantly decreasing the porosity. Further investigation of such an effect on the macroscale parameters of a cemented porous medium is warranted.