X-ray CT-based interpretation of microbial-induced carbonate precipitation and local hydraulic behaviors

Microbially induced carbonate precipitation (MICP) has been extensively studied at both lab and field scales for its functional applications, such as the shear strength enhancement or the hydraulic conductivity reduction for geological engineering. However, investigating the pore-scale interaction between calcite formation and hydraulic properties has been a challenge. In this study, we proposed an effective methodology that allows for non-destructive direct numerical simulations based on three-dimensional X-ray computed tomographic images of MICP-treated sands to capture those pore-scale interactions at from mm- to sub-mm scales. The lattice Boltzmann model was implemented to quantitatively evaluate the fluid velocity field through the realistic pore structures in the MICP-treated sands. It was confirmed that the calcite formation naturally led to a reduction in hydraulic conductivity, following a linear relationship with porosity. The spatial variability in hydraulic conductivity was finely mapped at different reaction stages, with vertical variation along the height being predominant over three orders of magnitude even for a 12 mm-length specimen. Additionally, we discussed the contribution of changes in hydraulic tortuosity and specific surface area to hydraulic conductivity variation and newly evaluated geometric factor in the Kozeny-Carman model for MICP-treated sands. Overall, the presented methodology demonstrated the feasibility of image-based characterization for 3D spatiotemporal changes during MICP via 'direct numerical simulation' and X-ray CT imaging, and the observation can be readily applied to porous geomaterials.