Impact of flow-direction and gravitational settling on microbially-induced calcium carbonate precipitation (MICP) in rock fractures

Microbially-induced calcium carbonate precipitation (MICP) is favored over traditional grouts for remediating fine rock fractures due to its water-like viscosity. We quantitatively investigate the spatial distribution and sealing efficiency of microbially-mediated CaCO₃ precipitation for horizontal-flow MICP-grouting in both horizontal (HHF) and vertical (HVF) fractures and for upflow grouting in vertical fractures (UVF)—for different width fractures (~ 0.2 mm and ~ 0.45 mm). The results indicate that the permeability reduction is most significant in horizontal fractures (HHF), with vertical fractures after horizontal MICP-grouting (HVF) being least affected and vertical fractures with upflow grouting (UVF) intermediate for identical MICP-grouting process. The morphologies of bio-CaCO₃ show a tortuous, dendritic distribution with a structure elongated along the MICP-grouting direction in the horizontal fracture, a triangular distribution in the vertical fracture with upflow grouting and layered precipitation at the bottom in the vertical fracture with horizontal grouting. These differences arise from the abscission, migration and redistribution of bio-CaCO₃ flocs generated on the fracture surfaces under the combined action of gravity and the flow field. Especially in the vertical fracture, the bio-CaCO₃ flocs generated on the fracture surfaces are readily detached by gravity, and both the flow direction and velocity of MICP-fluids determine the redistribution morphology of the precipitates. The results also indicate that the retained bio-CaCO₃ in larger-aperture fractures is significantly greater than that in smaller-aperture fractures with identical flow rates. The balance between the velocity of fluid and the fracture aperture controls the retention of bio-CaCO₃ within the fracture.