NMR Measurements of Fractured Sandstone Sealed with Microbially Induced Calcium Carbonate Precipitation: Hydraulic Properties and Cementation Distribution

Abstract

Microbially induced calcite precipitation (MICP) is an emerging ground improvement method. Although a reduction in permeability of intact and fractured rock is reported due to MICP, the heterogeneous distribution of the calcium carbonate precipitation and clogging near the injection source have posed challenges to using this technique in underground applications. Monitoring tools that characterize the precipitate distribution may be used to mitigate heterogeneity. This study investigated the effect of inoculation and incubation conditions on sealing fractured sandstone following a staged injection strategy. Nondestructive and noninvasive nuclear magnetic resonance (NMR) methods that are suitable for subsurface application were employed to monitor changes in pore structure and fracture aperture during the MICP process. The results showed that the staged injection strategy resulted in a uniform precipitation distribution along the fractures regardless of inoculation and incubation condition. Though the sensitivity of NMR was lower than that of gas expansion and conventional gravimetric methods, NMR was demonstrated to be a reliable tool for monitoring precipitation distribution. This study promotes MICP as a promising biotreatment that could advance underground engineering. Recently, the number of underground projects has increased due to the difficulties of expanding aboveground infrastructure in dense urban areas. A major underground construction hazard is ceiling collapse due to the disintegration of the rock mass in the presence of fractures. Thus, underground projects often require a sufficient rock support system to keep the underground structure stable, safe, and operable over the design life of the project. Rock fracture grouting is one of the most widely used methods for repairing fractures in underground applications. However, traditional grouts can be toxic, expensive, and have a high carbon footprint. Recently, a biobased grouting technique has been reported as an innovative ground improvement. This approach relies on microbes to induce precipitation of minerals as cementitious material. However, the heterogeneous distribution of the precipitated cement and clogging near the injection source have posed challenges to using this technique underground. In this study, different practices that have been shown to promote uniform precipitation were integrated, and an imaging technique suitable for subsurface exploration was employed to monitor the precipitation process. Results showed that biobased grouting treatment led to homogeneous cement precipitation in fractured rock and subsurface imaging is a helpful tool to nondestructively monitor the treatment. This study promotes biobased grout as a promising treatment to advance underground engineering.