Microscale Analysis of Fractured Rock Sealed With Microbially Induced CaCO3 Precipitation: Influence on Hydraulic and Mechanical Performance

• Published in: Water resources research 2018-10, Vol.54 (10), p.8295-8308
• Main Authors: Tobler, Dominique J., Minto, James M., El Mountassir, Gráinne, Lunn, Rebecca J., Phoenix, Vernon R.
• Format: Article
• Language: English
• Subjects: Apertures; Bridges; Calcite; calcite growth; Calcium carbonate; Chemical precipitation; Computed tomography; Cores; Crystal growth; Crystals; Electron microscopy; fracture grouting; Fractures; Granite; Grout; Hydraulics; Injection; Mechanical properties; Permeability; Porosity; Precipitation; Repositories; Rock masses; Rocks; Sandstone; Sedimentary rocks; Shear strength; Stability; Stone; Tomography; Transmissivity; ureolysis; Waste storage; water protection; X‐ray computed tomography
• ISSN: 0043-1397; 1944-7973
• DOI: 10.1029/2018WR023032

Microbially induced CaCO3 precipitation (MICP) has shown great potential to reduce permeability in intact rocks as a means to seal fluid pathways in subsurface ground, for example, to secure waste storage repositories. However, much less is known about how to apply MICP to seal fractured rock. Furthermore, there is limited information on the hydraulic and mechanical properties of MICP filled fractures, which are essential criteria to assess seal performance. Here MICP injection strategies were tested on sandstone cores, aimed at obtaining a homogeneous porosity fill that reduced permeability by 3 orders of magnitude. The injection strategy resulting in the most homogenous calcite distribution was then applied to fractured granite cores, to yield transmissivity reduction of up to 4 orders of magnitude. Microscale analysis of these sealed granite cores using X‐ray‐computed tomography and electron microscopy showed that >67% of the fracture aperture was filled with calcite, with crystals growing from both fracture planes, and bridging the fracture aperture in several places. Shear strength tests performed on these cores showed that the peak shear strength correlated well with the percentage of the fracture area where calcite bridged the aperture. Notably, brittle failure occurred within the MICP grout, showing that the calcite crystals were strongly attached to the granite surface. If MICP fracture‐sealing strategies can be designed such that the majority of CaCO3 crystals bridge across the fracture aperture, then MICP has the potential to provide significant mechanical stability to the rock mass as well as forming a hydraulic barrier. Key Points Microbially induced CaCO3 precipitation (MICP) was successfully applied to seal microsized fractures in granite cores Microscale analyses show CaCO3 strongly attached to both fracture surfaces, in places completely bridging the aperture MICP grouted fractures have the potential to provide additional mechanical stability to the rock mass as well as provide a hydraulic barrier