Microbially mediated rock fine fracture mineralization sealing: Evolution of flow channels and the resulting transmissivity reduction

The sealing of fine aperture fractures in geological repositories is an important consideration for long-term sealing integrity. Common grouting materials have some disadvantages, such as low sealing efficiency, environmental pollution, etc. Microbially induced calcium carbonate precipitation (MICP) has been proposed as a potential solution for sealing fine fractures. In this paper, the microbially mediated fine fracture mineralization sealing process was investigated by using a designed visual test device. The flow channel evolution and fracture transmissivity reduction during the sealing process were systematically monitored and analysed. Based on the experimental results, a sealing pattern of microbially mediated rock fracture mineralization was proposed. In addition, the influence of biomineralized floc particles formed by different bacterial concentrations on the critical sealing threshold of fracture was discussed. The results show that the sealing pattern can be divided into four stages: (a) flow channel formation; (b) preferred channel blocking; (c) finer channel blocking; (d) filtration zone formation. The four sealing stages correspond to different fracture transmissivity reduction trends, and the majority of transmissivity reduction occurs in stage III. However, the transmissivity decreases rapidly only after the preferred channels are blocked, which indicates that the critical threshold of sealing fracture is reached. The critical sealing threshold is mainly attributed to two factors: biomineralized floc particle size and fracture aperture. Larger floc particles and finer aperture are more likely to reach the critical sealing threshold, improving the sealing efficiency of fractures. It is crucial to further study the interaction between the biomineralized floc particle sizes and the fracture aperture at the critical sealing threshold to optimize the application of MICP-grouting in rock fine fractures.