Microbially Induced Carbonate Precipitation For Engineered Sealing Of Fractured Rock

Abstract:

Context

Engineering works in the subsurface often requires ground water ingress through fractures in the rock to be minimised. Traditionally cement based grouts have been used for this purpose, however chemical and alternative grouts are increasingly used. A promising alternative grout utilises bacteria to rapidly initiate the precipitation of calcium carbonate in a process termed microbially induced carbonate precipitation (MICP). Advantages of this process over conventional grouts are very low viscosity and small particle size allowing penetration into small aperture fractures, low toxicity and low associated carbon footprint. The use of MICP for soil stabilisation has been demonstrated and small scale fractures have been sealed in the lab, yet where the CaCO₃ precipitates within large fractures is poorly understood.

Aim

The aim of this project was to assess the feasibility of MICP for large scale fracture sealing and to identify the optimum injection strategy that would allow controllable precipitation of CaCO₃ over a large area and hence create a long-lasting reduction in fracture transmissivity.

A 3m² artificial fracture consisting of a rough-cut granite lower surface and transparent polycarbonate upper surface with a central injection well was created (Figure 1) and sealed twice with MICP: the first experiment to determine the transmissivity reduction achievable and the second experiment to optimise where CaCO₃ precipitation occurred. The sealing process was then simulated in a model developed with CFD software OpenFOAM.

Results

The transmissivity of the 300μm hydraulic aperture fracture was reduced from 1.7x10^-5 to 8.8x10^-9m²/s with 12 injections in the first experiment. A similar transmissivity reduction was achieved with only five injections in the second optimised experiment. Through the transparent upper fracture surface, CaCO₃ precipitation was visible over the entire 3m² fracture and attachment to both the top and bottom surfaces was very strong. This study has shown that when and where CaCO₃ precipitates can be controlled and that the MICP process may be an attractive alternative to cement based grouts for certain fracture sealing applications.