Fracture healing processes in upper crustal carbonates – insights from fluid percolation experiments

Geological observations, seismic data as well as laboratory experiments have shown that faults lithify and recover their strength (heal) during interseismic periods. The mechanical-chemical process of fault healing is a key in understanding many aspects of fault behavior, such as earthquake recurrence and rupture dynamics. Such processes do not only play an important role in understanding unconventional seismicity, such as ‘slow and low frequency earthquakes’ as observed at active plate boundaries, but are also pivotal for the application of deep geothermal energy, CO₂ sequestration and the underground storage of radioactive waste. In this study we investigate the mechano-chemical recovery of fractures in carbonates at upper crustal conditions. In the upper crust, fractures are dominantly sealed through mineral precipitation from supersaturated fluids that are chemically out of equilibrium with the host rock. In order to simulate the healing process, we performed fluid percolation experiments on intact as well as pre-fractured carbonates with varying timescales representing different healing rates. In order to quantitatively document the healing process, the selected rock samples are analyzed by X-ray microtomography before and after the experiments. In addition, optical as well as scanning electron microscopy is applied to document the mechanical-chemical processes of healing. The role of the initial (micro)fracture network, the effect of the initial chemistry of the injected fluid and the effect of temperature on the healing process will be investigated. The experiments on both intact and pre-fractured rock are carried out with a percolation cell that allows the fluid-rock interaction to be reproduced at confining pressures up to 100 MPa, pore pressures up to 100 MPa and temperatures up to 250°C. This work will advance knowledge about the damage-recovery cycle in fractured carbonates through the investigation of healing processes active at different timescales using a unique experimental approach.