  Serpentinization and carbonation of ultramafic formations is a ubiquitous
phenomenon, which deeply influences the biogeochemical cycles of water,
hydrogen, carbon... while supporting the particular biosphere around the
oceanic hydrothermal vents. Carbonation of peridotites and other mafic and
ultramafic rocks is also a hot topic in the current energy landscape as the
engineered sequestration of mineral CO2 in these formations could help reduce
the atmospheric emissions and cope with climate change. In this study, we
present two reactive percolation experiments performed on a natural
serpentinite dredged from the ultraslow South-West Indian Oceanic Ridge. The
serpentinite cores (length 3-4 cm and dia. 5.6 mm) were subjected for about 10
days to the continuous injection of a NaHCO3-saturated brine at respectively
160{\deg}C and 280{\deg}C. Petrographic and petrophysical results as well as
outlet fluid compositions were compared to numerical batch simulations
performed with the PHREEQC open software allowing to reconstruct the
mineralogical evolution of both cores. The most striking observation is the
fast and dramatic decrease of the permeability for both experiments principally
due to the precipitation of carbonates. On the contrary, serpentine was found
to be less impacting as it precipitates in low-flow zones, out of the main
percolation paths. In total, about 5.6% of the total injected CO2 was retained
in the core, at 280{\deg}C. In the same time, hydrogen was consistently
produced with a total recovered H2 corresponding to 0.8% of the maximum H2
possible. The global behavior of the cores is interpreted as the result from an
interplay between interacting spatio-temporal lengthscales controlled by the
Damkohler number.
