Space and time distribution of subsurface H2 concentration in so-called “fairy circles”: Insight from a conceptual 2-D transport model

Andrey Myagkiy , Isabelle Moretti (1, 2) , Fabrice Brunet

1 LFCR - Laboratoire des Fluides Complexes et leurs Réservoirs
2 ENGIE, Centre de Recherche et Innovation Gaz et Energie Nouvelle

Natural H 2 emissions from the ground have now been measured in many places worldwide. These emissions can be localized on faults or be more diffuse in some sedimentary basins, usually of Proterozoic age. In such a case, emanation zones are often visible from aerial images or on high-resolution topographic maps since they correspond to slight depressions of circular to elliptic shape. Furthermore, the rounded depressions are covered with a scrubby vegetation which often contrasts with the surrounding vegetation. Although the emission structure displays a very regular shape, the distribution of H 2 concentration in the first meter of soil in such a structure does show a clear pattern. For example, the maximum concentration is almost never measured in the center of the structure and the few time-resolved data show that the soil H 2 concentration is variable with time. Here, the time and space evolution of H 2 concentration is simulated using a 2-D advective-diffusive model of H 2 transport in porous media. Several parameters have been tested as the depth and periodicity of the H 2 point source (pulsed), bacterial H 2 consumption and permeability heterogeneities of the soil. The radius of the structure is linked to the time spent by the H 2 in the soil that depends on the soil permeability, the depth of the gas leakage point and the pressure of the bubble. To account for field observations, the case of a shaly, less permeable, heterogeneity in the center of the structures has been modeled. It resulted in an increase of the concentration toward the rim of the structure and a close to zero signal in its center. If the deep signal is periodic with a frequency smaller than a few hours, H 2 concentration within the soil is almost constant; in other cases, the near surface concentration wave reflects the concentration periodicity of the source with a delay (in the range of 12 h for 30 m of soil) and so the near surface H 2 concentration values will be highly dependent on the time at which the measurement is performed. H 2 monitoring through a sensor network is thus mandatory to characterize the H 2 dynamics in the soil of fairy circles. Keywords: natural H 2 / H 2 emission / soil micro-seeps / heterogeneous soil permeability Résumé-Variabilité dans le temps et l'espace dans le sol des structures émettrices dits « ronds de

Domaines: Géophysique [physics.geo-ph]

Andrey Myagkiy, Isabelle Moretti, Fabrice Brunet. Space and time distribution of subsurface H2 concentration in so-called “fairy circles”: Insight from a conceptual 2-D transport model. Bulletin de la Société Géologique de France, 2020, 191 (1), ⟨10.1051/bsgf/2020010⟩. ⟨hal-02938377⟩