The concentration transients of CO2, H2, and He in the volcanic gases: theoretical model and experimental results

Abstract

The thorough understanding of the gas transport through the porous media is of considerable interest in several environmental issues. Potential applications of this knowledge include the topics of contaminant transport through the soils, and the gas leakages from carbon sequestration and storage plants. In volcanology, the gas transport process through the porous media affects the compositions of the ground gases, that are the proxy of the magmatic activity. Herein we targeted the transients of the chemical composition of the ground gases through the formulation of a theoretical model, and the gas flux simulations carried out in the laboratory. The model accounts for the gas released by soils in volcanic areas, and takes into account the process of the mass transfer triggered by disequilibrium conditions in the system. The theoretical results describe the time-dependent evolution of the composition of the ground gases as the result of the pristine gas mixture, the thickness of the medium, the gas velocity, and the diffusivity of the chemical, that is a substance-dependent quantity. Moreover, the results indicate that the difference in the diffusivities of the different mixture components produce asynchronous transients of the concentrations in the ground gases. Carbon dioxide (CO2), hydrogen (H2), and helium (He) were used in a laboratory-scaled flux simulator with the purpose of investigating the time-dependent evolution of the gas composition profile through a porous medium. The structure of the porous medium (porosity and tortuosity) in the flux simulator, the composition of the pristine mixture, the gas flux velocity, and the thickness of the medium were fully constrained. The results of this study indicate that the theoretical computations fulfill the experimental data for the compositional range of the gases typically emitted by soils of volcanic and geothermal areas. Furthermore, the measurement of the delays between the concentration transients of two or more components with large difference in diffusivity (e. g. H2 and CO2) provide constraints between the chemistry of the ground gases and the depth of the gas reservoir.