PARTITIONING OF NOBLE GASES DURING SUBSURFACE FLOW

Noble gases are important tracers of subsurface flow processes with important applications in geological carbon storage and shale gas development. The interpretation of noble gas isotope distributions is typically based on zero-dimensional mixing and Rayleigh fractionation processes and the explanation of field observations commonly requires multi-stage models. To clarify the relation between these models the physical flow processes occurring in the subsurface we study the fractionation of noble gases in two important flow processes: First, we show that the migration of a gas plume in the subsurface leads to accumulation of both co-genetic and dissolved atmospheric noble gases at the front of the migrating gas. The underlying process is a chromatographic separation between gases of differing solubility. As such it is not related to the commonly assumed Rayleigh fractionation. Second, we study the fractionation of noble gases during convective dissolution of CO₂ into underlying brine. It is generally assumed that this process leads to Rayleigh fractionation between different gases. However, we show that the fractionation only approached the Rayleigh limit, if both the gases have the same aqueous diffusivity. Gases with differing diffusivities do generally not experience Rayleigh fractionation, even when the convective mass transport dominates over diffusion. However, in the limit of extreme solubility differences, such as between Helium and CO₂ the deviation from Rayleigh fractionation is small. Our results therefore outline when common assumptions are appropriate and when our interpretations need to be improved with new physical models.