Natural hydrogen and blend gas: a dynamic model of accumulation

Abstract

Natural hydrogen is accumulating in the form of “blend gases”, with the association mainly of methane, nitrogen, and helium. The extreme variation of proportion between these four gas compounds is explained through a model of dynamic accumulation, where some compounds as helium are chemically inert in the reservoir, nitrogen almost inert, methane is altered into CO2, and hydrogen is severely altered mainly in protons, and in a smaller degree in methane. All the gas compounds may also leak out of the reservoir, either through advection without any chemical fractionation, or through solubilization in water and diffusion, inducing a variable composition for the residual gas remaining in the accumulation.The model can explain the concomitance of high concentration of helium, clearly accumulated through geological time, with hydrogen, supposedly renewable in human time scales. With the best choice of parameters for the model, the ages of hydrogen accumulation known in the World are small, the hydrogen being diluted more and more in nitrogen and methane through geological time. Diffusive leakage through water appears negligeable compared to advective leakage and compared to hydrogen reactivity in the reservoir. The example of the hydrogen field of Bourakébougou would present an age of 500 years according to the model. Diffusive leakage through water solubilization has a small effect on gas composition. Advective leakage, even not fractionating, has a significant effect on accumulated gas composition, because of its mixing with active deep fluxes. During future production of natural hydrogen accumulations, it is predicted that the proportion of hydrogen should increase during production time, whereas the helium and nitrogen concentrations should decrease.