Modeling deep control pulsing flux of native H2 throughout tectonic
fault-valve systems
Abstract
Pulsing seepages of native hydrogen (H2) have been observed at the
surface on several emitting structures. It is still unclear whether this
H2 pulsed flux is controlled by deep migration processes,
atmosphere/near-surface interactions or by bacterial fermentation. Here,
we investigate mechanisms that may trigger pulsating fluid migration at
depth and the resulting periodicity. We set up a numerical model to
simulate the migration of a deep constant fluid flow. To verify the
model’s formulation to solve complex fluid flows, we first simulate the
morphology and amplitude of 2D thermal anomalies induced by
buoyancy-driven water flow within a fault zone. Then, we simulate the H2
gas flow along a 1-km draining fault, crosscut by a lower permeable rock
layer to investigate the conditions for which a pulsing system is
generated from a deep control. For a constant incoming flow of H2 at
depth, persistent bursts at the surface only appear in the model if: (I)
a permeability with an effective-stress dependency is used, (II) a
strong contrast of permeability exists between the different zones,
(III) a sufficiently high value of the initial effective stress state at
the base of the low permeable layer exists, and (IV) the incoming and
continuous fluid flow of H2 at depth remains low enough so that the
overpressure does not “open” instantly the low permeability layer. The
typical periodicity expected for this type of valve-fault control of H2
pulses at the surface is at a time scale of the order of 100 to 300
days.