A First Look at Hydrogen Generation in an Ultramafic Rock with Micro-CT and SEM-BEX

TL;DR

This study uses advanced imaging techniques to observe hydrogen gas formation at the pore scale during water-rock interactions in ultramafic rocks, providing insights into natural hydrogen generation processes.

Contribution

It presents the first in situ 4D micro-CT imaging of hydrogen generation in ultramafic rocks, linking reaction kinetics with gas flow at the pore level.

Findings

Gas phase appears after 8 hours of heating.

Hydrogen likely dominates the generated gas.

Pore-scale imaging links water-rock reactions to gas flow.

Abstract

Natural hydrogen generated by water-rock interaction in ultramafic rocks is increasingly recognised as a potentially important primary energy resource, but the pore-scale processes that control the initiation and early transport of a free gas phase remain poorly constrained. Here we present an in situ X-ray micro-tomography experiment in which an ultramafic granular pack of dunnite from West Papua, Indonesia, saturated with KI-doped brine, is heated to 100C with a pore pressure of 4bar under 10bar confining pressure inside a micro-CT scanner. Time-resolved 4D imaging captures the transition from a fully liquid-saturated pore space to the appearance and growth of a distinct gas phase after an 8h induction period. Bubbles first nucleate near the top of the sample before becoming distributed throughout the imaged volume as a connected ganglia. The nucleating gas phase is most plausibly dominated by molecular hydrogen generated by low-temperature fluid-rock reaction, as indicated by independent hydrogen-presence detectors, although we cannot yet fully exclude minor contributions from other gases. SEM-BEX imaging reveals textural alteration and local changes in elemental signals between reacted and unreacted material. Taken together, these observations provide spatially and temporally resolved evidence for gas generation during low-temperature alteration of ultramafic grains and demonstrate that pore-scale imaging can directly link water-rock reaction kinetics, gas generation and multiphase flow behaviour in natural hydrogen systems.