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

TL;DR

This study addresses how hydrogen is generated and begins to migrate within ultramafic rocks during water–rock reaction. It employs in situ time-resolved 4D X-ray micro-CT on KI-doped brine saturated, crushed ultramafic grains at $100^ op$C and 4 bar pore pressure to visualize gas nucleation and growth, complemented by SEM-BEX mineralogical analysis. The results reveal a three-stage evolution: a ~8 h delay with no resolvable gas, rapid nucleation and connected gas ganglia formation by ~23 h, and slower, diffusion-limited growth to ~30 h, with gas behavior consistent with hydrogen production from low-temperature serpentinization and independent gas detectors confirming H$_2$ presence. These findings provide direct, spatially resolved links between reaction kinetics and pore-scale multiphase transport, offering insights for natural hydrogen systems and underground hydrogen storage, while underscoring the qualitative nature of the single-experiment observations and the need for broader parameter exploration.

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.

Figures (6)

• Figure 1: Experimental set-up for in situ micro-CT imaging of H$_2$ generation and gas exsolution. A Hassler-type core holder containing crushed ultramafic rock is mounted on the rotation stage inside a Pb-lined micro-CT enclosure. The sample is confined by a Viton sleeve and aluminium foil within an aluminium outer sleeve, with steel end caps and end fittings connecting to the flow lines. Heating tape and a thermocouple control and monitor the sample temperature, and H$_2$-indicator tape is placed around the core to detect any leakage. Pore fluid is injected from the injection pump, while the receiving pump maintains pressure and collects effluent. Confining pressure is applied via the confining pump. The red dashed box indicates the imaged region within the core.
• Figure 2: Representative 2D slice showing the micro-CT image processing workflow. (a) Raw greyscale image, (b) the same slice after non-local means filtering to reduce high-frequency noise while preserving phase boundaries, and (c) final three-phase segmentation into solid grains (dark blue), brine-filled pore space (red) and gas voxels (light blue).
• Figure 3: Time series of segmented gas phase (false-colour red) within the ultramafic granular pack at (a) 0, (b) 8, (c) 23, and (d) 30 hours. The grains are rendered as translucent grey and the brine is transparent, so that the red overlay highlights only the voxels classified as gas in the micro-CT segmentation.
• Figure 4: Vertical profiles of gas saturation $S_g$ as a function of depth $z$ along the core axis at 8, 23 and 30 h. Depth increases downward from the top of the imaged sample and is expressed in micrometres; line colours correspond to the times indicated in the legend.
• Figure 5: Evolution of bulk gas saturation $S_g$ (left axis, solid blue line) and the fraction of gas in the largest cluster $f_{\max}$ (right axis, dashed orange line) as a function of time during the experiment. Shaded bands indicate the three stages inferred from the image-derived metrics: (i) no resolvable gas (0--8 h), (ii) rapid gas generation and connectivity growth (8--23 h), and (iii) slower, diffusion-limited growth (23--30 h).