Microfluidic study of evaporation-driven crystallization of saline and ammonia brines under hydrogen flow

Underground hydrogen storage in geological formations is critical for renewable energy integration, but salt precipitation during gas injection can impair storage performance. While well studied for CO 2 systems, precipitation mechanisms in hydrogen–brine and ammonia–brine systems remain poorly understood. This study presents a systematic microfluidic investigation of salt crystallization during hydrogen injection into saline and ammonia-containing brine aquifers under high pressure with varying compositions (1–5 mol/kg NaCl), chemical additives (surfactants, alcohols, ammonia), and hydrogen flow rates (200–1300 mL/min). Quantitative imaging shows hydrogen induces physical precipitation through evaporation and capillary trapping, resulting in discrete deposits, whereas CO 2 -ammonia systems produce interconnected ammonium bicarbonate networks. Interfacial tension controls brine distribution and crystal coverage: high-IFT fluids form large pools promoting crystallization, low-IFT fluids form isolated pools reducing coverage by 50%. Alcohols and surfactants suppress precipitation, while ammonia increases crystal fraction. Higher flow rates accelerate crystallization, highlighting gas-specific mitigation strategies for underground hydrogen storage. • H 2 injection triggers salt precipitation via evaporation and capillary trapping. • Ammonia-rich brines accelerate precipitation on secondary substrates. • Post-breakthrough water saturation is spatially variable due to high H 2 mobility. • Crystal shape and distribution follow residual brine size and connectivity. • Additives change surface tension, shifting brine flow and growth sites.