Microfluidic Study of Evaporation-Driven Crystallization of Saline and Ammonia Brines under Hydrogen Flow

TL;DR

The paper presents a pore-scale, high-pressure microfluidic study of evaporation-driven salt precipitation during hydrogen injection into brines containing NaCl and ammonia, contrasting it with reactive CO$_2$-ammonia systems. By varying brine composition, additives, and hydrogen flow rates across 81 experiments, it demonstrates that H$_2$-induced precipitation is governed by physical processes (evaporation and capillary trapping) and yields discrete, localized salt deposits, whereas CO$_2$-ammonia systems form extensive ammonium bicarbonate networks that clog pores. Interfacial tension strongly shapes brine pool connectivity and final precipitation, with low-IFT additives suppressing crystallization by up to ~50%, though ammonia can paradoxically increase crystallization. The findings emphasize gas-specific risk assessments for underground hydrogen storage and suggest mitigation strategies (flow-rate control, low-IFT additives) while noting the limitations of translating 2D microfluidic results to 3D reservoirs.

Abstract

Underground storage of hydrogen and ammonia in geological formations is essential for renewable energy integration, but salt precipitation during gas injection may threaten storage performance. While extensively studied for CO2 systems, precipitation mechanisms in hydrogen-brine and ammonia-brine systems remain poorly understood. This study presents a comprehensive microfluidic investigation of salt crystallization during hydrogen injection into saline and ammonia-containing brines using high-pressure microfluidics. We conducted 81 high-pressure experiments systematically varying brine composition (1-5 mol/kg NaCl), chemical additives (surfactants, alcohols, ammonia), and hydrogen flow rates (200-1300 mL/min). Quantitative image analysis reveals that hydrogen-induced precipitation differs fundamentally from CO2 systems. Hydrogen drives physical precipitation via evaporation and capillary trapping, producing discrete, localized deposits. In contrast, CO2-ammonia systems generate extensive reactive precipitation of ammonium bicarbonate with interconnected crystal networks. Interfacial tension (IFT) controls both residual brine distribution and final crystal coverage: high-IFT fluids form large, interconnected brine pools promoting extensive crystallization, while low-IFT fluids create isolated pools reducing crystal coverage by 50\%. Alcohol and surfactant additives suppress precipitation by enhancing brine mobility, whereas ammonia paradoxically increases crystal fractions despite lower IFT. Higher flow rates accelerate crystallization across all compositions, enabling operational mitigation strategies. and demonstrate that gas-specific, rather than CO2-analog, risk assessments are essential for underground hydrogen storage design. The effectiveness of chemical additives offers promising pathways for near-wellbore protection in underground hydrogen storage operations.

Figures (13)

• Figure 1: Photograph of high-pressure microfluidic setup utilized for crystallization process imaging. The setup consists of high-pressure syringe pumps used for brine injection and backpressure control, a high-speed microscopic camera used for image recording, a separator, and a flow meter. Chip is placed on the XYZ motorized stage. Flexible PEEK tubing allows chip moment. The first insert shows the tubing connection and chip holder. The second insert shows a microscopic image of the chip with a low-resolution objective. The setup is a modified version of the system used in dkabrowski2025surfacedkabrowski2025microfluidic
• Figure 2: High-resolution microscopic images of a microfluidic chip: (a) fully saturated with brine, (b) partially desaturated after H$_2$ breakthrough, (c) completely dried after brine dryout, with individual phases indicated by arrows (W5 fluid, 400 ml/min flow rate, 10 mm from chip inlet); (d) differential image highlighting crystal positions; (e) differential image highlighting brine positions; (f) segmented image with detected phases from (b) and (c) marked in distinct colors.
• Figure 3: Time evolution of brine evaporation and crystal growth for W5 fluid at a 400 ml/min flow rate: (a) time-sequence images showing the onset of evaporation, crystallization, and final dryout; (b) stacked segmented images illustrating brine evaporation, with the colorbar indicating the time from evaporation onset to final dryout; (c) stacked segmented images depicting crystal growth, with the colorbar representing the time from detection of initial crystallites to final dryout.
• Figure 4:
• Figure 5: Panoramic images of initial brine saturation (20$\times$2 mm, stack of 10 positions) for W5 fluid: (a) raw microscopic image at 400 mL/min flow rate; (b) contour plots of brine pools for three experiments at 400 mL/min, with colors indicating different runs; (c) contour plots of brine pools for three H$_2$ flow rates; (d) contour plots of brine pools for W1, W2.5, and W5 fluids at varying NaCl concentrations and 400 mL/min.