Dynamics of brine drying and salt precipitation and growth in porous media during CO2 injection

Subsurface CO2 storage in deep saline aquifers, owing to their capacity, containment efficiency, and availability, is a promising strategy to enable the global net-zero target. Maintaining CO2 injection into these storage sites is crucial for the success of carbon capture and storage (CCS) projects. However, dry CO2 injection leads to brine evaporation in porous rock, resulting in salt precipitation once the concentration exceeds solubility limits, which may lead to dramatic injectivity loss. Understanding the complex synergic effect of rock properties, injection parameters, and system thermodynamics on pore-scale brine drying and salt precipitation remains a challenge.<br/><br/>In this work, we conducted a series of microfluidic experiments to provide fundamental understanding of evaporation and salt precipitation in two-dimensional (2D) porous media. The experiments were conducted using custom-designed microfluidic chips with varying pore space heterogeneity, under atmospheric pressure and different temperatures and injection rates to shed light on key mechanisms controlling brine drying and salt precipitation in porous media. The results signify that (i) the residual brine distribution, determined by gas injection rate and pore structures, significantly controls the evaporation rate and salt distribution, (ii) depending on the injection rate and system temperature, two distinct types of salt crystal structures (monocrystalline structures nucleating within the aqueous phase and polycrystalline structures forming in the evaporated regions) may appear in porous media. The role of capillary backflow, driven by capillary pressure gradients during evaporation, was also elucidated in the context of salt crystallisation processes. This work provides a comprehensive understanding of the fundamental mechanisms of brine drying and salt precipitation, offering insights for optimising CO2 injection strategies in saline aquifers.