Pore-scale mechanisms of salt precipitation in heterogeneous media under geological carbon storage conditions

Salt precipitation, driven by CO₂-induced brine dry-out in deep saline formations, poses a significant risk to the long-term efficiency and safety of geological carbon storage. We developed a porous media lab-on-a-chip platform that mimics intrinsic heterogeneity by embedding small-scale features into a high-permeability matrix. The model effectively reproduces dual-permeability zones with permeability values comparable to real rock samples. Using this platform, we investigated the porescale dynamics of salt precipitation and dissolution under a contrasting permeability condition. High-resolution microscopy and three-dimensional confocal laser scanning enabled visualization of salt, brine, and CO₂ phases as well as volumetric quantification of salt crystals. Distinct stages of salt crystallization were found as the initial nucleation locations within residual brine in high-permeability zones, followed by sustained growth of crystals from bulk fluid in brine layers, and the salt formation along permeability transition boundaries due to enhanced evaporation of bypassed residual brine in low-permeability regions. Due to increased capillarity caused by crystallization in low-permeability regions, brine flows out from the downstream end of the low-permeability region. This leads to the invasion of CO₂ from the downstream portion of the heterogeneity rather than the upstream. Precipitated salt modifies imbibition pathways and impacts long-term dissolution behavior when the injection stops. The findings highlight the capability of the microfluidic system to replicate complex geological salt precipitation and provide insight into mechanisms for CO₂ storage.