Effects of micro pore structures and macro reservoir structures on salt precipitation during CO2 geological storage

Good reservoir injectivity is a fundamental requirement for high-quality CO₂ geological storage sites. Salt precipitation near injection wells, induced by brine evaporation and crystallization, is one of the key factors impairing reservoir injectivity. To clarify the differences in salt precipitation and consequent reservoir damage across distinct reservoir types, this study selected sandstone samples with varying micro pore structures and macro reservoir structures, and conducted salt precipitation simulation experiments based on a high-temperature and high-pressure core-flooding system. Employing thin-section analysis, scanning electron microscopy (SEM) with energy-dispersive spectroscopy (EDS), micro-area X-ray fluorescence (μ-XRF) spectroscopy, high-pressure mercury intrusion (HPMI), nuclear magnetic resonance (NMR), and micro-computed tomography (micro-CT) scanning, the salt crystal characteristics, distribution patterns, as well as variations in sandstone pore structures were systematically investigated. High-porosity and high-permeability reservoirs with favorable pore structures are characterized by a small number of salt precipitates, small-sized salt crystals, and dispersed single crystals. With the deterioration of the pore-throat size, sorting and connectivity, the quantity and size of salt crystals and their aggregates increase. With the change of the reservoir pore structures from the homogeneous large pore-throat type to the heterogeneous small pore-throat type, the distribution patterns of salt precipitation vary from weak homogeneous salt precipitation dominated by in-situ brine evaporation, to intensive local salt precipitation dominated by brine capillary backflow, and to intensive homogeneous salt precipitation controlled by brine capillary backflow and salt solute diffusion. Compared with macroscopically homogeneous massive sandstones, heterogeneous sandstones with low- and high-permeability zones generate a greater amount of salt precipitation both in low- and high-permeability segments. This is because the low-permeability zone lack effective brine displacement by injected CO₂, thereby retaining more residual brine for salt precipitation. Meanwhile, the salt crystal characteristics and contents indicate the existence of capillary backflow of brine from low-permeability zone to high-permeability zone. Reservoirs with different initial micro- and macro-structures have different characteristics of salt precipitation and thus varied risks of injectivity impairment. Heterogeneous reservoirs, both at the microscale and macroscale, carry a higher risk of salting-out induced reservoir damage, thus requiring the formulation of appropriate salting-out mitigation methods for these reservoirs.