Capillary pinning refers to the immobilization of CO₂ at capillary barriers when the uprising CO₂ pressure is lower than the capillary entry pressure of the overlaying pore throats. Also known as local capillary trapping, it has been proposed as a fifth geologic CO₂ storage mechanism, alongside structural, solubility, residual, and mineral trapping. Despite extensive research, the fragmented terminology surrounding capillary pinning has led to confusion, making it challenging to synthesize findings effectively. Often conflated with mechanisms such as residual and hysteresis trapping, capillary pinning is commonly underestimated or completely overlooked in reservoir-scale models. Furthermore, difficulties in characterizing and upscaling small-scale geologic heterogeneities that influence capillary pinning contribute to significant uncertainties, with estimates of CO₂ trapped via this mechanism ranging from 3 % to 100 % of total CO₂ trapped via capillary actions. This review explores the fundamental mechanisms, experimental findings, and modeling approaches for assessing CO₂ capillary pinning in carbon capture and storage (CCS). It seeks to bridge the gap between the reservoir engineering community, with its extensive expertise in hydrocarbon recovery but that needs adjustments for CCS applications, and the subsurface storage community, which stands to benefit from this knowledge but often lacks access to relevant literature. Additionally, the study identifies key research opportunities to advance the understanding of capillary pinning in sedimentary rocks, ultimately enhancing the efficacy and reliability of CCS operations.

The North Sea’s potential as a Green Energy Hub depends on large-scale CO2 storage in shallow-marine sandstones, but the effects of geologic heterogeneity, such as permeability barriers and capillary entry pressure contrasts, remain underexplored. This study uses multiphase flow simulations on geologically realistic, surface-based reservoir models informed by outcrop analogue data from wave-dominated shoreface sandstones. We investigate how sedimentological heterogeneity influences CO2 plume migration, pressure evolution, and storage capacity.

Preliminary results show that capillary barriers tied to facies architecture and early cementation, conditioned to clinoform geometries, significantly control plume movement. These barriers promote lateral spreading and residual trapping, representing a potential upper limit on long-term CO2 storage when stable. Clinoform-related heterogeneity also induces flow compartmentalization, limiting pressure dissipation and enhancing anisotropy, which may reduce injectivity and cause spatially variable pressure buildup.

Comparisons with waterflood simulations reveal contrasting dynamics: water advances more uniformly, while CO2 migration is more sensitive to fine-scale architecture due to its lower interfacial tension and capillary entry pressures. These findings underscore the need to incorporate realistic sedimentological heterogeneity in dynamic models to avoid misestimating injectivity, pressure behavior, and storage security. This approach offers a robust framework for early-stage screening and risk assessment in complex storage settings.

As the products of chemical sedimentation in the Archean oceans, Banded Iron Formations (BIFs) have been interpreted to record (bio)geochemical transitions in Earth’s ancient biosphere. Nonetheless, the effects of diagenesis and metamorphism over the long history of these rocks make it difficult to identify the minerals involved in the earliest stages of BIF formation. A series of recent studies has suggested that greenalite (Fe2+3Si2O5(OH)4), formed through hydrothermal fluid-seawater interactions, was among the primary mineral components of BIFs. However, the reactivity of greenalite as a function of relevant environmental parameters has not yet been mechanistically studied. The plausibility of its role in forming BIF deposits therefore remains speculative. Here, we fill this knowledge gap by conducting a series of kinetic experiments using a novel Si isotope doping method with hydrated, amorphous Fe(II)-silicate (a precursor to crystalline greenalite). The advantage of this technique is that it permits simultaneous determination of near-equilibrium forward and reverse reaction rates of Fe(II)-silicate-fluid interaction in plausible Archean ocean compositions. Reaction rate calculations indicate that the system’s behavior is governed by Fe(II)-silicate saturation state, with SiO2 sorption becoming dominant once a saturation threshold is exceeded. Combining kinetic data and thermodynamic calculations for the Fe-silicate-seawater system permits determination of a new solubility product for amorphous Fe(II)-silicate as log(K) = 24.9 ± 0.25. This value indicates maximum Fe2+ concentrations in Archean ocean waters at 25 °C would range from ∼ 1 mmol/kg at pH 7 to ∼ 10 µmol/kg at pH 8. Combining these observations with calculations of Stokes’ settling velocity implies that long-distance transport of greenalite nanoparticles – e. g., from deep-ocean hydrothermal vent sources to loci of BIF deposition – would have been feasible. Coupled with SiO2 sorption behavior on greenalite surfaces and the background SiO2 flux associated with the unique styles of Archean chert deposition, these results suggest that periodic waxing and waning of greenalite nanoparticle transport to BIF depositional environments can help to explain the Fe- and Si-enriched layers preserved in BIFs. Our results also provide a mechanistic underpinning for the exceptional preservation of greenalite in Archean sediments and its frequent association with chert. Ultimately, the readiness with which greenalite would have precipitated from Archean seawater and its apparent ability to be preserved despite transport across ocean basins suggests that it is time to reassess the traces of Earth’s early oceans recorded in BIFs and the ways in which these may be interpreted in light of new depositional models.