Exsolution, Evolution, and Hysteresis of CO ₂ Nanobubbles in Water‐Saturated Sandstone

Abstract Understanding CO 2 nanobubble formation in water‐saturated sandstone is critical for understanding fluid behavior in CO 2 storage systems. Here, CO 2 exsolution from an aqueous phase in a sandstone was investigated using small‐angle neutron scattering at 50°C during cyclic depressurization from 12 to 0.7 MPa. Nanoscale heterogeneities consistent with CO 2 clusters and nanobubbles (5–200 nm) were resolved during pressure reduction. Although bulk phase diagrams predict exsolution at ∼8 MPa at 50°C, detectable exsolution emerged only at 2.4 MPa, indicating strong confinement and surface effects. A progressive loss of signatures associated with nanobubbles <∼15 nm suggests preferential disappearance of nanobubbles, consistent with curvature‐driven coarsening (e.g., Ostwald ripening). Repeated cycling revealed partial qualitative reversibility, implying nucleation and saturation‐history (hysteresis) effects relevant to operational pressure transients in CO 2 storage. Our findings improve assessment of CO 2 mobility, trapping, and leakage risk under dynamic pressure in CO 2 storage systems.