Exsolution, Evolution, and Hysteresis of CO₂ Nanobubbles in Water-Saturated Sandstone

Understanding CO₂ nanobubble formation in water-saturated sandstone is critical for understanding fluid behavior in CO₂ storage systems. Here, CO₂ 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 CO₂ 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 CO₂ storage. Our findings improve assessment of CO₂ mobility, trapping, and leakage risk under dynamic pressure in CO₂ storage systems.