CO2 Exsolution and Nanobubble Evolution in Sandstone under Cyclic Depressurisation

Understanding the nucleation, growth, and persistence of CO₂ gas phases in water-saturated porous media is critical for predicting fluid transport, trapping efficiency, and integrity in geological CO₂ storage systems. Gas exsolution under depressurisation remains poorly constrained at the nano- to micro-scale, where capillarity, confinement, and surface chemistry strongly influence phase behaviour. In this study, we investigate CO₂ exsolution from water saturating a clay-rich sandstone using small-angle neutron scattering (SANS) under realistic reservoir conditions, providing direct, in situ insights into gas phase evolution within the pore space.

SANS experiments were conducted at the EQ-SANS instrument at Oak Ridge National Laboratory using a pressure cell allowing for exsolution testing at 50 °C under cyclic depressurisation from 12 MPa to 0.7 MPa. The pore fluid consisted of a contrast-matched H₂O–D₂O mixture (68:32 vol.%), yielding a stable scattering length density of 4.17 × 10¹⁰ cm^-2, similar to that of the matrix. The H₂O:D₂O mixture was saturated with CO₂ at 12 MPa and room temperature (~22 °C) prior to controlled pressure reduction. Under these conditions, the scattering signal arises from exsolved CO₂ nanobubbles. SANS profiles were obtained continuously during pressure decrease.

The scattering data reveal the emergence and evolution of nanoscale heterogeneities consistent with CO₂ gas clusters and nanobubbles forming within pores between 5 and 200 nm. Although phase diagrams predict CO₂ exsolution at about 8 MPa and 50 °C, this is only observed at ~2.4 MPa. Changes in scattering intensity and slope indicate pressure-dependent growth and coalescence processes, influenced by pore confinement and clay mineral surfaces. Notably, a progressive loss of scattering signatures associated with pores smaller than ~15 nm during pressure reduction suggests the preferential disappearance of CO₂ nanobubbles in the smallest pores. This is potentially driven by Ostwald ripening, whereby gas diffuses from high-curvature, unstable nanobubbles toward larger, more stable gas clusters. Repeated pressure cycling highlights the partial reversibility of exsolution and the persistence of gas features, suggesting potential hysteresis effects relevant for cyclic injection and pressure management strategies.

These findings demonstrate the capability of SANS to resolve nanoscale CO₂ exsolution processes in complex geomaterials and provide critical constraints for pore-scale and continuum models of multiphase flow and transport. The results have direct implications for assessing CO₂ mobility, trapping mechanisms, and leakage risk in clay-rich storage formations and caprocks under dynamic pressure conditions.