Background: Gas exsolution from supersaturated water injection (SWI) into porous media is of primary interest and has important applications (CCS, residual NAPL-remediation, CO2-enhanced oil recovery). Bubble formation and growth kinetics are typically studied in 2D-micro-models (sandstones-analoges; Zuo et al., 2013; Zuo and Benson, 2013). The only 3D-study was conducted by Li et al., 2017, using the NMR-method, and indicated a strong dependency of the exsolution process on the initial CO2-gas phase. However, the NMR-method was not able to attribute these different fluid-fluid-saturations to key parameters of the dissolution process such as gas cluster morphology and gas cluster size distribution. Methods: We conduct a series of column experiments to study the gas exsolution and gas cluster formation in 3D-porous media (natural sands, glass beads) using μ-CT. Based on this high-resolution non-invasive visualization method followed by image processing, we quantify (i) gas-cluster morphology, (ii) gas-cluster size distribution, (iii) correlation between pore structure and bubble formation, and (iv) the impact of surface roughness on exsolution efficiency. Results: We found that CO2-saturated water equilibrated under ambient pressure, pCO2 = 1.013 bar, (no supersaturation was measured and no pressure reduction was applied), already leads to gas exsolution of significant amount (about 10–12% gas saturation) in the presence of untreated SiO2-surfaces (natural sands, glass beads) which exhibit heterogeneous wettability. To the best of our knowledge this exsolution phenomenon was not observed before, and we assume it to be caused by fluid-rock interactions, i.e. by hydrophobic nucleation sites at the siliceous surface. The heterogeneous wettability has a dramatic impact on capillary trapping efficiency and was experimentally observed in Glass-beads monolayer (Geistlinger and Ataei, 2015, Influence of the heterogeneous wettability on capillary trapping in glass-beads monolayers: Comparison between experiments and the invasion percolation theory, J. Colloid Interface Sci., 459, 230 - 240).