Abstract: The change in nanopore structure of coal with the presence of water due to supercritical CO₂ (SCCO₂) exposure could significantly affects CO₂ storage in target coal seams. Thus, the interaction of SCCO₂-H₂O mixture fluid with long flame coal, gas coal and anthracite was simulated on a fluid geo-reaction system. All the samples were exposed to fluid at 45 ℃ and 12 MPa. The small angle X-ray scattering (SAXS) was used to address the response of nanopore structure and heterogeneity of coal samples to SCCO₂-H₂O exposure. The results indicate that SCCO₂-H₂O exposure reconstructs nanopore structure of coal samples. However, the responses of nanopores of different rank coals to the exposure are different. After the exposure, the porosity, specific surface area (SSA) and microporous content with diameter below 5 nm of long flame coal decrease, while mesoporous content with diameter greater than 5 nm increases. For gas coal after the exposure, a reduction in microporous content is recorded, but remarkable increases occur in mesoporous content and SSA. The increase in both porosity and SSA is found for anthracite. The multiple effects of coal matrix swelling, extraction effect due to SCCO₂ exposure, and dissolution and mobilization of mineral matters account for the different variation in coal nanopores. SCCO₂-H₂O fluid enhances the heterogeneity of pore size distribution of low-medium bituminous coal samples with the remarkable change in gas coal, while decreases the irregularity of pore surfaces of all the coal samples.