<p>Multi-thermal fluid stimulation has been acknowledged as an effective recovery solution for heavy oil production. However, the effects of the main fluid, CO₂, on heavy oil formation are still under debate. To explore the responses of heavy oil formations to injected CO₂ concerning multi-thermal fluid stimulation, we performed CO₂ exposure experiments on heavy oil sandstone samples under high pressure, high temperature, and varying durations. The pore structure and mineral composition of the sandstone samples were checked before and after CO₂ exposure. The quartz-rich sandstone samples with no carbonate showed increased contents of quartz and clay, the maximum of which were from 66 to 77.6% and from 10.8 to 18.6% respectively, and decreased content of feldspar with a maximum from 23.2 to 8.1%. The increased clay content was manifested by a decreased proportion of illite and an increased proportion of the chlorite-montmorillonite mixed layer. The mineral alterations were not controlled by exposure temperature, but impeded by increasing pressure and promoted by longer duration. Micropore and mesopore structures of the sandstone samples changed consistently with reduced pore volume and were sensitive to exposure temperature and pressure rather than exposure duration. The mesopores smaller than 8&#xa0;nm were reduced remarkably due to CO₂ exposure. On the contrary, the macropore structure of the sandstone samples kept relatively stable. Permeability of the sandstone samples decreased after CO₂ exposure in general. Therefore, the permeability enhancement for heavy oil formations after CO₂ exposure is not always true and needs to be checked for the targeted formation.</p>