A Study on the Macroscopic and Microscopic Mechanisms of Physical Property Changes in Tight Sandstone under CO₂ Miscible and Immiscible Displacement

Abstract China’s tight sandstone oil reservoirs are abundant in reserves. Due to low permeability, strong heterogeneity, and insufficient natural energy, CO 2 injection is an important approach to enhance oil recovery and achieve carbon sequestration in such reservoirs. Considering the complexity of CO 2 -fluid-rock interactions in tight sandstone reservoirs, there was a lack of consensus on changes in physical properties influenced by CO 2 . This study investigated the extent of CO 2 -fluid-rock interactions at different locations in tight sandstone from Ordos basin through CO 2 miscible or immiscible displacement and rock cutting into slices. The changes in porosity, permeability, wetting angle, and mineral composition of sandstone after reaction were explored. The variation patterns influenced by different CO 2 injection pressures (3 MPa, 6 MPa, 9 MPa) and initial physical property were compared. The experimental results indicated that under CO 2 miscible and immiscible displacement, the porosity at the inlet of different tight sandstone cores generally decreased, while permeability increased; the porosity and permeability at the outlet decreased. When CO 2 injection pressure was high, dissolution and fine particle migration blockage were strong, and the porosity and permeability at the far end of some cores decreased. When CO 2 reached the supercritical state, CO 2 -fluid -rock reactions intensified, and the impact on pore-throat changes was pronounced. The concentration ratio of feldspar, calcite, and clay minerals in sandstone decreased and oil wettability increased after the action of CO 2 , when the CO 2 pressure reached the supercritical state. This paper explored the mechanisms of micro-scale and macro-scale physical property changes at different locations in tight sandstone during CO 2 injection. It could provide a theoretical basis for the development of tight sandstone reservoirs driven by CO 2 injection.