Microscopic Mechanism of CO₂ Displacement Oil in Different Pore Throat Radius Segments of Tight Sandstone Reservoirs

The micropore structure of tight sandstone affects the efficiency of CO₂ displacement of crude oil. As the pressure changes, the oil displacement efficiency (E _d) in segments with different pore radii changes, and the asphaltene precipitation in the pores causes alterations in the pore structure and wettability, which constrain E _d. Ten samples of tight sandstone from the Yanchang Formation in the Ordos Basin were selected for this study. A variety of methods, including X-ray diffraction (XRD), casting thin sections (CTS), scanning electron microscopy (SEM), high-pressure mercury intrusion (HPMI), CT scanning, and nuclear magnetic resonance (NMR) combined with CO₂ displacement, were used to study the efficiency of crude oil utilization and the amount of asphaltene deposited at different pore-throat radii, and then the impacts of pressure, pore structure, and wettability changes on E _d were discussed. The findings indicate that samples have three types: macropore-fine throats (MF), medium pore-tiny throats (MT), and small pore-microthroats (SM). The MT exhibits a favorable configuration. The pore-throat radius of each sample can be divided into two segments, namely, large pore segments (P_L) and small pore segments (P_S), and the P_L has a significant E _d. The E _d of the MF-type P_S is constrained by pressure. The E _d of P_L is significantly affected by the pressure sensitivity for the MT, while the E _d of P_L for the SM structure is more affected by pressure. Changes in wettability and the precipitation of asphaltene are the results of the reaction between crude oil and CO₂. In the MF, asphaltene precipitates from the P_L, while in the MT and SM, asphaltene precipitates both from the P_L and P_S. The amount of asphaltene precipitation strongly affects the E _d in P_S. The oil wettability increases more obviously with better pore-throat configurations. This study offers a reference and foundational understanding for evaluating CO₂ displacement in tight sandstone reservoirs.