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