Microscopic Mechanism of CO 2 Displacement Oil in Different Pore Throat Radius Segments of Tight Sandstone Reservoirs.

The micropore structure of tight sandstone affects the efficiency of CO ₂ 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 CO ₂ 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 CO ₂ . 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 CO ₂ displacement in tight sandstone reservoirs.
Competing Interests: The authors declare no competing financial interest.
(© 2024 The Authors. Published by American Chemical Society.)