Effect of pore structure and CO2-oil-rock interactions on sweep efficiency of CO2 EOR in tight sandstone reservoirs

CO₂-oil-rock interactions and complex pore structure affect the sweep efficiency (E_S) and wettability, thus having a significant impact on CO₂ enhanced oil recovery in tight oil reservoirs. In this study, we selected 10 rock plugs from the Yanchang Formation, Ordos Basin in China. First, casting thin sections and mercury intrusion capillary pressure were performed to investigate the microscopic pore structure characteristics of the tight rock samples. The results show that pore structure can be divided into three types (RT-I, RT-II, and RT-III) from good to poor qualities. On this basis, CO₂ floodings using the Nuclear Magnetic Resonance technique were performed to investigate the influence of pore structure on the E_S in large (P_L) and small (P_S) pore throat intervals. With the increase of displacement pressure, the oil recovery of RT-I, RT-II and RT-III are about 70.9%, 67.8% and 10.16%, respectively. The E_S of P_L of all samples are similar, while the E_S of P_S decrease subsequently for the three types. Pressure, mineral composition and the complex pore structure are attribute to the differences. On one hand, higher displacement pressure leads to lower interfacial tension and viscosity, resulting in higher oil recovery. On the other hand, CO₂ is more likely to vaporize the light oil components, resulting in the asphaltene precipitation. Quartz with a smooth surface is not easy to precipitate, while most clay minerals are easier to absorb asphaltene and are likely to alter the wettability of pore surfaces. Thus, in comparison to RT-III, the E_S of RT-I with a higher quartz content is higher in P_S. In addition, the worse the relationship between pore structure configurations, the greater the capillary pressure, causing the different E_S between RT-I and RT-II. The findings in this study shed a light on the understanding of complex mechanisms for CO₂ EOR in tight oil reservoirs.