Experimental study of oil recovery from pore of different sizes in tight sandstone reservoirs during CO2 flooding

The investigation of tight oil reservoirs has become a significant area of interest in unconventional oil development. CO2 flooding is considered an effective oil recovery method for tight oil reservoirs as it can significantly increase oil recovery when it reaches supercritical condition and becomes miscible with oil. Although extensive research has been conducted on CO2 flooding, the oil recovery from pore of different sizes in tight sandstone reservoirs at different pressures has not been thoroughly investigated. In this study, we analyzed the petrology features and the pore structure of a reservoir using casting section and scanning electron microscope (SEM) images. Subsequently, three cores with different permeability from the reservoir were subjected to CO2 flooding experiments at different pressures. A nuclear magnetic resonance (NMR) spectrometer was used to quantify the oil recovery. Amott-Harvey index was measured to study the effect of CO2 flooding on core wettability. Results indicate that the total oil recovery and the oil recovery of smaller pores increase as the pressure increasing. The oil recovery of the larger pores does not increase continuously. The oil recovery of the smaller pores is more dependent on pressure than that of the larger pores, and the total oil recovery is related to the oil recovery of smaller pores. Meanwhile, the supercritical and miscible CO2 has a positive effect on oil recovery. After CO2 becoming supercritical and miscible, the cores still have potential to produce more oil with the pressure increasing. As the pore structure is playing a significant role in oil recovery, the core with a higher proportion of the volume of larger pores is relatively easy to produce more oil at low pressure. Amott-Harvey index shows CO2 flooding can significantly reduce the hydrophilicity of cores. This study reveals the mechanism of pressure effect on oil recovery during CO2 flooding in tight sandstone reservoirs. The results can be used to improve the efficiency of reservoir development.