Experimental Study on Pore Structure Quantitative Characterization and Enhanced Oil Recovery During Air/CO2 Flooding of Shale Reservoir With Online NMR

Injecting air or CO2 into shale reservoirs can significantly enhance oil recovery (EOR) following the initial depletion. However, effectively characterizing the complex pore structure of shale reservoirs poses a challenge, leading to an incomplete understanding of the seepage mechanism and microscopic production characteristics of air/CO2 flooding at different pore scales. In this study, we characterized the microscopic pore structure of shale reservoirs through the reconstruction of visual and quantitative digital cores in multiple dimensions. Subsequently, the online nuclear magnetic resonance (NMR) air/CO2 flooding experiments were conducted, and the production characteristics and influencing factors of microscopic pore crude oil were quantitatively studied. The results show that the pore structure characteristics and connectivity of shale reservoirs are highly intricate and the deterioration of reservoir physical properties correlates with a decreasing trend in pore-throat coordination numbers and heterogeneity. Shale oil primarily occurs in three types of pores (< 0.1, 0.1–1, and 1–10 μm), and improving micronanopore recovery is urgent for EOR. Crude oil production is observed during the air and oil molecule generation low-temperature oxidation (LTO) reaction. Additionally, CO2 accelerates mass transfer and oil and gas extraction through molecular diffusion effects, substantially improving shale oil recovery; however, significant differences exist in the microscopic production characteristics of air/CO2 flooding. High-oxygen-concentration air flooding or high-pressure CO2 proves beneficial for EOR, especially for small pores and macropores, which contribute 45.75%–53.42% recovery. This study provides scientific and theoretical support for clarifying the microscopic production characteristics and efficient development of shale oil.