Abstract: To study the injection process of CO₂-hydrocarbon gas mixtures in shale oil and understand the impact of various parameters and the fluid production degree in nano-micron-sized pore throats, nuclear magnetic resonance (NMR) experiments were conducted to reveal the huff-n-puff characteristics such as changes in final recovery, T₂ distribution curves, and the mobilization of crude oil in different pore sizes during injection, and explain the effects of gas mixture type, injection method, and total organic carbon (TOC) on the huff-n-puff efficiency. The results show that CO₂ + C₃H₈, compared with CO₂ + CH₄, has stronger miscibility with crude oil. Under the same gas injection pressure conditions, the final recovery of CO₂ + C₃H₈ huff-and-puff can be increased by 3.21 percentage points, with C₃H₈ contributing more to shale oil recovery. Combining the strong compressibility of CO₂ + CH₄ and the strong miscibility of CO₂ + C₃H₈, injecting CO₂ + CH₄/CO₂ + C₃H₈ can effectively produce crude oil in large, medium, and small pores of the shale, with the final recovery reaching 28.97 percentage points. As TOC of the shale increases, more crude oil is adsorbed and dissolved during saturation, reducing recovery but increasing gas mixtures diffused into kerogen. This study provides a theoretical basis for the field implementation of huff-n-puff injection of CO₂-hydrocarbon gas mixtures in shale oil.

Key words: shale oil, huff-n-puff of gas mixtures, nuclear magnetic resonance, nano- and micron-scale pore throats