Shale oil, as one of the unconventional resources, has become increasingly important to supplement conventional oil and gas. Although CO 2 has great potential to enhance shale oil recovery by the huff and puff method, the experimental results show that the enhanced values of shale oil recovery are unstable and have great fluctuations for different cores. In order to increase the stability of CO 2 huff and puff and further enhance oil recovery, cosolvents such as ethanol, isopropanol, ethyl acetate, and acetone were chosen to construct CO 2 /cosolvent mixed fluids and the experiments of huff and puff by the mixed fluids were carried out. The results show that CO 2 /cosolvents can enhance the shale oil recoveries of the matrix and fracture. CO 2 /ethanol has the highest oil recovery among the mixed fluids. In particular, for cycles 2 and 3 in the huff and puff process, CO 2 /ethanol has a much larger oil recovery than CO 2 . Moreover, the effects of ethanol concentration in the mixed fluids on the enhanced shale oil recovery were investigated. The enhanced oil recoveries by CO 2 /ethanol mixed fluids increase with the increase of ethanol concentration. Furthermore, the mechanisms to enhance shale oil recovery by CO 2 /cosolvent fluids were discussed by measurements of the pressure decay process and interfacial tension. The presence of ethanol can not only accelerate the transfer of CO 2 to the oil phase in the porous media during the huff process, to make more CO 2 transfer into the oil phase, but also decrease the interfacial tension between oil and CO 2 as well as the minimum miscibility pressure, to make oil and CO 2 mixed fluids have a longer time to flow out in one phase than CO 2 in the puff process. Therefore, the CO 2 /cosolvent mixed fluids, especially CO 2 /ethanol, can be used to further enhance shale oil recovery during the huff and puff process.

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