Abstract: In order to evaluate the oil displacement effect of CO₂ huff-n-puff of shale oil reservoirs and the mobility of crude oil under different scales of pore throats, a natural core of Chang7 reservoir in Zhidan Oil Region of Yanchang Oilfield was selected, based on high temperature and high pressure CO₂ huff-n-puff in long cores and residual oil in short cores online NMR test experiments, and on the basis of evaluating the oil displacement effect, the mixed-phase state of CO₂ and the displacement law at different pressure measurement points were analyzed, the effect of mixed-phase huff-n-puff was compared with that of non-mixed-phase huff-n-puff at nano- and micron-scale pore throats, and the mobility of crude oil in different scale spaces of shale oil reservoirs was evaluated. The study shows that the effect of CO₂non-mixed-phase huff-n-puff was poor, the pressure inside the core increase was not obvious, and the pressure affecting range is small; under the condition of injecting the same pore volume multiplier, the recovery efficiency was increased by 8.24 percentage points compared with CO₂ non-mixed-phase huff-n-puff; the production pressure difference in the first 2 cycles of CO₂ mixed-phase huff-n-puff was large, the distance between CO₂ mixed-phase huff-n-puff intervals was long, and the characteristics of CO₂ mixed-phase displacement were obvious; in the whole pore throat of the rock sample, the proportion of micron-level pores was low, and nano-and submicron-level pores were the the main contributor of the crude oil. The study results can provide an important reference for shale reservoirs to replenish formation energy and improve recovery efficiency.

Key words: mobility evaluation, long core, online NMR, CO₂ huff-n-puff, Zhidan Oil Region