Based on the interactions between CO

2

and crude oil

CO

2

and conglomerate

the enhanced oil recovery（EOR） mechanism of CO

2

huff-n-puff has been revealed in Huanmahu tight conglomerate reservoir. Meanwhile

a high temperature and high pressure nuclear magnetic system was applied to monitor the change of oil saturation during experimental processes at pore scale in cores of substrates. Core fracturing and displacement system were used to investigate the effects of fractures on EOR during CO

2

huff-n-puff. The experimental results indicated that CO

2

could effectively dissolve in crude oil to replenish formation energy

reduce oil viscosity and improve wettability. Moreover

the crude oil produced in depletion development was in large pores

while that in medium pores and small pores could be effectively activated by CO

2

huff-n-puff. After depletion development

three cycles of huff-n-puff could enhance oil recovery by 23.1 %

among which the first cycle contributed most with factor of 15.1 %. The existence of fractures could increase the contact area between crude oil and CO

2

and decrease the flow resistance of crude oil

and finally the EOR is up to 27.1 %. All the results reveal the feasibility of CO

2

huff-n-puff to improve oil recovery in tight conglomerate reservoirs from pore scale.