Serious retrograde condensation has occurred before the development of gas cap in South A condensate gas reservoir of Zanarol Oilfield

resulting in serious loss of condensate oil. It is particularly important to explore the method of improving the production of condensate oil for the further development of oilfield. Therefore

taking a typical well in South A gas reservoir as a example

the phase equilibrium theory and numerical simulation are applied to study the retrograde condensation rule of formation fluid and the distribution of condensate oil in formation. On this basis

the mecha

nism and effect of condensate oil enhancement by CO

2

huff-n-puff are researched. The results show that retrograde condensation in South A gas reservoir has two stages: constant composition expansion（CCE） and constant volume depletion（CVD）. Condensate oil distributes mainly within 30 m of wellbore

and its saturation ranges from 22.44 % to 29.51 %. The mechanism of CO

2

increasing condensate production is to change the retrograde condensation rule

reduce pressure condition of retrograde condensation

and evaporate condensate oil. However

if the injection volume of CO

2

is low

the volume of vaporized condensate oil will be small

which limits the effect of EOR. Therefore

the cumulative injection of CO

2

should be more than 500×10

4

m

3

at a cycle to ensure the effect of CO

2

huff-n-puff. This study has guiding significance for condensate oil enhancement and plug removal in the middle and later stages of similar condensate gas reservoir development.