The water cut of heavy oil reservoir in Dagang Oilfield increased rapidly in the middle and late period of water flooding

and the production effect became worse. CO

2

huff and puff technology is an effective method to develop heavy oil reservoirs. But the parameter optimization and field effect in the late stage of high water cut needed to be studied urgently. Therefore

the heavy oil reservoir in Dagang Oilfield was used to carry out the experiments of CO

2

injection for solubilization and swelling with viscosity reduction. Based on the experiments and well logging data

a single-well numerical simulation model was established to simulate the reservoir parameters and CO

2

injection parameters

and analyzed the CO

2

oil stimulation mechanism. Based on the results of theoretical researches

the in-site CO

2

huff and puff experiments were carried out in Banqiao and Liuguanzhuang. The research results show that the mechanism of water control and oil increase by CO

2

is mainly to expand the volume of crude oil and reduce the viscosity of crude oil

and the viscosity can be reduced by up to 98 %. The impact of injection volume

injection speed

and throughput cycle is relatively large on the CO

2

throughput effect. It is recommended that the CO

2

injection volume of a single well is 600~1 000 t（0.22~0.37HCPV）

the injection speed is 40~80 t/d

and the throughput runs 3~4 cycles. In Banqiao and Liuguanzhuang

CO

2

huff and puff have been carried out 12 times on wells

with an average increase of 3.4 times of oil production per well and a 52.2 % reduction of comprehensive water cut. Thus

CO

2

huff and puff technology is an effective water-control and oil-increasing technology

which has important reference significance for improving oil recovery in the later stage of water injection in similar heavy oil reservoirs.