关键词: CO₂混相驱, 极限关井时间, CO₂水合物, 冻堵

Abstract: The development of low permeable reservoir by means of alternate water gas injection can not only enlarge the swept volume, supplement formation energy, but also effectively prevent gas channeling. However, the problem of plug in gas well often occurs during the construction. By simulating the distribution of internal pressure and temperature in the wellbore of the gas injection well, the cause of freezing and plugging is analyzed. The main reason is the long time shut in of the gas injection well, resulting in the formation of fluid flowing back to the frozen plugging section and the CO₂ reaction in the wellbore to form a frozen plug. Therefore, the well test method is used to calculate the pressure change rule in the wellbore after closing the well, and the time of formation fluid to freeze plugging is obtained. The results show that the gas injection rate has great influence on the wellbore temperature field distribution. As the injection speed increases, the low temperature in the wellbore extends longer. The phase state of CO₂ gas can change within 1 000 m of the wellbore, which causes low temperature and high pressure in the wellbore and makes hydrate easily. Finally the limit closure time of injection well is determined to be 25 d according to the law of oil and gas twophase seepage and the law of pressure change after well closure.

Key words: CO₂ miscible flooding, Maximum shut in time, CO₂ hydrate, Freeze plug