榆树林油田CO2混相驱注气井极限关井时间研究

采用水气交替注入的方式开发低渗透油藏既可以扩大波及体积，补充地层能量，又能有效的防止气窜。但是现场施工过程中经常出现注气井的冻堵问题。通过软件模拟注气井井筒内部压力、温度分布规律，分析注气井冻堵原因主要为注气井长时间的关井导致地层流体返出到易冻堵段与井筒内CO₂反应形成冻堵物，因此采用试井的方法计算关井之后井筒内压力变化规律，进而得出地层流体到冻堵段的时间为该井的极限关井时间。研究结果表明,注气速度对井筒内温度场分布影响较大，随着注入速度的增加井筒内低温延伸越长；井筒内部1 000 m以内会发生CO₂气体的相态变化，造成该位置低温高压，极易形成水合物；最后根据油气两相渗流规律和关井后井筒内压力变化规律确定注气井极限关井时间为25 d。

The Maximum Shut in Time of Gas Injection Wells in CO2 Miscible Flooding in Yushulin Oilfield

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.