超临界二氧化碳钻井流体关键技术研究

超临界二氧化碳钻井技术是利用超临界二氧化碳作为钻井流体的一种新型钻井方法,具有能有效驱动深井井下马达,控制井底压力容易,破岩门限压力低、破岩速度快,能防止储层损害等优点,但成功利用超临界二氧化碳钻井技术的关键是充分了解超临界二氧化碳钻井过程中井筒中二氧化碳流体的温度和压力分布。为此,建立了考虑井筒流体与地层换热对井筒流体温度影响的井筒传热模型,根据能量守恒原理,推导出了井筒流体温度计算模型,并考虑到钻井过程中可能钻遇水层的情况,对该计算模型进行了修正;利用有限元方法,推导出了井筒内二氧化碳钻井流体的压力计算公式。实例计算表明:钻杆内二氧化碳流体的温度和压力随井深增深而增大,但与井深的关系是非线性关系;钻杆内二氧化碳流体的密度随井深的增加而减小,但到近钻头处开始增大。环空中的压力随井深的增加而增大,但两者的关系也是非线性关系;环空中的温度随井深增加先升高后降低;环空中的二氧化碳密度随井深增加而增大,但两者为非线性关系。 

Key Technology on Drilling Fliud of Supercritical Carbon Dioxide

The supercritical carbon dioxide(SC-CO2) drilling technology is a new type of drilling method,which has the advantage of effective driving of downhole motor in deep underbalance drilling,easy control of downhole pressure,low threshold pressure and fast speed of rock breaking,preventing reservoir damage etc.The key in successful use of SC-CO2 drilling technology is fully understanding the temperature and pressure distribution of the carbon dioxide fluids in wellbore during the drilling process.Therefore,the mathematical model taking into account of the effects of heat exchange between the wellbore fluid and the formation on well fluid temperature was set up.The model for wellbore temperature calculation was derived based on energy conservation principle.In the case of encountering water layer,the model was rectified.With the finite element method,the wellbore pressure and pressure lose can be calculated.Case calculation results show that temperature and pressure of carbon dioxide fluid in the drill pipe increases with the depth,but the relationship of temperature and pressure with depth is non-linear.The density of carbon dioxide fluid in drill pipe decreases with the increase of depth,but it begins to increase near drill bit.Annulus pressure increases with the increase of the depth,but the relationship between the two is non-linear.With the increase of depth,the temperature in annulus increases first and then decreases.The density of carbon dioxide increases with the depth in annulus,but the relationship between the two is also non-linear.