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.