Study on Effects of Fluid Properties on Borehole Pressure under Temperature and Pressure Coupling in High Temperature Deep Wells
High temperature high pressure(HTHP)environment in deep wells plays an important role in mud rheology control;omitting its effects will lead to an inaccurate knowledge of hole pressures and a negative influence on safe drilling.Based on the principle of energy conservation,a borehole temperature computation model is established.In this model a method of computing borehole temperature by coupling the effects of temperature and pressure is built up taking into account the effects of the fluid flow state on temperature and pressure.The reliability of the model is verified using data acquired from field operations.The study results show that the effect of temperature on the density and rheology of a drilling fluid is more important than the effect of pressure.As a well becomes deeper,the density and yield point of the drilling fluid in the annulus are also increasing.As the circulation time increases,the bottom hole temperature is decreasing,the density,yield point and flow index of the drilling fluid in the annulus are increasing,while the thickening index is decreasing.The ECD of the drilling fluid in the annulus under coupled temperature and pressure condition is lower than the ECD of the fluid in the same annulus without considering the coupling of temperature and pressure,the difference between the two ECD is 0.067 g/cm3.Hence,if the effects of temperature and pressure coupling on the density and rheology of a drilling fluid are not considered,a mud density lower than that is necessary to balance the formation pressure will be designed,and well kick and blowout may be induced.The results and understanding of this study provide a key theoretical base for precise evaluation of temperature and pressure in an ultra-deep well.
High temperature high pressureCoupling of temperature high pressureFluid densityRheological parameterPressure distribution
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