Study On Friction Reduction Mechanism Of Drillstring Longitudinal Vibration In Extended Reach Wells

The extended reach well has the characteristics of large inclination,long slant depth and long open-hole section,which make the strings weight generate great contact force to the wellbore wall as well as increase the friction force between the strings and the wellbore wall.The additional friction force due to the couple effect of axial force and buckling deformation is generated once the pipe string turns into buckling instability.The high friction force between the strings and the wellbore wall has a strong impact on the transfer efficiency of weight-on-bit.It has been proved at drilling site that down-hole vibration technology can reduce the friction force effectively,and the longitudinal vibration has a better efficiency of friction reduction than transverse vibration and torsional vibration.Although the longitudinal vibration down-hole tools occupy the major part,the effect of string longitudinal vibration on friction reduction is still in a lack of adequate research.Here,the effect of longitudinal vibration on static friction and dynamic friction is studied respectively from macroscopic and microscopic perspectives.Based on double line friction hysteretic model,the extended reach well string longitudinal vibration model is established,and the simulation experiment device of drilling string longitudinal vibration anti-friction referring to actual field conditions is established.According to experimental and theoretical results,the effect of vibration anti-friction tool vibration parameters(frequency,amplitude and the excitation amplitude,etc.),drilling working condition parameters(drilling fluid viscosity and inclination,etc.)and position of tools on friction reduction is studied,and the suggestion to the rational use of vibration anti-friction tool is given.All researches show that:(1)The main principles of extended reach well string longitudinal vibration anti-friction include changing the static friction into kinetic friction,reducing average friction;(2)The amplitude and frequency have significant impacts on friction force,and ensuring the vibration frequency as far as possible near to the string resonance frequency with the excitation amplitude relatively large;(3)As for the efficiency of longitudinal vibration anti-friction,the tool vibration parameters have better effect than the drilling working condition parameters,therefore,give priority to achieve the tool vibration parameters to the best as far as possible by choosing vibration anti-friction technology,and using the relative solutions to optimize drilling working condition parameters at the same time;(4)The efficiency of vibration anti-friction can be improved significantly by putting the position of vibration anti-friction tool near the middle area of the inclined straight string;(5)The additional contact force due to buckling can reduce the efficiency of friction reduction of string longitudinal vibration tool significantly.