Research on the characteristics of earthworm-like vibration drilling

The load transfer difficulty caused by borehole wall friction limits the rate of penetration (ROP) and extended-reach limit of complex structural well enormously. A new friction reduction technology called “earthworm-like drilling”, which can improve the load transfer and extended-reach limit, is proposed in this paper. A mathematical model based on “soft-string” model is developed to analysis the characteristics of this technology. Simulation results indicate that more stick-slip and load transfer issue are caused by higher friction. The earthworm-like drilling is more effective in reducing friction than single-point vibration drilling and less effective than multi-point vibration drilling because of the pulse pressure attenuation. However, this disadvantage can be offset by adding the number of axial oscillators. Amplitude and frequency of pulse pressure and the installation position of axial oscillators have great impact on the friction reduction and load transfer. An optimizing model based on projection gradient method is developed and used to optimize the position of three axial oscillators in a three-dimensional horizontal well. The weight on bit (WOB) increases significantly after the optimized position and the new position of axial oscillators move towards bottom of well and close to each other. Results verify the feasibility and advantages of earthworm-like drilling, and lay a solid theoretical foundation for its application in oil field drilling.