Research On The Flow Field Of The Two-stage PDC Bit In The Bottom Hole

Two-stage PDC(polycrystalline diamond compact)bit is a rock breaking tool composed of a pilot with a small diameter and a reamer with a large diameter.With its unique rock breaking structure,this bit can improve rock breaking efficiency in deep hard and abrasive strata.Bit hydraulics affects drilling life of bit.Predecessors only studied the flow distribution of two-stage PDC bit with formula derivation,not studied the bottom-hole flow field with numerical simulation.In this paper,the following work was done for drilling bottom-hole flow field of two-stage PDC bit:(1)Using Solid Works to establish the 3D model for two-stage PDC bit(pilot Φ 171.45 mm + reamer Φ 215.9mm)bottom-hole flow field.Grid generation,numerical simulation and post-processing are completed on ANSYS software platform.(2)the flow distribution of two-stage PDC bit,and velocity and pressure field characteristics in the nozzle position outflow channel were studied from four aspects,i.e.,the area ratio of the pilot nozzle to the total nozzle,the area ratio of the central nozzle to the peripheral nozzle of the pilot,the drilling fluid displacement and viscosity.(3)the cuttings cleaning and carrying effect of two-stage PDC bit drilling in bottom hole were studied from seven aspects,i.e.,the area ratio of the pilot nozzle to the total nozzle,the area ratio of the center nozzle to the peripheral nozzle of the pilot,drilling fluid viscosity and displacement,ROP(rate of penetration),rotary speed of bit,and cuttings particle size.The results show that: when the area ratio of the pilot nozzle to the total nozzle is 0.75～0.8,and the area ratio of the central nozzle to the peripheral nozzle of the pilot is 1.22～1.5,the average cuttings mass concentration in the bottom hole flow field is low,and the cuttings cleaning and carrying efficiency is high.This study is expected to provide a reference for the optimal design of the hydraulic structure of the two-stage PDC bit.