基于DPM模型的PDC钻头井底岩屑黏附规律研究

Study on Downhole Cuttings Adhesion of PDC Bit Based on DPM Model

为提高页岩气钻井效率、减少钻头泥包,根据现场常用PDC钻头建立三维流域模型,将岩屑模拟为从井底面射入流场的球状颗粒,用拉格朗日粒子追踪法描述固相,将钻头体DPM边界条件设置为捕获,以岩屑颗粒捕获率作为反映泥包可能性的评价参数,结合井底流场结构特点,模拟分析了岩屑输运与黏附规律。数值模拟结果表明,井底岩屑输运时沿程存在碰撞与反弹的不规则三维运动,这导致部分岩屑黏附到钻头体表面;对于不同尺寸的岩屑颗粒,各刀翼和排屑槽的岩屑颗粒捕获率Ra呈类似的变化规律,当岩屑粒径为10 mm时Ra最大,当粒径小于1 mm时,Ra较小且变化不大。岩屑容易发生黏附的情况为:流通面积较小处,流场异常复杂、三维特性明显处及存在大面积的涡旋处,涡旋导致流线偏向钻头体。增大排屑槽最小流通面积,提高切削齿附近流速,减小涡旋数量和强度,有利于预防和减少钻头泥包。

To improve shale formation drilling efficiency and reduce bit balling, a three?dimensional flow vol?ume model of onsite typical PDC bit has been established?The cuttings are treated as spherical particles flow into the flow field incident from the bottomhole face, and are controlled by the Lagrangian particle tracking method?DPM boundary condition of bit body is set as capture?The capture rate of cuttings is set as the evaluation parameter of bit balling possibility?The cuttings transport and adhesion rule are analyzed considering the downhole flow field structure?The simulation results show that the bottomhole cuttings transport is an irregular three?dimensional motion along with the collision and rebound, which led to some cuttings adhering to the surface of the bit body?For differ?ent sizes of cuttings, the cutting capture rate (Ra) of each blade and junk slot is in similar variation pattern?The Ra has the maximum value with the cuttings size of 10 mm in diameter, and becomes small and changeless when the cutting size is small ( less than 1 mm ) ?Situations that are prone to cuttings adhesion include: where has a small flow area; where exist a very complex flow field and strong three?dimensional characteristics; and where present a large area of vortex which makes flow line deviate to the drill bit body?Increasing the minimum flow area of junk slot, improving the flow rate near the cutter, and reducing the number and intensity of vortex are in favor of helping prevent and eliminate bit balling.