超声波辅助PDC切削齿振动破岩仿真分析

Rock breaking simulation analysis for the ultrasonic vibration‑assisted PDC cutter

PDC钻头在软至中硬地层钻进时具有钻速高、使用寿命长、设计灵活等显著优点,在钻井领域中的需求量逐年增加。而超声波钻进作为一种新型碎岩技术,由于在钻进过程中具有穿透能力强、钻进效率高等优点而获得了广泛关注。以超声波振动辅助PDC钻头破岩有望取得良好的钻进效果。为此,基于线型Drucker-Prager模型,利用ABAQUS软件建立了超声波辅助PDC钻进振动切削岩石的二维有限元模型,分析了不同超声波振动频率下PDC钻进破岩比功和切削力的变化规律,比较了超声波振动切削与常规切削岩屑形成过程的差异。研究结果表明,当激励频率从20 kHz至40 kHz增长的过程中,破岩比功与平均切削力都呈现先减少后增加的变化趋势,即存在一个最优频率位于25~30 kHz间,使破岩比功最小,钻进效率最高;超声波辅助振动切削的破岩方式与常规切削的塑性破坏不同,主要以脆性破坏为主,其切屑形成过程共分为4个阶段,且切削力保持为零的阶段较常规切削更为明显;当激励频率接近岩石固有频率时,超声波振动切削的平均切削力较常规切削小20.5%,并更易产生大块岩屑,使岩石产生更多体积破碎,从而提高破岩效率。

PDC bits have the advantages of high rate of penetrate(ROP),long service life and flexible design when drilling in soft to medium‑hard formation.The demand of PDC bits in drilling field is increasing yearly.As a new type of rock breaking technology,ultrasonic drilling has attracted wide attention due to its strong penetration and low drilling cost.However,the understanding of the formation mechanism of rock cuttings at micro level and the change of cutting force in the cutting process is not enough,which makes the wide application of this technology limited.In order to deeply understand the cutting mechanism of ultrasonic drilling,based on the linear Drucker-Prager model and ABAQUS software,a two‑dimensional finite element model for ultrasonic vibration assisted rock cutting is established,and the formation mechanism of rock cuttings at micro level of ultrasonic vibration cutting is analyzed and compared with the conventional single tooth cutting process.In addition,the variation law of rock breaking specific work and cutting force at different frequencies is analyzed.The results show that,in the process of ultrasonic vibration assisted cutting,the cutting force decreases to zero more obviously than conventional cutting;when the excitation frequency is close to the natural frequency of rock,ultrasonic vibration cutting has smaller cutting force than conventional cutting,which is conducive to improve the stress state of the tool and reduce the wear degree of the tool;ultrasonic vibration cutting is easier to produce large blocks than conventional cutting when the frequency increases from 20kHz to 40kHz;the rock breaking specific work and average cutting force decrease first and then increase,indicating that there is an optimal frequency at which the specific work of rock breaking is minimum while drilling efficiency is highest.