锥形聚晶金刚石复合片齿破岩特征与机制研究

Rock breaking characteristics and mechanism of conical polycrystalline diamond compact cutter

锥形聚晶金刚石复合片(polycrystalline diamond compact,PDC)齿是一种具有较强抗冲击性和耐磨性的新型PDC齿,在坚硬、强研磨性和软硬交错地层中取得了非常好的钻井提速效果。为了揭示锥形齿破碎硬岩机制,开展锥形齿破碎花岗岩试验与数值模拟研究,分析了切削深度和前倾角对锥形齿切削力和破岩比能的影响规律,采用高速摄像机和透明K9玻璃观测了锥形齿作用下岩屑形成过程及微裂纹萌生与扩展过程,通过数值模拟分析了破岩过程中岩石应力响应与损伤演化特性,结合对切削槽和大尺寸块体岩屑表面形貌及断口微观特征分析,建立了锥形齿破碎花岗岩机制模型。结果表明,锥形齿破碎花岗岩的过程可以分为挤压成核和块体崩裂两个阶段,前倾角对岩石破碎过程影响较小,切削深度的影响显著;锥形齿周围的裂纹主要由压实核、纵向裂纹和横向裂纹组成,纵向裂纹和横向裂纹扩展的最大深度分别为切削深度的6.69倍和4.53倍;齿尖周围压应力集中,岩石发生压剪破坏,压应力区外围形成弧形条带状拉应力区,并在齿尖及压应力区边界处诱导出拉伸微裂纹;微裂纹向齿前扩展形成弧形拉伸主裂纹,发生块体岩屑崩裂,提高破岩效率,向岩石内部扩展劣化岩石强度,形成底部损伤区,提高后续破岩效率。

Conical polycrystalline diamond compact(PDC)cutter is a new type of PDC cutter with high impact and wear resistance,which has a very good drilling effect in hard,strong abrasion,and soft-hard interbedded formations.In order to reveal the hard-rock breaking mechanism of conical PDC cutters,the laboratory test and numerical simulation on the granite broken by the conical PDC cutter were carried out.The influence law of cutting depth and front rake angle on cutting force and rock breaking specific energy of the conical PDC cutter was analyzed.In the laboratory test,a high-speed camera and transparent K9 glass were used to observe the formation process of cuttings and the initiation and propagation of microcracks under the action of conical PDC cutters.The stress response and damage evolution characteristics during the rock breaking process were analyzed by numerical simulation.The surface morphology and fracture microscopic characteristics of cutting grooves and large-size cuttings were analyzed,and the mechanical model of conical PDC cutters to break granite was established.The results show that the granite breaking process can be divided into two stages:crushing and chipping.The effect of the front rake angle on the rock breaking process is relatively small,while the influence of cutting depth is significant.The cracks around the conical PDC cutter are mainly composed of compaction nucleation,longitudinal crack and transverse crack.The maximum propagation depth of the longitudinal and transverse cracks is 6.69 and 4.53 times the cutting depth,respectively.The compressive stress around the cutter tip is concentrated,the shear-compression failure occurs,an arc-shaped strip-like tensile stress zone is formed at the periphery of the compressive stress zone,and the tensile micro-cracks are induced at the boundary of the cutter tip and compressive stress zone.When the micro-crack propagates to the front of conical PDC cutters to form an arc-shaped tensile main crack,the block debris cracking occurs,which improves the rock breaking efficiency.Meanwhile,the tensile micro-cracks propagate to the rock inside and deteriorate rock strength,a bottom damaged area is formed,which improves the rock breakage efficiency of the subsequent cutting.