钻头冠部形状对破岩效果的影响

Influences of bit profiles on possible fracture modes

用有限元方法模拟了不同形状钻头的破岩效果,研究了地层应力分布及裂缝延伸模式。按照国际钻井商协会(IADC)分类标准设计了14种钻头形状。设计了地层模型,给出了钻头尺寸、地层性质、网格划分方法和边界条件等。采用拟静态条件和质量缩放法以减少模拟时间、提高模拟精度。模拟结果表明,对于9种常用钻头形状,增加锥体高度高应力区面积会增大,而保径高度与产生的应力之间没有明显关系。减小保径高度,高应力区从鼻端和中心部位转移到保径区。对于非常用钻头形状,凸形钻头产生的应力场较大,双心钻头导眼体底部应力较集中。裂缝主要产生在保径区和鼻端,锥体高度和保径高度较大的钻头产生的垂直裂缝较长。使用平形和凸形钻头时没有产生较长的垂直裂缝。双心钻头可抑制保径区的垂直裂缝,却使地层更易出现水平裂缝,有中间段扩眼器的钻头造成的井壁损伤更小。

The influence of different bit profiles on possible fracture modes was investigated using Finite Element Method. The International Association of Drilling Contractors （IADC） classification was used to design 14 types of profiles. The formation model was built, with given bit size, formation properties, meshing method and boundary conditions, etc. Moreover, the pseudo-static state and mass scaling were used to reduce the simulation time and increase the simulation accuracy. The simulation results showed that, for nine common bit profiles, larger cone height led to larger area of high stress zone, and the gauge height was not apparently related to the stress generated. When the gauge height decreased, the high stress zone turned from the nose and center to the gauge zone. For the non-common bit profiles, the convex bit produced larger high stress field, and the pilot section bottom of bicentre bits had concentrated stress. Fractures were created in the gauge zones and noses, and the bits with larger cone height and gauge height induced longer vertical fractures. The flat and convex bits did not generate longer vertical fractures. The bicentre bit can hinder the vertical fractures in the gauge zone, but enables the horizontal fractures easier. The bit with intermediate reamer has less damage to sidewall.