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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。
Finite element method was used to simulate the rock breaking effect of different shape drill bits, and the stratigraphic stress distribution and fracture extension mode were studied. According to the International Association of Drillers (IADC) classification criteria designed 14 kinds of bit shape. The stratigraphic model is designed and the bit size, stratum property, meshing method and boundary conditions are given. Quasi-static conditions and mass scaling methods are used to reduce simulation time and improve simulation accuracy. The simulation results show that for the 9 kinds of commonly used drill shapes, the area of high stress area increases with the height of the pyramid, and there is no obvious relationship between the height of the gauge and the stress generated. Reduce the gauge height, high-stress zone from the nose and the central part of the transfer to the gage area. For the unusual shape of the drill bit, the stress field generated by the convex drill is larger, and the stress at the bottom of the dual-core drill guide body is more concentrated. Cracks mainly occur in the guage zone and the nose, and the vertical fractures produced by the drill with larger pyramid height and gauge height result in longer cracks. The use of flat and convex drill did not produce long vertical cracks. Biconical drill bits can restrain the vertical cracks in the gauge-retaining zone, but make the formation more prone to horizontal cracks. The borehole wall damage caused by the drill bit with the middle reamer is smaller.