The complex formations such as soft-hard interbedded and strong abrasive strata in ultra-deep wells make the PDC bit inefficient in rock-breaking, which greatly increases the development cost of oil and gas reservoirs. There are two basic ways to solve the above problems: one way is to relieve the force condition of the bit by changing the working mode and the other is to release the stress of the rock at bottomhole with special tools. A method is proposed in this paper to make full use of the stress-releasing effect to reduce the value of compressive stress of the rock at bottom hole, even turn it into partially tensile stress, and take advantage of the cutting method of ‘Hybrid Bits’ to improve rock-breaking efficiency (RBE). A supporting Weight-on-bit (WOB) Self-adjusting Dual-diameter PDC Bit (WSADB) is invented to realize the above idea. Based on Biot's theory of poroelasticity and considering the fluid-solid coupling effect, numerical calculation of the stress field at well bottom is performed. The results show that the WSADB can improve the RBE by enlarging the volume of the stress unloading area at well bottom; The rock damage function based on the 3D Mohr-Coulomb Criterion is used to analyze the effect of structural parameters of the bit on RBE and the optimal structural parameters are obtained. When the diameter ratio of the pilot bit to the reaming bit is 0.5, the rock to be drilled at the well bottom will be damaged to the greatest extent; When the length of the pilot bit is longer than 0.5 times of the borehole diameter, the longer the length is, the greater the degree of damage to the rock at well bottom will be. To verify the accuracy of the above calculations, a comparative test between the WSADB and conventional PDC bit is performed. The results show that the WSADB has great superiorities in improving the rate of penetration (ROP), lowering the torque on bit (TOB), reducing the mechanical specific energy (MSE) and enhancing the RBE. Making full use of the advantages of higher ROP, smaller TOB and higher mechanical energy utilization of the WSADB is expected to provide a solution to the problem of low RBE of conventional PDC bit in ultra-deep wells.

超深井中复杂的地层，如软硬夹层和强磨蚀性地层，使得PDC钻头的岩石破碎效率低下，这极大地增加了油气藏的开发成本。解决上述问题的基本方法有两种：一种是通过改变工作方式来减轻钻头的受力状态，另一种是用专用工具来释放井底岩石的应力。本文提出了一种充分利用应力释放效应来减小岩石在井底的压应力值，甚至将其转变为部分拉应力的方法，并利用“混合钻头”的切割方法。以提高破岩效率（RBE）。为了实现上述思想，发明了一种支持钻压（WOB）自调节双直径PDC钻头（WSADB）。基于Biot的孔隙弹性理论，并考虑流固耦合效应，对井底应力场进行了数值计算。结果表明，通过扩大井底应力卸载区的体积，WSADB可以提高RBE。利用基于3D Mohr-Coulomb准则的岩石破坏函数，分析了钻头的结构参数对RBE的影响，并获得了最佳的结构参数。当导向钻头与扩孔钻头的直径比为0.5时，将在井底最大程度地破坏岩石。当导向钻头的长度大于钻孔直径的0.5倍时，长度越长，对井底岩石的破坏程度越大。为了验证上述计算的准确性，在WSADB和常规PDC位之间进行了比较测试。结果表明，WSADB在提高钻速（ROP），降低钻头扭矩（TOB），降低机械比能（MSE）和增强RBE方面具有巨大优势。充分利用WSADB的更高的ROP，更小的TOB和更高的机械能利用率的优势，有望为超深井中常规PDC钻头的RBE低的问题提供解决方案。