基于CFD-DEM耦合的水平井PDC钻头水力结构研究

钻采深层页岩气的水平井段多为泥岩、页岩等黏性地层，为提高钻井效率和减少钻头泥包的发生，必须保证井底岩屑被快速运移，这对PDC （polycrystalline diamond compact，聚晶金刚石复合片）钻头的水力结构提出了要求。为此，基于CFD-DEM （computational fluid dynamics-discrete element method，计算流体动力学-离散单元法）耦合方法，对水平井PDC钻头井底流场进行数值模拟分析，并以岩屑滞留量和岩屑运移比作为评价指标，对直径为152.4 mm的五刀翼PDC钻头的流道形状、喷嘴数量、喷嘴排布方式及喷嘴直径等水力结构参数进行分析。最后，对水力结构改进后的PDC钻头进行现场应用。数值模拟结果表明：流道形状为抛物线型的PDC钻头具有更好的携岩性能；在总喷射面积相同的情况下，PDC钻头应采用较少数量的喷嘴；喷嘴等径排布的PDC钻头的携岩性能优于非等径排布的；在一定范围内，适量减小PDC钻头喷嘴直径有利于岩屑运移。现场应用结果表明，在同区块、同地层的多个水平井段钻进时，水力结构改进后PDC钻头的钻井效率更高。研究结果可为PDC钻头结构优化提供参考。

Research on hydraulic structure of PDC bit in horizontal well based on CFD-DEM coupling

The horizontal well sections where deep shale gas is drilled and extracted are mostly mudstone, shale and other viscous formations. To improve the drilling efficiency and reduce the occurrence of bit balling, it is necessary to ensure that the cuttings in the downhole must be moved quickly, which puts forward requirements for the hydraulic structure of PDC bit. For this reason, based on the CFD-DEM coupling method, the numerical simulation analysis of downhole flow field of PDC bit in the horizontal well was carried out. Then, taking the cuttings retention and the cuttings migration ratio as evaluation indicators, the hydraulic structure parameters such as the flow path shape, the number of nozzles, the nozzle arrangement and the nozzle diameter of the five-blade PDC bit with a diameter of 152.4 mm were analyzed. Finally, the PDC bit with improved hydraulic structure was applied in the field. The numerical simulation results showed that the PDC bit with parabolic shape flow path had better cuttings-carrying performance; in the case of the same total injection area, the PDC bit should use a smaller number of nozzles; the PDC bit with equal-diameter arrangement of nozzles had better cuttings-carrying performance than that with nonequal-diameter arrangement; in a certain range, a moderate reduction of the nozzle diameter of PDC bit was beneficial to the migration of cuttings. The field application showed that the PDC bit with improved hydraulic structure had higher drilling efficiency when drilling in multiple horizontal well sections in the same block and same formation. The research results can provide reference for the structure optimization of PDC bit.