在深井、超深井钻井过程中常出现“大小井眼”的复杂井身结构，即“大差异环空”井筒。上部大尺寸环空钻井液流速过慢，导致携岩困难；而下部小尺寸环空流速则过快，显著增加了循环压耗。为了解决上述问题，提出了一种新型分流工具，旨在实现“上能携岩，下能降耗”的功能。然而，目前尚缺乏针对该工具的井筒流动数学模型，影响了分流工具结构的设计与优化。基于此，依据流动与传热基本原理，建立了适用于分流工具的井筒流动数学模型，并以ZS102井为例，证实了工具的应用效果。结果表明：安装分流工具后，井底压力从85.08 MPa降至80.30 MPa，立压从20.97 MPa大幅降至7.22 MPa，环空压耗从7.16 MPa降至2.40 MPa。研究结果为深井、超深井钻井作业中复杂井身结构携岩参数优化及防漏治漏提供了新的解决方案，对深井、超深井钻井技术发展具有重要意义。

"Large-difference annulus" is a common characteristic encountered in complex wellbore structures with "varying diameters" during (ultra-) deep well drilling. This characteristic leads to slow drilling fluid velocity in the upper large-diameter annulus, posing challenges for cuttings removal, while the higher velocity in the lower small-diameter annulus results in significant circulating pressure loss. To address these issues, a novel flow diverter tool was designed to carry cuttings in the upper section and the loss of circulating pressure in the lower section. However, the lack of a specific wellbore flow model tailored for this tool in current research has hindered its design and optimization. Based on the fundamental principles of fluid flow and hear transfer, a valid mathematical model was proposed to be compatible with the flow diverter tool. Then, via a case study on Well ZS102, the tool was proved to be effective and perform well in practice. The results show that with the installation of the flow diverter tool, the bottomhole pressure is lowered from 85.08 MPa to 80.30 MPa, the standpipe pressure is significantly reduced from 20.97 MPa to 7.22 MPa, and the annulus pressure loss is decreased from 7.16 MPa to 2.40 MPa. The research presents a novel approach for optimizing cuttings removal parameters and preventing leaks in complex wellbore structures during deep and ultra-deep well drilling operations, contributing significantly to the advancement of related drilling technologies.