Small-size turbine drilling tools have better application prospects in small borehole drilling and so on. Based on the SST model, the influence of a Φ73 mm turbine knuckle-shape parameters on the mechanical energy output characteristics was simulated, and the vortex structure of the turbine internal flow field was analyzed to find the law. First, the influence of leading-edge radius on the turbine internal flow field is concentrated on the rotor suction surface. Second, as with the axial clearance, there is a regular effect of the trailing-edge radius on the flow field in the rotor as a whole and in the middle and rear parts of the stator. Third, the change in the installation-staggering angle does not change the turbine output performance. The output performance is optimal when the leading-edge radius of the Φ73 mm turbine blade is 0.8 mm, the trailing-edge radius is 0.4 mm, and axial clearance is 6 mm. At the same time, the effects of rotational speed, displacement, and fluid viscosity on the output performance of the turbine were simulated, and the output performance of the turbine of this size was predicted under the conditions of low rotational speed, small displacement, and high fluid viscosity. Under the working conditions of conventional drilling parameters, the output pressure drop of a single-stage turbine can be up to 0.018 MPa or less, and the torque is more than 1.6 Nm. If 100–200-stage turbines are used as the power, the output torque can reach 150–300 Nm, which can meet the demand of rock-breaking in the mine.

During ultra-deep well drilling, the bottomhole circulating temperature (BHCT) can easily exceed 150 °C, causing a series of problems such as drilling-fluid degradation, downhole instrument failure, and intensified well-control risks. Conventional surface-cooling methods experience sharply diminishing effectiveness under deep well conditions, while an insulated drill-pipe (IDP) offers good engineering feasibility as a passive cooling technique. However, existing studies lack a transient wellbore-formation coupled model validated by field data, and the influence patterns and interaction mechanisms of key parameters of the insulation coating under varying well depths remain unclear. Therefore, this study integrates a thermal-resistance representation into a transient wellbore-formation heat-transfer framework to characterize the insulation effect via an overall heat-transfer coefficient. Based on this framework, the cooling mechanism is systematically investigated from two perspectives: heat-absorption rate and cumulative blocked heat. Based on this model, the cooling mechanism is systematically investigated from two perspectives: heat-absorption rate and cumulative blocked heat. The results show that a 2000 m IDP section reduces BHCT from 161.81 °C (with a conventional drill-pipe, CDP) to 144.15 °C after 50 h of circulation. This yields an additional cooling of 17.66 °C and a cumulative blocked heat of 180.03 GJ. Parameter analysis further shows that lower thermal conductivity, longer coating length, and placement 200–400 m above the bottomhole enhance cooling, whereas coating thickness exhibits a marginal benefit threshold of 1 mm. More importantly, Sobol’ global sensitivity analysis reveals a distinct evolution of the dominant parameter controls with increasing well depth: at 6000 m measured depth (MD), coating length is the absolute governing factor; at 8000 m, coating position and length become equally important; at 10,000 m, the coupling between thermal conductivity and length emerges as critical; and at 12,000 m, thermal conductivity, thickness, position, and length jointly determine the cooling performance. This evolutionary pattern provides a depth-dependent priority framework for IDP parameter design, offering clear guidance for engineering application across varying well depths.