In the drilling process of ultra-deep wells with large-diameter boreholes, the transport and deposition behavior of cuttings plays a critical role in maintaining wellbore cleanliness and ensuring operational safety. Due to the geometry of enlarged boreholes and their complex annular flow characteristics, conventional single-parameter control methods often fail to achieve effective cuttings transport. This study aims to identify the dominant influencing factors and optimize key parameters by focusing on the cuttings volume fraction as a primary evaluation metric. A numerical simulation approach is employed to systematically investigate the influence of stabilizer geometry and hydraulic parameters. Five variables—drilling fluid velocity, drill pipe rotational speed, number of stabilizers, flow area, and helical angle—are selected for analysis. An initial one-factor sensitivity analysis is conducted to evaluate local impacts and to establish relative sensitivity indices, thereby identifying key variables. A variance-based global sensitivity analysis is further applied to quantify first-order effects, full-order effects, and interaction contributions, revealing nonlinear coupling and synergistic mechanisms. The results indicate that drilling fluid velocity and rotation speed exhibit the most significant first-order influences, while stabilizer-related parameters show strong interaction effects that are often underestimated by traditional methods. Based on these findings, an optimized cuttings transport scheme for large-diameter boreholes is proposed. Additionally, a multi-parameter response model for the cuttings volume fraction is developed using sensitivity-weighted analysis, offering theoretical support and methodological reference for enhancing cuttings transport performance and structural design in large-diameter borehole drilling operations.

Natural gas hydrates occupy an important position in the development of clean energy around the world in the 21st century. It is of great significance to research the mechanical properties of methane hydrate-bearing sediment (MHBS). In this paper, gas-saturated MHBS were synthesized based on the self-developed triaxial compressor apparatus. The triaxial shear tests were performed at temperatures of 2 °C, 3 °C, and 5 °C and confining pressures of 7.5 MPa, 10 MPa, and 15 MPa. Results indicate that the axial strain process can be divided into three stages: initial elastic deformation, initial yield deformation, and strain softening. When confining pressure is increased, the shear strength of MHBS increases at a lower confining pressure. In contrast, shear strength appears to decrease with increasing confining pressure at a higher confining pressure. There is a negative correlation between temperature and shear strength of MHBS. The initial yield strain of MHBS increases in condition due to the increase in confining pressure and the decrease in temperature. The change in strength degradation is kept within 2 MPa. Using test data, the Duncan-Chang model was modified to describe the strength behaviors of gas-saturated MHBS. The accuracy of the model was verified by comparing calculated values with test data.

During the drilling processes of a 10,000-meter-deep well, cutting removal becomes difficult in the 32-inch borehole, which significantly increases downhole risks and affects drilling efficiency. To address this, a numerical simulation method based on the Eulerian two-fluid model was established for cuttings transport simulation in ultra-large boreholes. This method revealed the cuttings transport behavior in the 32-inch borehole of the SDCK1 well, analyzed the actual return velocity and the critical return velocity required for cuttings transport, and examined the cuttings transport characteristics near the bottom stabilizer. The results show that under the maximum flow rate of 160 L/s, the actual return velocity in the annulus is only 0.32 m/s, while the critical return velocity for 10 mm cutting particles is 0.57 m/s. Except for the stabilizer position, the actual return velocity throughout the entire well section is lower than the critical return velocity required for 10 mm cutting particles transport, which is one of the main reasons for the poor cutting removal in the wellbore. Near the bottom stabilizer, the annular flow is altered by the large outer diameter of the stabilizer, causing drilling fluid backflow and resulting in cuttings accumulation. The cuttings backflow and accumulation are more pronounced with the double stabilizer tool combination compared to the triple stabilizer tool combination. The small annular gap near the stabilizers makes it difficult for large cuttings to pass through, leading to blockages. A low annular return velocity and cuttings accumulation near the stabilizer are the primary reasons for poor cuttings removal in the 32-inch borehole.

Ultra-high-temperature and pressure downhole environments pose challenges for conventional electronic instruments to adapt to high-temperature formations, thereby restricting the application of downhole electronic tool technology in deep and ultra-deep wells. Given the aforementioned limitation of electronic inclination measurement systems, specifically their poor temperature resistance, this study proposes a novel shunt flow control method. This method employs a mechanical structure to overcome temperature constraints: gravitational torque generated by the mechanical structure is utilized to control valve opening and regulate flow rate. By converting sensed well inclination information into changes in flow rate, this approach enables the transformation of well inclination sensing and its associated signals. In this study, a kinetic analysis model of the shunt-regulating valve spool was established. Using computational fluid dynamics (CFD) simulations, the flow characteristics of the regulating spool were analyzed under varying valve openings. The structure of the flow control valve was optimized with the goal of maximizing internal flow. Finally, the reliability of the designed structure for well deviation sensing and flow control was verified using simulation experimental studies and theoretical analyses.