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Axial and torsional impact drilling technology is used to improve the drilling efficiency of hard rock formation in the deep underground. Still, the corresponding theory is not mature, and there are few correlative research reports on the rock-breaking mechanism of axial and torsional coupled impact drilling tools. Considering the influence of the impact hammer geometry and movement on the dynamic load parameters (i.e., wavelength, amplitude, frequency, and phase difference), a numerical model that includes a hard formation and single polycrystalline diamond compact cutter was established. The Riedel-Hiermaier-Thoma model, which considers the dynamic damage and strength behavior of rocks, was adopted to analyze the rock damage under axial and torsional impact loads. The numerical simulation results were verified by the experimental results. It was found that compared with conventional drilling, the penetration depths of axial, torsional, and axial-torsional coupled impact drilling increased by 31.3%, 5.6%, and 34.7%, respectively. Increasing the wavelength and amplitude of the axial impact stress wave improved the penetration depth. When the bit rotation speed remained unchanged, increasing the frequency in the axial and circumferential directions had little effect on the penetration depth. However, as the frequency increased, the cutting surface became increasingly smooth, which reduced the occurrence of bit vibration. When the phase difference between the axial and circumferential stress waves was 25%, the penetration depth significantly increased. In addition, the bit vibration problem can be effectively reduced. Finally, the adjustment of engineering and tool structure parameters is proposed to optimize the efficiency of the axial-torsional coupled impact drilling tool. [ABSTRACT FROM AUTHOR]

The multiphase flow characteristic is one of the most concerning problems during solid fluidization exploitation of marine natural gas hydrate reservoirs. In this research, a new transient gas--liquid--solid multiphase flow model with hydrate phase transition was developed. Meanwhile, this model considered the coupling relationship among convective heat transfer, hydrate dynamic decomposition, and multiphase flow. The model can simulate the change of flow pattern from solid--liquid to gas--liquid--solid flow, and describe the distribution character of volume fraction of phase, wellbore temperature and pressure, and hydrate decomposition rate during transportation. The simulation results indicate that the hydrate decomposition region in the wellbore gradually expands, but the hydrate decomposition rate gradually decreases during the solid fluidization exploitation of hydrate. When mining time lasts for 4 h, and the bottom hole pressure decreases by about 0.4 MPa. Increasing NaCl concentration in seawater helps expand hydrate decomposition regions and improves the wellbore hydrate decomposition rate. When the NaCl mass fraction in seawater reaches 15%, it will raise the hydrate decomposition regions to the whole wellbore. In addition, the higher the wellhead backpressure, the lower the decomposition area and decomposition rate of hydrate in the wellbore. When wellhead backpressure reaches 2 MPa, the volume fraction of gas near the wellhead will reduce to about 12%. This work is expected to provide a theoretical basis for the development of marine hydrate reservoirs. [ABSTRACT FROM AUTHOR]