Mechanical characteristics and energy consumption of shale dynamic crushing under dynamic loading

Shale gas in the Sichuan Basin has become an important field and object of large-scale benefit development. The southern Sichuan Basin has become the main battlefield of shale gas exploration and development in China, and its annual shale gas production in 2022 is 223.23×108 m3, revealing great exploration and development potential of shale gas in this area. In the southern Sichuan Basin, however, shale gas long horizontal wells face severe challenges of low rate of penetration (ROP) and long drilling cycle. In order to clarify the shale dynamic crushing behaviors and energy consumption laws in the rock breaking process of shale-gas horizontal well drilling, this paper tests the stress wave propagation process of dynamic rock breaking by virtue of the Hopkinson pressure bar test technology. In addition, the evolution characteristics of shale dynamic strength, the response laws of crushing energy consumption and the dynamic propagation behaviors of rock cracks under dynamic loading are evaluated by using the stress wave balance method. And the following research results are obtained. First, in the process of shale crushing, dynamic strength and energy consumption directly influence the crushing and ROP improvement effect. Shale strength changes dynamically under different loading rates and loading modes. Dynamic loading can activate more microcracks inside the shale and penetrating cracks. Second, as the loading rate increases, the compressive strength and tensile strength of shale increase slowly at first and then rapidly when the critical strain rate is exceeded. Third, there is a critical strain rate interval corresponding to the energy consumption of shale compression crushing. As the strain rate increases, rock breaking energy consumption increases slowly. When it exceeds the critical interval, energy consumption increases rapidly. The energy consumption of shale tensile crushing increases logarithmically, and it also has a critical strain rate interval. Fourth, in the process of bit-rock interaction, excessively high loads can directly increase the contact strain rate, resulting in bit damage and increased contact strength, so as to reduce the rock breaking efficiency. Fifth, the comprehensive evaluation indicators of efficient rock breaking should include static uniaxial compression strength, rock dynamic strength and energy consumption. In conclusion, the research results provide a theoretical support for the ROP improvement and optimization of shale-gas horizontal well, and is of important guiding significance to improve the rock breaking efficiency in deep shale and conducive to the large-scale benefit development of shale gas. © 2023 Natural Gas Industry Journal Agency. All rights reserved.