Analysis of Differences in Mechanical Properties and Main Controlling Factors of Deep Coal Bearing Rocks

Coal-bearing strata are a key focus in the exploration and development of unconventional natural gas in China. The implementation of drilling and fracturing operations is significantly influenced by the mechanical properties of the strata rocks. Through extensive rock mechanics experiments on sandstone, coal, and mudstone under different loads and temperature-pressure conditions, combined with acoustic emission, fracturing simulation, and CT experiments, a deep analysis was conducted on the differences in rock mechanical properties and the main influencing factors of deep coal-bearing strata in the eastern margin of the Ordos Basin. The results indicate that, rock mechanical parameters are internally controlled by lithology and the development of pores and fractures. Under uniaxial conditions, the elastic and strength mechanical parameters of mudstone are higher than those of sandstone and coal. An increase in internal “defects” such as pores and fractures leads to a significant reduction in rock elasticity and strength parameters. Externally, rock mechanical properties are influenced by confining pressure, fluid properties, pressure, and temperature. As confining pressure increases, rock elasticity and strength parameters show an upward trend. The presence of fluids and an increase in fluid pressure reduce rock strength. At low temperatures, the effect of temperature on rock mechanical parameters is weak, and the influences of confining pressure, pore pressure, and temperature on rock mechanical parameters gradually diminish. Based on the analysis of influencing factors of rock mechanics parameters, a logging interpretation model for mechanical parameters of deep coal-bearing strata based on lithology has been established, with a logging interpretation error < 10%. This provides a more precise mechanical parameter foundation for the design of drilling, fracturing, and other engineering processes.