Micromechanical properties of coal and its influencing factors based on nanoindentation

The microscopic mechanical properties of coal are closely related to its macroscopic mechanical properties. They are key indexes for analyzing the mechanism of macroscopic mechanical properties and important factors affecting the fracturing of coalbed methane(CBM). At present, the research on the micromechanical properties of coal is limited to mechanical parameter characterization and phenomenon description, and the evolution mechanism and influence mechanism is lack of research. Taking the coals with different metamorphic degrees in China as research objects, the key parameters of nanoindentation experiment are optimized, the microscopic mechanical properties and main controlling factors of vitrinite and inertinite of coal are clarified by means of nanoindentation experiment, liquid nitrogen adsorprtion and in-situ laser Raman test. The research shows that:(1) The pre-experimental result shows that the parameter setting of the maximum indentation depth of about 1 000 nm(corresponding to the peak load of about 9 mN) is more suitable for the nanoindentation experiment of coal vitrinite and inertinite under low load and small range.(2) The elastic modulus and hardness of inertinite are generally higher than vitrinite, and the elastic modulus and hardness of macerals at different positions in the same coal are similar, indicating that the distribution of mechanical properties of coal matrix macerals in the local range has certain similarity.(3) The macerals and maturity of coal affect its micro-mechanical properties by controlling the pore structure and chemical structure in the form of “one main and one auxiliary”,that is, the micro-mechanical strength of vitrinite increases first, then decreases and then increases with the deepening of coalification degree. The descending section is mainly affected by the change of pore structure caused by the increase of gas pores, and the chemical composition and structure of the ascending section are the main controlling factors. The micro-mechanical properties of inertinite increase with the increase of coal maturity, and are always controlled by chemical composition and structure. The study of micro-mechanical properties of coal can be widely used in the analysis of deformation mechanism of coal matrix at micro-scale, the generation and expansion of micro-cracks during hydraulic fracturing, and the optimization of favorable development areas.