Microstructure and nanomechanical characterization of tectonic coal based on SEM, AFM, XRD and DSI

The microstructure and mechanical properties of tectonic coal are the key to the efficient development of tectonic coal reservoir coalbed methane. The microstructure and mechanical properties of coal samples were studied and analyzed by SEM-EDS-PCAS, AFM, XRD and DSI experiments. The results show that the fractal dimension (D) of WJB, QL, and FR coal samples of pores is 2.356, 1.946, and 1.721 respectively. The fractal dimension (Ds) of PSD is 2.26, 2.25, and 2.23 respectively. The porosity (φ) values are 2.64%, 2.76%, and 3.98% respectively. and porosity (Ф) values are 2.55%, 2.67%, and 3.16% respectively. With the enhancement of tectonic action, the number of pores in coal increases, the fractal dimension of pores increases, and the surface morphology and pore structure become more complex. The depth and residual depth of nanoindentation increase with the increase of load. The hardness (Hx), fracture toughness (Kc) and creep stress exponent (n) of coal are negatively correlated with the aromatic interlayer spacing d002, and positively correlated with the degree of graphitization (Gd), while the elastic modulus (Ex) has no obvious relationship with it. The linear correlation between Hx and Ex is less than that between Kc and Ex. The relationship between Ex and Hx of coal samples and different peak loads is not obvious, but the Kc and n of coal samples increase with the increase of peak load. Under a certain load, the plastic energy, fracture energy and elastic energy increase with the increase of load. The tectonic coal with high metamorphic degree is prone to elastic deformation, while the coal with low coal quality is prone to plastic deformation. The research results can provide theoretical guidance for coalbed methane mining in tectonic coal reservoirs.