Micromechanical Properties of Different Rank Coal and Their Impact on Fracture Compressibility

The micromechanical properties are crucial factors influencing coal fracturing and efficient gas extraction. This paper employs nanoindentation techniques to investigate coal micromechanical properties across various ranks, analyzing their impact on fracture compressibility. The results indicate significant variations in the distribution of micromechanical parameters of lignite, exhibiting the strongest heterogeneity. Average contact depth and creep displacement follow the order lignite > bituminous coal > anthracite. In terms of average hardness, reduced modulus, and fracture toughness, the ranking is anthracite > bituminous coal > lignite. Guhanshan Mine (GHS) anthracite demonstrates the highest values, exceeding Simengou Mine (SMG) lignite by factors of 2.73, 1.82, and 1.83, respectively. Both hardness and fracture toughness show a significant linear relationship with the elastic modulus. The average fracture compressibility coefficients for Daliuta Mine (DLT), Pingdingshan Mine (PDS), Jiaozishan Mine (JZS), and GHS are 0.144, 0.098, 0.09, and 0.08 MPa–1, respectively. Coal with greater micromechanical strength demonstrates enhanced resistance to compressive deformation fractures. This indicates that higher fracture toughness not only resists fracture propagation but also effectively withstands compressive deformation. Quantitative relationship models between the fracture compressibility coefficient and contact depth, hardness, reduced modulus, and fracture toughness are obtained through fitting analysis. These models facilitate the calculation of coal fracture compressibility coefficients under varying micromechanical strengths. The research findings offer theoretical guidance for optimizing coal seam fracturing and enhancing permeability schemes.