FRACTAL CHARACTERISTICS OF DIFFERENT-SCALE STRUCTURES IN WATER INJECTION COAL AND THEIR CONTROL MECHANISM ON DYNAMIC WATER MIGRATION

Clarifying the control mechanism of structures in water injection coal on dynamic water transport is fundamental for improving the effect of pre-wetting and dust reduction. Using nuclear magnetic resonance experimental system, we simulated realistic water injection coal seam conditions. We obtained the real mechanical environment of water injection coal seam, the [Formula: see text] spectrum curves of the pore structure at different scales, realized a fine measurement of liquid permeability, proposed a method for calculating the fractal dimension of the specific surface area which combined a tortuosity fractal dimension algorithm, delineated micro-holes and medium–large holes, clarified the quantitative relationships between the structural porosity, the specific surface area, tortuosity, and permeability, and investigated the control mechanism of structural characteristics on the dynamic water transport process. The results showed that, under different mechanical environments, the fractal dimension of the specific surface area of micro–small pores was close to 2, while that of medium–large pores was close to 3. The former fractal dimension increased more obviously, indicating that medium–large pores are more easily compressed than micro–small ones. Under such conditions, the complexity of pore size distribution increased. Additionally, the fractal dimension increase of tortuosity was higher for medium–large pores than for micro–small pores, indicating that the hydraulic reconstruction process was more obvious at a large scale. According to the quantitative relationship between structural porosity, the specific surface area, tortuosity, and permeability, medium–large pores control most of the dynamic water migration in the coal bodies of Daliuta Coal Mine, while the whole pore size range has an important influence on dynamic water migration in the coal bodies of Luling Coal Mine. Our findings provide a solid scientific basis for the development of hydraulic permeability technologies for difficult-to-penetrate coal seams.