Microscopic pore of sandstone in Longde Coal Mine based on FHH fractal theory

The coal resources in western China are mainly Jurassic coal fields, and the Jurassic coal measure strata are generally affected by roof composite water, so the water-rich evaluation of roof sandstone aquifer is the premise of safe mining. Taking Longde Coal Mine as the research object, the pore structure characteristics of sandstone were analyzed through nitrogen adsorption test and whole rock analysis. Based on FHH fractal model, the correlation between fractal dimension and rock composition, pore structure and burial depth was analyzed. The results show that the sandstone in Longde Coal Mine has strong heterogeneity, and the fractal dimension D2 is larger than D1, indicating that the pore structure in the sandstone is more complex than that on the surface. The composition of rock is an important factor affecting the complexity of pores, and clay minerals and quartz are the main factors affecting the fractal dimension. The adsorption hysteresis of sandstone in the region is approximately H3 and H4, mainly narrow and plate-like fissure pores, and micro-medium pores. Sandstone pores have obvious fractal characteristics in phases. The fractal dimension is positively correlated with clay mineral content, quartz, BET specific surface area, illite content and hydraulic radius, weakly positively correlated with the total pore volume of quartz and BJH, and negatively correlated with rock-forming mineral content, potassium feldspar, plagioclase, kaolinite content, average pore size and porosity. There was a weak negative correlation with permeability, but no correlation with chlorite content.