Analysis on correlation between nanopores and coal compositions during thermal maturation process

The evaluation of nanopore that links to methane accumulation space and migration channel is vital for coalbed methane development and underground gas drainage. This work aims to uncover the evolution of coal compositions and nanopores with thermal maturation and to evaluate quantitatively the influencing degrees of coal compositions on pore structure parameters based on gray correlation analysis using 498 coal samples from different countries, covering a very wide and continuous thermal maturation range (R-o = 0.22-5.18%) and representing commercially valuable coal seams, typical coal basins and coalfields and CBM development sweet spots. As thermal maturation increases, pore structure parameters and coal compositions changes regularly. As thermal maturation increases, PV-N-2, SSA-N-2 show a rough W-shaped trendency with left steepness and right slowness while fractal dimensiosshow a rough U-shaped trend with left steepness and right slowness, APD-N-2 present an approximate M-shaped trend, PV-CO2 and SSA-CO2 both present an asymmetrical V-shaped tendency with a minimum value at R-o between 1.0 and 1.3%. Concerning coal compositions, with progressive thermal maturation, there are the L-shaped evolution of moisture, the increasing envelope line of vitrinite and the decreasing envelope lines of inertinite, exinite and minerals with the same trend of first steepness and then slowness. In essence, the alteration of pore structure parameters is closely linked to the evolution of coal compositions. Fractal dimensions are controlled by coal macerals while inorganic compositions like moisture and minerals have a significant impact on PV-N-2 and SSA-N-2 within whole thermal maturation process.