Influencing factor analysis of the coal matrix compressibility of middle-high rank coals

The pore structure and coal matrix compressibility evolution law and their influencing factors of middle-high rank coals was evaluated based on the combined application of high pressure mercury injection, low-temperature nitrogen gas adsorption and low-temperature carbon dioxide adsorption. The middle-high rank coals are dominated by macropores and micropores, along with lower developed mesopores. The porosity, total pore volume and macropores have an increasing trend with increasing coalification degree. Meanwhile, micropores had gradually become more dominant. Coal matrix compressibility of middle-high rank coals is between 0.55 × 10−10 and 1.42 × 10−10 N/m2, which had significant effects on mesopores, while its influence on macropores rise with the increases of coal rank. There was a negative correlation between coal matrix compressibility and coalification degree, resulting from the increasing peak intensity and elastic modulus during the coalification processes. The polarity of the water molecules tends to reduce the peak strength and elastic modulus of coal. As a result, the deformability becomes more enhanced resulting in increases in coal matrix compressibility. Organic compositions and inorganic minerals have opposite effects on coal matrix compressibility. Inorganic matter can effectively resist coal matrix compressibility, resulting from the higher intensities and elastic features and their roles in supporting cleats and pores while organic matter has the opposite effect. Pore structural features have important influences on coal matrix compressibility. Coal matrix compressibility has a trend of increasing with the increasing in the porosity, total pore volume and macropores or the decreasing in micropores. In the maceral composition, the vitrinite was positively correlated with the peak intensity and elastic modulus of medium rank coal, while inertinite tends to be the opposite. There is a positive correlation between coal matrix compressibility and vitrinite, but the opposite occurs for inertinite, resulting from differences in their mechanical strength and elastic properties. •The coal strength showed a rising trend as coal rank increase, resulting in decreases in the coal matrix compressibility.•The coal matrix compressibility displayed a decreasing trend with increases in mineral content levels.•Coal matrix compressibility displayed a decreasing trend with inertinite increase or vitrinite decrease.