Analysis of physical properties and influencing factors of middle-rank coal reservoirs in China

Middle-rank coal mainly refers to bituminous coal with the vitrinite maximum reflectance (Ro,max) between 0.65% and 2.0%. Based on the results of a proximate analysis, maceral composition, high-pressure mercury intrusion porosimetry (MIP) experiments and isothermal adsorption experiments, combined with gas content and permeability derived from well test, the pore size distribution (PSD), adsorption characteristics and the evolutionary paths of middle-rank coal were analyzed, and the influencing factors of the gas-bearing property and permeability were discussed. The results indicated that as Ro,max increases, the porosity shows first decreasing and then increasing, with the minimum values reached when Ro,max = ∼1.0%. The volume and specific surface area of the total pores have the same evolutionary paths with each class of pores, first decreasing and then increasing with the increases of Ro,max. The minimum value appears when Ro,max is between 1.2% and 1.3%. As Ro,max increases, VL increases while PL first increases and then decreases, the maximum value is again reached at near Ro,max = 1.3%. The gas content increases with the increases of Ro,max, first increasing and then decreasing with the increases of buried depth, and the buried depth at 950 m is the critical depth of gas content. There is a positive correlation between gas content and gas saturation; the permeability decreases with the increases of buried depth, and increases with the increases of coal reservoir porosity. Ground stress is the main controlling factor of reservoir permeability.