Study on joint characterization of pore structure of middle-rank coal by nitrogen adsorption-mercury intrusion-NMR

In order to accurately characterize the pore of middle-rank coal reservoirs,four sets of coal samples of different coal ranks were taken as the research objects,and the characteristic parameters were quantitatively studied by experimental methods such as mercury intrusion,liquid nitrogen adsorption and nuclear magnetic resonance. The results show that coal reservoirs are dominated by micropores,followed by mesopores and the least macropores. Micropores are the main contributor to specific surface area and pore volume. The pore volume of middle-rank coal is between(3.19～5.21)×10-2 mL/g,the specific surface area is 6.25～9.81 m2/g,the porosity is between 5.171%～8.307%,and the permeability is between(0.008～1.253)×10-3 μm2. The pores of fat coal and coking coal are mainly open pores,and the pores of lean coal and meagre coal are mainly closed or semi-closed pores. The tiny pores of fat coal are mostly parallel plate holes,coking coal and lean coal are double-tapered pipe holes or tapered holes,and meagre coal are split-shaped holes or cylindrical holes. With the increase of coal rank,the proportion of micropores and adsorption pores gradually increases,the proportion of medium and large pores and seepage pores gradually decreases,the connectivity between pores is getting worse,and the permeability gradually decreases. With the decrease of coal rank,the specific surface area gradually decreases,the effective porosity gradually increases,and the residual porosity gradually decreases. The pore volume and total porosity show a trend of first decreasing and then increasing.