Abstract: In order to explore characteristics of coal reservoir structure and internal fluid distribution, with 10 sets of coal samples from different regions as research objects, low-field nuclear magnetic resonance tests of coal under different conditions were carried out by using saturated, centrifugal and low-field nuclear magnetic resonance system. Based on conventional single T₂ cutoff value, double T₂ cutoff value was proposed to be used to distinguish non-recoverable fluid, capillary bound fluid and movable fluid in coal. Then, a fluid saturation model was established, and pore structure and fluid characteristics of coal at different ranks were studied. The results show that with increase of coal rank, proportion of micro pores gradually increases, and that of adsorption pore throat increases from 51.77% to 90.19%, while that of medium and large ones gradually decrease with seepage pore throat dropping from 48.23% to 9.81%. However, as coal rank decreases, connectivity between pores becomes better, and permeability increases from 0.03 mD to 1.88 mD. Moreover, porosity of coal samples ranges from 0.73% to 11.50%, main distributions of T_2CI and T_2CII are 0.17～2.31 ms and 0.97-15.70 ms, and corresponding saturation distribution is 4.21%-53.33%, 34.67%-66.60%, and 6.19%-37.84%.

Key words: pore structure, fluid characteristics, low-field nuclear magnetic resonance, cutoff, fluid saturation