Abstract
To study water occurrence in coal under different negative pressure conditions, two coals (Coal DF and Coal SH) were conducted with NMR and LP-ArAD experiments. Water exists mainly in adsorption pores in the two saturated coals. When the absolute pressure is slightly lower than 0.1 MPa for coals DF and SH, the removed water in seepage pores is 75.7 % and 99.4 %, respectively, but only 5.7 % and 8.7 % in adsorption pores, respectively. When the absolute pressure drops from 0.03 MPa to 1.37x10−4 MPa for coals DF and SH, the water removal rates of adsorption pores are changed from 5.16 % to 87.19 % and from 26.45 % to 57.18 %, respectively. The difference in water migration capacity under variable negative pressure conditions is mainly related to water migration phase. As the absolute pressure decreases below the saturated vapor pressure, the water migration phase changes from liquid to gas, improving the water migration capacity of adsorption pores. The higher gas production rate, the lower water vapor partial pressure. Water tends to migrate in gas phase according to Dalton's law in high production CBM wells, which mitigates water block damage and improves CBM productivity under negative pressure.
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