Atmospheric-variational pressure-saturated water characteristics of medium-high rank coal reservoir based on NMR technology

Abstract The fluidity of medium-high rank coal reservoirs is different at the microscopic scale, which can lead to differences in coalbed methane (CBM) production. Taking the medium-high rank coal reservoirs in eastern Yunnan and western Guizhou as examples, the characteristics of atmospheric-variational pressure-saturated water based on nuclear magnetic resonance (NMR) technology were studied. The results show that under atmospheric pressure and vacuum pressure, the water saturation (Sw) of medium rank coal is lower than that of high rank coal; the former is less than 50%, and the latter is more than 60%, which is attributed to the higher developed micropores and hydrophilicity of high rank coal. With an increase in the saturation pressure, the Sw of medium-high rank coal is not evident. A conversion method of the T2 spectrum and the low-temperature liquid nitrogen average pore size are proposed. Thus, the calibration method of the minimum pore size that water can enter under atmospheric-variational pressure conditions is established, and the saturable pore diameter (d*) under atmospheric-variational pressure conditions is obtained. Under natural atmospheric pressure, water can enter the micropores and transition pores (pore diameter d