Permeability dynamic variation of coal under various confining pressures using LFNMR relaxation method

Although an extensive literature has emerged on the use of low-field nuclear magnetic resonance (LFNMR) relaxation spectrometry for the determination of various petrophysical properties of conventional reservoir rocks, relative few papers have reported the use of LFNMR discovering coal characterizations, especially the permeability dynamic variation. In this study, LFNMR measurements were conducted to analyze the permeability dynamic variation under various confining pressures of three coal samples with different metamorphic grades and were compared with the conventional experimental and empirical calculating methods. The results show that low rank coal (MLC) has relatively higher permeability, poorer adsorption capacity, and weaker compressibility compared with medium (TLC) and high rank coals (DQC). The permeability of coal reservoir decreases under the effect of effective stress during coalbed methane (CBM) development, of which the decreasing rate is reduced with confining pressure increasing and Ro, max increasing. Both the empirical calculating and LFNMR evaluating methods can be used to reflect permeability of coal reservoirs under low confining pressure condition, while the LFNMR evaluating method is more suitable than the empirical calculating method to reflect coal reservoir permeability under high confining pressure condition.