Evaluating method of gas content of coalbed methane based on nuclear magnetic resonance technology

China has abundant coalbed methane (CBM) resources, which is a good reserve of natural gas. The development and utilization of CBM has multiple values. Gas content of coal is the key to evaluate coal seam, and it is also an important factor to determine the exploration and development area and productivity potential of CBM. Using logging data to evaluate gas content is an important means in CBM exploration and development. In order to avoid the density difference caused by different methane states and the underestimation of gas content caused by Langmuir monolayer adsorption model in coal seam, a new method is adopted to separate the NMR signal of adsorbed natural gas in coal seam, calculate the molecules mole number of natural gas in unit volume formation, and convert into the gas content in standard state by density logging. The experimental results of methane isothermal adsorption and NMR show that differentT₂ cut-off values can be used to distinguish methane states in coal seam. The adsorpted methane content calculated by using new method based on NMR is in good agreement with the measurement of coal sample, which proves the effectiveness of the method. The actual logging data processing and interpretation results show that the new method based on NMR logging is more effective than the method based on conventional logging in calculating gas content of coal seam.