Division of coalbed methane desorption stages and its significance

To quantitatively analyze the effect of desorption characteristics on productivity, a division method of coalbed methane (CBM) desorption stages was established based on the Langmuir adsorption isothermal theory, and the indicating significance of the method was analyzed with cases of CBM production. The desorbed efficiency was used to quantitatively characterize the CBM desorbed rate under different reservoir pressures. On the basis of key nodes on mathematical curves, the starting pressure, turning pressure and sensitive pressure were defined. For different coal samples, the corresponding desorbed efficiencies of the three nodes were always constants, therefore the CBM desorption process was divided into four stages, i.e., inefficient, slow, fast and sensitive desorption stages. Studies reveal that, fast and sensitive desorption stages contribute a lot to CBM productivity, while inefficient and slow desorption stages have very little contribution; absorption capacity, gas-bearing property and reservoir pressure are the key factors for the desorption characteristics. The production status of southern Qinshui Basin and Liulin area in Ordos Basin in Northwest China demonstrates that the method can guide the CBM development effectively.