Understanding the dynamic changing patterns of the permeability of coal seams is one of the key issues in coalbed methane development. In this study, we divided the drainage and production stages of coalbed methane wells according to the non-dimensionalized gas production rate. By combining these figures with the coalbed methane desorption process identified in isothermal adsorption experiments, we further determined the interfacial location between the drainage and production stages. A model for evaluating the permeability of medium-rank coal reservoirs was also developed using the dynamic balance theory of material and energy. Finally, the dynamic characteristics and control mechanisms of coal reservoir permeability in different drainage and production stages of medium-rank coalbed methane wells were explained from multiple perspectives, including the trend of permeability variation, the dominant mechanism, and the productivity dynamics. The results show that the absolute permeability of the coal reservoir experiences a dynamic change of"first decreasing, then reverting, and finally increasing" during the drainage and production processes. A rapid reduction in the effective permeability of water phase and a zero effective permeability of gas phase are observed during the drainage stage. The reservoir enters the gas production stage once the reservoir pressure drops to the critical desorption pressure. During this stage, the effective permeability of gas phase increases rapidly while the effective permeability of the water phase drops slowly. The absolute permeability begins to decrease during the gas production reduction phase. Influenced by the slippage effect, the effective permeability of the gas phase continues to increase slowly while the effective permeability of water phase reduces.
