Research on permeability variation law of coal reservoir in drainage process of CBM vertical wells

Based on the effect of geological parameters differences in coal bed gas reservoirs on the internal stress state, the mechanical structure properties and the energy system changes of coal reservoir in drainage process, the permeability variation mathematical model in drainage process has been established according to the pore structure, elastic plasticity, damage mechanics and the balance of energy conversion theory. The differences of permeability variation law under different conditions were quantitated by proposing the concept of rebound coefficient. The results show that, the permeability variation law presents the general trend of decrease first and then increase in drainage process. But there would be large differences in permeability decrease level and rebound level when the reservoir parameters are different. The shallower the depth is, the smaller the elastic modulus for permeability recovering to initial value is. When the depth is above 400 m the permeability can recover to initial value with 4-5 GPa elastic modulus, while the 7-8 GPa elastic modulus is needed when the depth is about 600 m, and the permeability cannot recover to initial value within 8 GPa elastic modulus when the depth is up to 800 m. The quantitative geological parameters in the critical state of permeability variation during the drainage process were very important theoretical basis for engineering and theory research. ©, 2015, China University of Mining and Technology. All right reserved.