Dynamic evolution and its influencing factors of coal seam permeability during joint gas production of superimposed CBM system

According to the coalbed methane ( CBM) reservoir-forming model commonly developed in the coal-bed group stratum of “superimposed CBM system”,with the help of the “physical simulation test device for joint produc- tion of superimposed CBM system” independently developed by Chongqing University,the simulated initial gas pres- sures of No. Ⅰ,No. Ⅱ,No. Ⅲ and No. Ⅳ coal seams are designed to be 1. 0,1. 4,1. 8 MPa and 2. 2 MPa,respective- ly. Based on the physical simulation experiment of joint production in the superimposed CBM system,the dynamic evo- lution characteristics of coalbed permeability and its influencing factors during joint production are analyzed. The results show:① The permeability curves of CBM joint production in the superimposed CBM system can be divided into two types:one is characterized by increasing permeability first,then decreasing and then increasing,represented by No. I and No. II coal seam;the other is characterized by first decreasing and then increasing,represented by No. Ⅲ and No. Ⅳ coal seam. ② It can be seen that the difference coefficient changes significantly in the initial stage of joint pro- duction,but not significantly in the middle and later stages. At the end of production,the difference coefficient of No.Ⅰ, No.Ⅱand No. Ⅲ coal seam relative to No. IV coal seam are 37. 5% ,26. 5% and 13. 6% respectively,which indicates that the gas pouring into high reservoir pressure coal seam during joint production process is reversed. The low reservoir pressure coal seam produces interlayer interference,which is significant in the initial stage,not obvious in the middle and later stages,and the interference degree increases with the increase of initial gas pressure difference. ③ The coupling of effective stress,temperature and deformation of coal seam affects the evolution of permeability,and makes the permeabili- ty evolution of four seams change in stages. The effective stress of No. Ⅳ coal seam occupies the dominant position in the early stage of production,while the temperature and strain of coal seam are under the permeability respectively. The effect on permeability evolution begins when the permeability ratio decreases to 0. 87 and 0． 75.