Factors affecting production capacity and production system optimization of deep coalbed methane in Daning-Jixian area

The deep coalbed methane has the characteristics of "ultra-low porosity, ultra-low permeability and super-saturation", which makes it impossible to copy the productivity models of shallow coalbed methane and shale gas, and the production effects of different sections of deep coalbed methane are quite different, and the influence degree of different factors on productivity is unclear. Moreover, numerical simulation analysis of deep coalbed methane productivity is carried out according to the geological characteristics of Daning-Jixian block on the eastern margin of the Ordos Basin to explore the influence degree of its key productivity factors, especially to determine the appropriate cluster spacing and scientific drainage system.Based on the geological characteristics of deep coalbed methane in Daning-Jixian block, this paper comprehensively considers the change of coalbed methane occurrence state, multiphase fluid seepage and stress-strain constitutive relation of reservoir, and establishes a productivity mathematical model considering the coupling effect of gas-water two-phase flow and coal deformation by introducing the dual-path migration model of free gas seepage diffusion and adsorption gas desorption diffusion in view of the multi-physical field coupling mechanism of coal reservoir deformation and fluid migration during drainage and production. The model is solved by COMSOL Multiphysics software. And the accuracy of the model is fully verified by comparing the simulation results with the actual on-site data in the study area, and the main factors affecting the production capacity of the study area are analyzed by using the grey correlation method, and the effects of different well spacings and bucking rates on the production capacity are revealed according to the results of the production capacity prediction. The simulation results show that the initial permeability ratio k₀, Langmuir volume V_L, Langmuir strain coefficient ε_L and elastic modulus E are directly proportional to the adsorption gas production, while the initial water saturation S_w0, Langmuir pressure p_L and Poisson ratio v are inversely proportional to the adsorption gas production. The initial permeability k₀, Langmuir pressure p_L, Langmuir strain coefficient ε_L and elastic modulus E are directly proportional to the free gas production, while the initial water saturation S_w0, Langmuir volume V_L and Poisson's ratio v are inversely proportional to the free gas production, among which Langmuir volume V_L has little influence on the free gas production. The grey relational analysis shows that the main factors affecting the productivity of the study area are ε_L and k₀ of coal seam. By analyzing the adsorbed gas production, free gas production and water production under different cluster spacing conditions, it was concluded that the optimal cluster spacing was 20 m and the optimal pressure reduction rate was 0.18 MPa/d in the Daning-Jixian block.