Study On Multi-field Coupling Seepage Characteristics Of Coalbed Methane Wells In Eastern Yunnan And Western Guizhou

The production of coalbed methane(CBM)wells mainly comes from the adsorption gas in the matrix pores,but the permeability of matrix is very small,and the seepage resistance is very large.Therefore,the gas flow in the matrix is crucial for the process of the whole migration of CBM.The study of multi-physics fields coupling in the process of CBM development is of positive significance to the stimulation and drainage system formulation of CBM wells.In this paper,physical experiments and numerical simulations methods are comprehensively used to study the seepage characteristics of coal samples from eastern Yunnan and western Guizhou and multi-field coupling effect of coal-rock deformation,matrix and fracture seepage in the process of CBM mining.The specific work and conclusions are as follows:(1)Based on the Coreval 700 unsteady state gas permeameter and porosimeter at overburden pressure,the seepage characteristic experiment under different effective stress is carried out for the coal samples from eastern Yunnan and western Guizhou.Through the experiment,the volume compression coefficient of coal fractures is obtained,and the negative exponential relationship between permeability and effective stress is verified.The stress sensitivity level of coal rock of this area is strong to extremely strong level,and the sensitivity of artificial fracture is stronger than natural fracture.In the production process of CBM wells,it is necessary to control the bottom hole flow pressure reasonably,and the effective stress is within 8MPa to avoid serious permeability damage.(2)Based on the experimental fitting results,considering the horizontal strain,an effective stress expression including the adsorption swelling stress is established,and the fracture permeability model under real stress state is obtained.The dynamic porosity and permeability model of matrix is derived from the mechanical and adsorption deformation.According to the characteristics of adsorption,storage and flow of CBM in the coal reservoir,the gas seepage equations of fracture and matrix pore in the coal reservoir are established respectively.According to the effective stress principle of the elastic theory of porous media,the reservoir stress field equation is established,which considers the influence of gas pressure and adsorption stress.The multi-field coupling process of coal seam stress,matrix and fracture seepage is realized through the gas pressure,volume strain,adsorption strain and other variables.(3)Based on the multi-field coupled model of stress-matrix and fracture seepage,using COMSOL solves the coupling model in order to simulate the process of CBM production.Compared with stress and fracture seepage coupling model,it was found that considering the resistance effect of gas seepage in the low permeability matrix will reduce the cumulative gas production by 6.48%,but it can characterize the actual flow of methane gas in the coal reservoir better.The larger the fracture spacing is,the smaller the matrix permeability is,the more severe the damping effect of matrix is.The analysis result shows that shows that the influence extent of various factors on gas production from high to low is: fracture permeability,in-situ stress,fracture spacing,matrix permeability.The engineering practice of CBM production in eastern Yunnan and western Guizhou indicates that the simulation results is in good agreement with the change trend of production on-site,and the error of cumulative gas production is less than 26%.This model can simulate the CBM development process well.