Study On Characteristics And Effective Factors Of Coalbed Methane Desorption And Diffusion

The coal reservoir has a dual-pore system with strong heterogeneity in which CBM production goes through a complex flow process of multiphase fluids.Desorption and diffusion have essential effects on coalbed methane(CBM)production and productive life.Carrying out the research on desorption and diffusion will be helpful to establish effective plans for CBM drilling,well completion,drainage and development.It also provides a theoretical reference for improving the CBM recovery.Therefore,this thesis studied desorption and diffusion law of CBM and their effective factors based on the in-situ desorption data of No.9 coal seam in Xishan coalfield,Qinshui Basin.By analyzing the in-situ desorption data of the samples,the desorption process of CBM can be summed up as three stages: unstable desorption,rapid desorption and slow stagnation stage.In the first stage,the desorption efficiency is the largest.The second stage goes through stable desorption.While the third stage has a long period of low efficient desorption.Unstable desorption and rapid desorption contribute most of gas desorption volume.Based on the single pore diffusion model,a diffusion model combining the shrinkage effect of matrix is established,which aims to calculate the diffusivity coefficient and to study the diffusion properties of coal samples.The diffusivity distribution of coal seams in the west of the study area is better than those in the east.The Malan mine,located in the western margin of the study area,has higher diffusivity coefficient of coal samples;While Dongqu mine has lowest diffusivity coefficient.It is because that the Dongqu mine,located at the southeast of study area,is far from the axis of the main tectonic unit(Malan syncline).The influence factors of coal diffusivity and desorption characteristics include coal structure,moisture,ash and coal rank.Coal structure is the main factor influencing the diffusivity coefficient.Moderate deformation is beneficial to the production of CBM.Compared with primary coal,the structural-deformed coal has better desorption and diffusivity.The coal rank is the secondary control parameter for desorption time.With increasing coal rank the amount of gas flow channel decrease,because the space of the macropore and mesopore become much more closer.