1.National Engineering Research Center of Coalbed Methane Development and Utilization,Beijing ,China;2.National Engineering Research Center of China United Coalbed Methane,Beijing ,China；3.School of Geosciences,China University of Petroleum(Beijing),Beijing ,China;4.State Key Lab of Petroleum Resources and Prospecting(Beijing),Beijing ,China;5.Linfen Branch,PetroChina Coalbed Methane Company Limited,Linfen ,China

In order to improve the gas drainage mode of the coalbed methane well,with a long term site gas drainage practices in Hancheng Coalbed Methane Field,based on the sectional gas drainage mode of the available “two periods and five stages” coalbed methane well,from a view of the desorption-diffusion-seepage of the coal reservoir,the analysis held that at the stage of the pressure built-up water drainage-the production capacity released,the rapid fluctuation phenomenon of the casing pressure and dynamic fluid level occurred were caused by the uncompleted matching of the flow supply capability due to the present gas drainage system with the high rank coal and low permeability coal reservoir.According to the stable control of the pressure drop of the flow pressure at the bottom of the well and the gas drainage working system with a basic idea relative suitable to the flow supply capacity of the strata,a modification and improvement was conducted to the site applied mode and a gas drainage fine control mode of the coalbed methane well with “two periods and six stages” was provided.The site test showed that according to the fine control mode,the rapid fluctuation wave phenomenon of the casing pressure and dynamic flow level could be eliminated and the sand flow accident ratio could be reduced.