Study on gas drainage control of CBM well in stage of increasing production in Linfen Area

In order to find out the reasons that affect the CBM production difference between the stable and high-yield production wells and the post-peak production-descending wells in the Linfen Area, the speed of pressure drop in well bore and the growth rate of daily gas production in the production stage of both types of wells were contrasted and analyzed. The influence mechanism of fast gas production on pressure drop expansion of coal reservoir and its importance in drainage control was revealed and the control indexes and method for CBM well drainage parameters in the increasing production stage in Linfen were are formulated. The study results show that the reason for the poor stable production effect of the post-peak production-descending wells lies in the fact that the down hole pressure deceleration and the daily gas production increase rate are not slowly controlled during the production stage. If gas production rate is too fast, the migration of large pulverized coal particle and clogging will be induced by gas-water bubble flow. At the same time, gas saturation and its relative permeability in near borehole zones increase, and water relative permeability decreases as the flowing resistance is growing, which influence the effective discharge of water and cause pressure drop transmission becoming slower and hindered. The reasonable value of pressure drop speed in the increasing production stage of CBM wells in Linfen Area should not exceed 0.01 MPa/d, and the growth rate of daily gas production should be less than 15 m3/d. It is advisable to use stepwise stable production and alternate production.