Establishment and optimization of drainage system for deep coalbed methane in South Yanchuan CBM Field

In order to establish the drainage system suitable for deep coalbed methane in South Yanchuan CBM Field,on the one hand,the vitrinite reflectance test,mercury intrusion test and nuclear magnetic resonance test were used to analyze the coalbed porosity of the coalbed methane field in South Yanchuan coalbed methane field. On the other hand,the difference in geological conditions between South Yanchuan and Qinnan coalbed methane was analyzed by comparing with the pore development,in-situ stress characteristics and coal rock mechanics characteristics of the coal seams in South Qinshui Basin,and combined with the drainage parameters of coalbed methane in South Yanchuan,a suitable drainage system was established. The result show that the vitrinite reflectance value Ro of coal seam in South Yanchuan CBM field is about 2.5% in South Yanchuan,the mercury porosity and movable fluid porosity are the lowest among all coal grades,and the pore system is dominated by micropores with poor connectivity among all levels of pores,which is extremely unfavorable to the permeability of coal reservoir. Compared with southern Qinshui basin,the coal reservoir of South Yanchuan CBM field has the characteristics of poor porosity and permeability,high in-situ stress,and low compressive strength of coal rock itself. In the process of drainage control,a slower drainage system should be adopted in process of drainage control; the drainage process for deep coalbed methane in South Yanchuan CBM filed can be divided into five stages,namely the rapid depressurization stage,stable depressurization stage,gas yield rising stage,gas yield fluctuation stage,and stable yield stage. In the rapid depressurization stage,the daily downhole flow pressure should be controlled at about 0.100 MPa. In the stable depressurization stage,the production stage and the production fluctuation stage,refined drainage control should be adopted,and the daily downhole flow pressure in the stable decompression stage should be around 0.003 MPa. The daily downhole flow pressure during the up-production stage should be about 0.005 MPa,and the daily downhole flow pressure during the production fluctuation stage should be controlled at about 0.003 MPa.