Discussion about “critical depth” of deep coalbed methane in Zhengzhuang area,Qinshui Basin

Based on the in-situ stress measurement data of 35 CBM wells in Zhengzhuang area,the characteristics of in-situ stress was analyzed,and the different evolutions of permeability and gas content vertically were also discussed. The results show that the in-situ stress state transformation vertically:① Within 575 m,the stresses σH >σv >σh shows the earth stress field;② From 575 m to 675 m,the stress relationship is σH≈σv >σh ,and the recent in-situ stress state is situated in transition zone;③ From 675 m to 825 m,the stress relationship is σv >σH >σh , and the recent in-situ stress state is located in extension zone;④ When 825 m below the surface,the stress relationship is σH >σv >σh ,which shows the dynamic field. The change of the well test permeability of the coal reservoir is almost the same with the change of stress state,the essence of which is that the in-situ stress controls the function of original permeability by changing the width and density of cracks. There is a “critical depth” range (800 ~ 1 000 m) between gas content and depth,when below this depth,the gas content shows a decrease tendency with the increase of depth. Resources of coal- bed methane below 825 m is both under the depth of in-situ stress transformation and the “critical depth”of gas con- tent with poor gas recovery,its occurrence and development of geological conditions are converted,which belongs to the category of deep coalbed methane. In general,the development of CBM under 825 m faces the challenge of “high in-si- tu stress,low permeability,and low gas content”.