Abstract：Hydraulic fracture is one of the most effective methods to improve productivity of low permeability coal seam vertical well. In order to explore the time effect of interaction between gas and rock, and to explore the fracturing time evolution rule in gas containing soft coal seam, according to the stress characteristics of vertical wells, a model of initiation fracture pressure was set up on the base of elastic mechanics theory. Considering the influence of damage to the coal/rock body, the model of extension stress was established by taking damage factor into account. Combination of the nonlinear dropping pressure law of net pressure in crack extension, extension pressure calculation model and the classic PKN model, a new model of crack extension was established, which reflects the functional relationship between fracture time t and fracture radius L. The results of case study in Jiaozuo mines show that, when the fracture time is 40 ~ 60 min, the corresponding fracture radius can reach to 88. 04 ~ 108. 75 m. The theoretical value was consistent with the field monitoring data, which preliminarily proved the validity of the model. The conclusions provide a solid theoretical basis for the actual construction control and groundwater environment protection in mining area.

Keyword：gas vertical well;gas-containing soft coal;extension pressure;fracture extending model;fracturing time;