Injection time of injection/fall off test in the low-permeability coal seam

The purpose is to obtain the rough injection-time range when the radial flow happens during injection/fall-off test in low-permeability coal seam. Based upon the theory and experience of injection/fall-off test, studies of large amounts of real data and field experiments on the spot were conducted, and finally the feasible measures were presented, which is to reduce the well-bore storage time, prolong testing time and realize third-level well bore configuration in CBM well. The results show that within certain testing time, if radial flow happens during injection/fall-off test in low-permeability coal seam, it must guarantee t_inj> 3t_wb, t_fall≥2t_inj under conditions of ensuring well-bore safety.