Experimental investigation of flow regime transition characteristics of fractured coal bodies around gas extraction boreholes

The accurate determination of the seepage flow pattern of crushed coal around the perimeter of the borehole is an important technical parameter used in the determination of the gas extraction radius. To obtain the permeability parameters of the crushed coal system, a triaxial seepage test system was designed for the crushed coal. Three flow systems of interest (pre-Darcian, Darcian, and non-Darcian Forchheimer flow systems) were investigated. The conclusions can be drawn as follows: the pre-Darcian flow state is determined at a very low Reynolds number, and there is a nonlinear phase of the seepage process of the crushed coal. The infiltration process at a low Reynolds number is basically in the pre-Darcian flow state, and its flow resistance mainly comes from the viscous force of the fluid itself. The inertia zone of crushed coal body particles starts from vs > 0.086 m/s, and the infiltration process at a high Reynolds number indicates turbulent flow. Calculation of permeability of crushed coal gives the seepage flow dividing line and critical Reynolds number values (Rep > 0.18). The improved method of calculating the pore-permeability flow regime of perforated fractured coal was used. The research can provide theoretical basis for the rational determination of the gas extraction radius. •The pre-Darcy state of the crushed coal medium exists only in the range of flow velocity vs ≤ 0.062 m/s.•When ReK is greater than 3.29, the whole infiltration process is considered to be in the non-Darcy state.•The flow resistance of the broken coal rock body comes from the viscous force of the fluid itself.