Coal permeability evolution with the interaction between nanopore and fracture: Its application in coal mine gas drainage for Qingdong coal mine in Huaibei coalfield, China

Permeability plays a significant role on CBM. However, the gas flow mode in nanopore and fracture is different, resulting in the confusion of gas flow mechanism. In this work, a dual-permeability model was constructed with the nanopore permeability and fracture permeability. Then, the impact of adsorption layer, elastic modulus reduction ratio, adsorption effect and stress on the evolution of pore parameters were investigated to reveal the interaction mechanism between nanopore and fracture. Finally, this model was carried out to analyze the coal mine gas drainage of Qingdong coal mine. Results show that, the adsorption layer primarily controls the effective pore radius and thus impacts the nanopore permeability and total permeability. The elastic modulus reduction coefficient can affect the fracture porosity and thus change the fracture permeability, gas mobility and equilibrium pore pressure. Then, the effective pore radius, nanopore porosity and nanopore permeability are affected. For the adsorption effect, it can notably affect the effective pore radius, nanopore porosity and fracture porosity. It is noticed that its effect focuses on the evolution process. Furthermore, the effective pore radius, nanopore porosity and fracture porosity are controlled by stress and the fracture porosity is more sensitive to the stress. Finally, the permeability model can match the field test results well, indicating that our model can be used to estimate gas drainage effect and relief zone for gas pressure. In addition, for the different reservoirs, once the coal basic parameters were determined, the gas production and gas permeability can be estimated by using this model. Therefore, it is important for guiding the gas drainage in coal mine gas prevention and control.