Fracture and pore structure dynamic evolution of coals during hydraulic fracturing

Hydraulic fracturing is an effective method for producing coalbed methane. To better understand the evolution of the fracture and pore structure of coals that contain macro-fractures during hydraulic fracturing, we have performed a series of in-situ compression experiments under continuous injection liquid conditions and dynamically monitored the samples using nuclear magnetic resonance methods. The mechanism of fracture and pore structure dynamic evolution is identified in terms of spherical expansion theory. The total porosity increases dramatically within 10-120 min of the initial injection stage, and pore channels form. The number of micropores (< 10 nm) and transition pores (10-100 nm) continues to increase during the injection pressure reduction stage or increasing confining pressure stage. Total porosity tends to initially decrease and then increase when the confining pressure remains either constant or increases. Continuous liquid injection leads to a constant change of coal pore pressure and effective stress, such that pores break, close, and continuously reorganize. The results presented here provide a reference for permeability calculations, coalbed methane productivity predictions, and improved hydraulic fracturing techniques.