Experimental Study on Post-Fracturing Shut-in Time Optimization for Tight Coalbed Methane Reservoirs

Summary Shut-in time optimization is a critical determinant of coalbed methane (CBM) stimulation efficiency, governing fracture conductivity preservation and fracturing fluid invasion dynamics within coal matrices. This study pioneers an integrated experimental approach, which couples core displacement with low-field nuclear magnetic resonance (LF-NMR), to systematically investigate the fluid imbibition and flowback behaviors in high-rank anthracite from the Mabi Block of the Qinshui Basin, China. Experiments are conducted under both attenuated and constant intrusion pressures (IPs), with the specific aim of elucidating the significance of post-fracturing shut-in time. Results demonstrate that invasion volume is higher under attenuated IP, yet the liquid phase also flows back more readily. Water-phase trapping (WPT) is more severe under constant IP than under attenuated pressure. Fracture permeability initially fluctuates, then declines during shut-in, with potential for significant increase during the fluctuation phase. Liquid-phase invasion enhances coal pore structure, primarily occupying mesopores (2–50 nm), while clay swelling and stress sensitivity diminish the resultant permeability improvement. Maintaining shut-in time below 10 hours effectively preserves fracture permeability while minimizing fracturing fluid invasion. Field implementation of post-fracturing rapid flowback technology—optimized for shut-in duration—has succeeded in the Qinshui Basin, significantly boosting gas production. This study provides critical insights for developing strategies to maximize CBM well stimulation efficiency in the Qinshui Basin.