Effect of Low-Frequency Pulsating Water Injection Parameters on Coal Permeability: Quantitative Characterization Experiments and a Field Case Study

Abstract

Efficient stimulation of coal reservoirs is crucial for enhancing coalbed methane (CBM) well productivity. As an emerging unconventional stimulation technology, low-frequency pulsating water injection (LF-PWI) has attracted widespread attention for its ability to transform the pore-fracture system of coal reservoirs and its potential for engineering application. In this study, LF-PWI experiments are conducted using NMR, CT, and a triaxial apparatus to investigate the influence of LF-PWI parameters on pore-fracture properties of coal samples. The dynamic gray relational analysis (DGRA) method is employed to quantitatively evaluate the correlations of pore-fracture properties with porosity and permeability, respectively. Finally, the stimulation mechanism of productivity is elucidated and validated through a case study. The results demonstrate that LF-PWI can reduce the coal elastic modulus by 35.15%. The volume and connectivity of micropores and mesopores fluctuate, while macropores with diameters of 200–300 μm increase significantly, forming dominant flow pathways (>500 μm). Permeability continuously increases due to macropore development (k/k0 = 4.07). Porosity initially shows a rapid rise driven by macropore expansion (φ/φ0 peaks near 1.05); it then declines (by up to 9.95%) and gradually recovers toward unity as the micropores vary. Increasing the pressure amplitude simultaneously enhances the deterioration of mechanical properties, the fluctuation of porosity, and the improvement of permeability. A segmented control strategy employing high–medium–high pressure amplitude can help balance the pore-fracture network, preserve the gas source, and expand gas seepage pathways. Field application in the DFZ-1 well confirmed that LF-PWI mitigates coal fines flowback and increases cumulative gas production by 17.07%. This study provides a theoretical basis for employing LF-PWI to improve CBM well productivity.