R&D and application of an experimental system for coal permeability enhancement through resonance under the excitation of low-frequency vibration

Objective As an emerging green, efficient method for enhancing coal permeability, coal resonance under the excitation of vibration waves works by stimulating the development of pores and fractures in coals using the stress waves generated by low-frequency vibration. To investigate the impacts of excitation frequency, stress field, and resonance effect on permeability enhancement, as well as exploring the mechanisms behind coal permeability enhancement using the method, this study independently designed and developed an experimental system. Method The system, consisting of a mainframe control unit, a coal-sample clamping unit, a vibrational excitation unit, and a coal vibration parameter monitoring unit, allows for testing the natural frequency of coals, simulating excitation conditions with varying intensities, and real-time monitoring of the vibration response characteristics of coal under the vibrational excitation. Using this system, this study examined the anthracite samples from the Zhaogu No. 2 Coal Mine in Jiaozuo, Henan Province. Through experiments on the natural frequency of coals and their seepage under the excitation of low-frequency vibration, this study revealed the variation patterns of coal permeability under the excitation of low-frequency vibration with different excitation parameters and stress magnitudes, achieving the in-situ forced vibration of coals. This study observed the resonance effect of coals under the excitation of low-frequency vibration by monitoring the vibration response characteristics of coals. Based on this, as well as industrial CT, this study elucidated the mechanisms behind coal permeability enhancement through resonance-induced fracturing. Results and Conclusions The experimental results are outlined as follows: (1) Low-frequency vibration enhanced the coal permeability, and more effective permeability enhancement of coals was achieved in the case where the coal-rock damage approached its critical instability; (2) The coals produced a resonance effect when the vibrational frequency approached their natural frequency (20 Hz). The forced resonance of the coals was accompanied by increased acceleration response, the gradual propagation of microfractures inside coals and rocks, and the interconnection of pores and fractures inside the coal matrix, thus significantly enhancing the coal permeability. These experimental results and the R&D of the experimental system can reveal the mechanisms behind coal permeability enhancement through resonance under the excitation of low-frequency vibration, thus serving as a theoretical guide for efficient gas extraction from low-permeability coal seams.