Analyzing the permeability enhancement effect of hydraulic slotting for low-permeability coal seams based on a stress-diffusion-seepage coupling model

Authors

ZHANG Hongtu, School of Safety Science and Engineering, Henan Polytechnic University, Jiaozuo 454003, China; State Key Laboratory Cultivation Base for Gas Geology and Gas Control, Jiaozuo 454003, China; Collaborative Innovation Center of Coal Work Safety and Clean High Efficiency Utilization, Jiaozuo 454003, ChinaFollow
ZHOU Tian, School of Safety Science and Engineering, Henan Polytechnic University, Jiaozuo 454003, China
WANG Dengke, School of Safety Science and Engineering, Henan Polytechnic University, Jiaozuo 454003, China; State Key Laboratory Cultivation Base for Gas Geology and Gas Control, Jiaozuo 454003, China; Collaborative Innovation Center of Coal Work Safety and Clean High Efficiency Utilization, Jiaozuo 454003, ChinaFollow
LI Botao, School of Safety Science and Engineering, Henan Polytechnic University, Jiaozuo 454003, China; State Key Laboratory Cultivation Base for Gas Geology and Gas Control, Jiaozuo 454003, China; Collaborative Innovation Center of Coal Work Safety and Clean High Efficiency Utilization, Jiaozuo 454003, China
LUO Yong, Sichuan Furong Chuannan Construction Engineering Co., Ltd., Yibin 644000, China
PAN Rongsheng, Sichuan Furong Chuannan Construction Engineering Co., Ltd., Yibin 644000, China
TANG Jiahao, School of Safety Science and Engineering, Henan Polytechnic University, Jiaozuo 454003, China
LU Weiyong, Department of Resources and Mechanical Engineering, Lüliang University, Lüliang 033001, China

Abstract

Objective and Methods This study aims to improve the permeability enhancement effect of coal seams for gas drainage. Targeting the soft, low-permeability coal seams with complex gas occurrence conditions in the Baiping mining area and considering the impacts of effective stress, gas migration, actual gas effects, and tortuosity, this study constructed a stress-diffusion-seepage coupling model based on the pore-fracture dual medium assumption. Using the coupling model, this study simulated the variation patterns of gas content in coal seams during gas drainage under varying gas contents and borehole spacings. Through field tests under different permeability enhancement measures and unloaded coal amounts across borehole intervals, this study compared the permeability enhancement effects of hydraulic flushing, hydraulic slotting, and mechanical reaming to determine the optimal permeability enhancement technique.Results and Conclusions The stress-diffusion-seepage coupling model enables accurate characterization of the diffusion behavior and flow paths of gas in coal seams. The coal seam thickness and gas content produce significant impacts on the gas drainage effects, with a thicker coal seam and higher gas content associated with a smaller borehole spacing required to achieve effective gas drainage. Under gas contents of ≤7 m³/t, >7‒8 m³/t, and >8‒9.35 m³/t, borehole spacings sshould not exceed 11 m, 9 m, and 8 m, respectively in the case of coal seam thicknesses ≤1 m, should not exceed 10 m, 9 m, and 7 m, respectively if coal seam thicknesses range from > 1 m to 5 m, and should not exceed 10 m, 8 m, and 7 m, respectively if coal seam thicknesses vary from > 5 m to 9 m. Field tests indicate that hydraulic flushing, hydraulic slotting, and mechanical reaming all reduce the volume fraction of methane in coal seams and increase the effective gas drainage radius. Notably, these effects prove especially significant in the early drainage stage, while gradually tending to be gentle in the later stage. Under the same technical conditions, hydraulic slotting can deliver optimal performance, reducing the volume fraction of methane by 64.74 % and increasing the effective gas drainage radius by 0.35 m after 90 days of gas drainage.

Keywords

coalbed methane (CBM), hydraulic slotting, permeability enhancement measure, stress-diffusion-seepage coupling model, borehole arrangement

DOI

10.12363/issn.1001-1986.25.02.0132

Recommended Citation

ZHANG Hongtu, ZHOU Tian, WANG Dengke, et al. (2025) "Analyzing the permeability enhancement effect of hydraulic slotting for low-permeability coal seams based on a stress-diffusion-seepage coupling model," Coal Geology & Exploration: Vol. 53: Iss. 6, Article 13.
DOI: 10.12363/issn.1001-1986.25.02.0132
Available at: https://cge.researchcommons.org/journal/vol53/iss6/13

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