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Mechanism of Coal Permeability Enhancement by a Series of Mechanically Over-excavated Cavities Along Drainage Boreholes

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Abstract

To solve the difficult problems of gas drainage in coal seams with a high gas content and low permeability, a novel technology based on a series of mechanically over-excavated cavities along drainage boreholes in coal seams to increase permeability was proposed. In this paper, the fluid–solid coupled model of the mechanically over-excavated borehole and the gas drainage in soft coal seams was analyzed, and COMSOL Multiphysics numerical simulation software was used for validation. The influence of the parameters for the mechanically over-excavated borehole (i.e., cavity diameter, cavity length, and interspace between the cavities and cavity number) and the reservoir properties of coal seams (i.e., initial permeability, initial porosity and elastic modulus) on gas drainage were investigated. The results show that the gas drainage volume increases with cavity volume and has little effect when the interspace between the cavities exceeds 0.5 m. This indicates that mechanically over-excavated boreholes can significantly increase the coal seam permeability and gas drainage volume. Based on numerical analyses, a single-wing embedded mechanically over-excavated borehole drill bit was developed. The effectiveness of the drill bit was tested in situ at the Licun Coal Mine of the Shanxi Lu’an Group. The results indicate that the effective gas drainage volume of a mechanically over-excavated borehole is 147% higher than that of an ordinary borehole and is not much different from that of a hydraulic flushing borehole. However, mechanically over-excavated borehole technology has great advantages with respect to operational efficiency and cost.

Highlights

  • The influence of the parameters for the mechanically over-excavated borehole (i.e., cavity diameter, cavity length, interspace between the cavities and cavity number) on gas drainage was studied.

  • The influence of reservoir properties of coal seams (initial permeability, initial porosity and elastic modulus) on gas drainage was studied.

  • Combining the actual coal seam drilling conditions of the Licun Coal Mine of the Shanxi Lu’an Group and considering the economic investment, based on numerical simulation analysis, the optimal cavity diameter was determined.

  • A single-wing embedded over-excavated bit was developed and successfully applied in the #2302 workface of the Licun Coal Mine.

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Acknowledgements

This work was supported by the National Key Research and Development Program of China (2020YFA0711802), the National Natural Science Funds of China (51504252), the Fundamental Research Funds for the Central Universities (2017QNB15) and the Research Fund of State and Local Joint Engineering Laboratory for Gas Drainage & Ground Control of Deep Mines (Henan Polytechnic University SJF202001).

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Author notes

  1. Zhongyi Man

    Present address: The National Joint Engineering Laboratory of Internet: Applied Technology of Mines, China University of Mining & Technology (CUMT), Xuzhou, 221116, Jiangsu, People’s Republic of China

Authors and Affiliations

  1. The National Joint Engineering Laboratory of Internet: Applied Technology of Mines, China University of Mining & Technology (CUMT), Xuzhou, 221116, Jiangsu, People’s Republic of China

    Chun Liu & Mingyao Wei

  2. Key Laboratory of Gas and Fire Control for Coal Mines, China University of Mining & Technology (CUMT), Xuzhou, 221116, Jiangsu, People’s Republic of China

    Zhongyi Man, Fubao Zhou & Jiaxing Gu

  3. State and Local Joint Engineering Laboratory for Gas Drainage & Ground Control of Deep Mines, Henan Polytechnic University, Jiaozuo, People’s Republic of China

    YongLong Wang

Authors
  1. Chun Liu
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  3. Fubao Zhou
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  4. Jiaxing Gu
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Contributions

LC helped in guidance, investigation, funding, date curation, and writing—reviewing. ZM contributed to revision, writing—review and editing, polishing, data collection and application. FZ helped in direction, supervision, validation, editing and conceptualization. JG contributed to writing—original draft preparation, software, application and editing. MW helped in software. YW contributed to application.

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Correspondence to Fubao Zhou or Mingyao Wei.

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Liu, C., Man, Z., Zhou, F. et al. Mechanism of Coal Permeability Enhancement by a Series of Mechanically Over-excavated Cavities Along Drainage Boreholes. Rock Mech Rock Eng 56, 8979–8996 (2023). https://doi.org/10.1007/s00603-023-03535-5

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