Model for non-equilibrium evolution coal permeability of whole process under influences of gas pressure difference

XIAO Zhiyong; SUN Xiaoxiang; WANG Gang; WANG Mingzhen; JIA Wenwen; JIANG Feng; ZHENG Chengcheng

doi:10.11779/CJGE20231012

Volume 47 Issue 2

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Chinese Journal of Geotechnical Engineering > 2025 > 47(2): 355-364. > DOI: 10.11779/CJGE20231012

XIAO Zhiyong, SUN Xiaoxiang, WANG Gang, WANG Mingzhen, JIA Wenwen, JIANG Feng, ZHENG Chengcheng. Model for non-equilibrium evolution coal permeability of whole process under influences of gas pressure difference[J]. Chinese Journal of Geotechnical Engineering, 2025, 47(2): 355-364. DOI: 10.11779/CJGE20231012

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Model for non-equilibrium evolution coal permeability of whole process under influences of gas pressure difference

1.
Laboratory of Geological Hazards in Three Gorges Reservoir Area, Ministry of Education, China Three Gorges University, Yichang 443002, China
2.
Shandong Provincial Key Laboratory of Civil Engineering Disaster Prevention and Mitigation, Shandong University of Science and Technology, Qingdao 266590, China
3.
School of Civil Engineering, Fujian University of Technology, Fuzhou 350118, China

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Received Date: October 15, 2023
Available Online: July 23, 2024

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Abstract

Abstract

During the process of coalbed methane (CBM) extraction, gas pressure difference will exist for a long time due to different permeabilities between fracture and matrix, which makes coal in non-equilibrium state. The interaction between matrix and fracture in non-equilibrium state is constantly changing. At present, most studies do not fully explain the relationship between matrix adsorption deformation and change of fracture aperture. Therefore, a new non-equilibrium factor related to gas pressure difference is defined to describe the influence of matrix adsorption deformation on change of fracture aperture and coal deformation at different time. A theoretical model for non-equilibrium evolution of coal permeability is established, which includes the whole process from the initial equilibrium to the final one. Furthermore, the model is verified through the experimental data, and the spatial and temporal distribution of gas pressure and the evolution of coal permeability during gas injection and extraction are analyzed by the numerical simulation. The results show that: (1) The change of matrix pore pressure lags behind that of fracture pressure. The gas pressure difference exists for a long time, and its size increases rapidly from 0 and then slowly decreases to 0. (2) The permeability evolution can be divided into five stages. The first and the last stages represent the equilibrium state, the middle three stages represent the non-equilibrium state, and the permeability changes non-monotonically during the non-equilibrium evolution process. (3) The ratio of matrix width to matrix bridge width will affect the variation amplitude of permeability, and the different boundary conditions used in coal reservoir will affect the variation amplitude and the final value of permeability. The results are of certain guiding significance for CBM extraction reasonably.
- gas pressure difference,
- non-equilibrium state,
- matrix bridge,
- permeability,
- coalbed methane extraction

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Model for non-equilibrium evolution coal permeability of whole process under influences of gas pressure difference

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