Studying gas migration patterns under in situ extraction condition is essential for developing and utilizing unconventional gas resources. The in situ gas extraction environment can be characterized by uniaxial strain condition, where the overlying rock formation experiences constant stress and horizontal deformation remains stable. As gas depletion ensues, horizontal stresses diminish albeit to varying degrees. Currently, the influence of unloading gradients of horizontal stresses (σ2 and σ3) on the stability of coal seams and the mechanism of seepage evolution remains unclear. This study conducted seepage experiments at different dσ2/dp and dσ3/dp reduction gradients under various in situ stress and adsorptive gas scenarios to investigate the dynamic evolution of mechanical parameters, deformation and failure behavior, and the control mechanism on seepage characteristics of coal. Experimental findings revealed that when dσ2/dp and dσ3/dp were lower during helium (He) depletion, the effective stress in the horizontal direction rose, leading to compression of pores and fractures within the coal and a subsequent permeability decrease. However, as gas pressure decreased further, and the mean free path of gas molecules approached the characteristic length scale of coal structure, gas slippage emerged, causing an increase in permeability. The faster σ2 and σ3 were unloaded, the higher the increment of permeability and the faster the transition from lower to higher. At higher unloading gradients, coal damage ensued, manifesting as an abundance of fractures, which resulted in decreased elastic and hydraulic coupling moduli, an elevated Poisson's ratio, and an augmented damage variable, leading to an abnormal permeability increase. Notably, the high in situ stress enhanced the deformation resistance of coal, preventing its breakdown. The permeability of CO2 was notably lower than that of He due to adsorption-induced expansion, which narrowed the effective seepage channels and hindered gas flow by adhering gas molecules to these channels. During CO2 depletion, an exponential permeability increase was observed, attributed to the desorption of adsorbed gas. Building upon these insights, this study formulated a permeability evolution model under stress–pressure dual unloading conditions for different adsorbed gasses, offering a theoretical support for interpreting experimental results.

Deformation-Fracture Behavior and Permeability Evolution of Coal Under In Situ True Triaxial Stress–Pore Pressure Unloading

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发表日期：

2025年8月20日

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Beichen Yu,
Dongming Zhang,
Yingke Liu,
Changbao Jiang,
Yubing Liu,
Chao Liu,
Man Yuan

摘要

Studying gas migration patterns under in situ extraction condition is essential for developing and utilizing unconventional gas resources. The in situ gas extraction environment can be characterized by uniaxial strain condition, where the overlying rock formation experiences constant stress and horizontal deformation remains stable. As gas depletion ensues, horizontal stresses diminish albeit to varying degrees. Currently, the influence of unloading gradients of horizontal stresses (σ₂ and σ₃) on the stability of coal seams and the mechanism of seepage evolution remains unclear. This study conducted seepage experiments at different dσ₂/dp and dσ₃/dp reduction gradients under various in situ stress and adsorptive gas scenarios to investigate the dynamic evolution of mechanical parameters, deformation and failure behavior, and the control mechanism on seepage characteristics of coal. Experimental findings revealed that when dσ₂/dp and dσ₃/dp were lower during helium (He) depletion, the effective stress in the horizontal direction rose, leading to compression of pores and fractures within the coal and a subsequent permeability decrease. However, as gas pressure decreased further, and the mean free path of gas molecules approached the characteristic length scale of coal structure, gas slippage emerged, causing an increase in permeability. The faster σ₂ and σ₃ were unloaded, the higher the increment of permeability and the faster the transition from lower to higher. At higher unloading gradients, coal damage ensued, manifesting as an abundance of fractures, which resulted in decreased elastic and hydraulic coupling moduli, an elevated Poisson's ratio, and an augmented damage variable, leading to an abnormal permeability increase. Notably, the high in situ stress enhanced the deformation resistance of coal, preventing its breakdown. The permeability of CO₂ was notably lower than that of He due to adsorption-induced expansion, which narrowed the effective seepage channels and hindered gas flow by adhering gas molecules to these channels. During CO₂ depletion, an exponential permeability increase was observed, attributed to the desorption of adsorbed gas. Building upon these insights, this study formulated a permeability evolution model under stress–pressure dual unloading conditions for different adsorbed gasses, offering a theoretical support for interpreting experimental results.