Coalbed methane extraction: Characteristics of damage-seepage evolution and dynamic response of methane transport in non-homogeneous coal seams under cavitation

Abstract

Mechanical cavitation has been widely researched and applied as a coal seam permeability enhancement and modification technology in coal CH₄ extraction and utilization. The non-homogeneous nature of coal seams in actual projects has a remarkable influence on the space-time distribution of coal seam damage and CH₄ transport. To clarify the damage-seepage synergistic evolution and the dynamic response of gas transport in non-homogeneous coal seams before and after cavitation. A coupled mechanics-damage-seepage model (MDS) considering the non-homogeneous characteristics of coal was constructed based on damage mechanics and geostatistical methods. Numerical simulations of coal bed cavitation gas extraction under different vertical loading conditions were conducted. The findings indicate that with the vertical load increase, coal seam damage gradually changes from mainly tensile damage to mainly shear damage. The damaged area gradually increases in size. The permeability of coal seams develops parallel with damage. When t = 0, a sudden rise in the permeability of the coal is consistent with the damage zone distributive pattern. As extraction time rises, the Klinken-Berg effect causes permeability to continue to rise. The increase in permeability changes the transportation path of CH₄ within the coal. The transport path is deflected from the low permeability region to the high permeability region. The inhomogeneity of gas transport path deflection is more pronounced in non-homogeneous coal seams. The CH₄ pressure in the coal seam declines with increasing extraction time. The effective extraction radius of non-homogeneous coal in the borehole before and after cavitation was 0.996 m and 1.661 m, which was improved by 66.7 %. The non-homogeneous nature of coal seams mainly affects the distribution characteristics of stress and damage. The coupled MDS model considering the non-homogeneous nature of coal seams can more accurately predict the damage-seepage evolution characteristics of coal seams and the dynamic gas transportation law after the creation of cavities.