Study on permeability laws of coal under coupled fluid dynamics and pore elasticity
SUN Zhenping
Abstract：The evolution of coal reservoir permeability is one of the key scientific issues affecting productivity enhancement in coal-bed methane development.Permeability is not only controlled by the geometric characteristics of the fracture structure but is also sig-nificantly influenced by fluid properties and their interactions with the coal matrix.To systematically investigate the permeability re-sponse characteristics of coal under different gas types and stress conditions,this study selected two groups of coal samples(M1 and M2)from Dongwanan Coal Mine in Shanxi Province.A series of triaxial seepage experiments were conducted using inert gases(He,Ar)and adsorptive gases(N2,CH4,CO2)under multiple levels of effective stress.Experimental results reveal that:the apparent per-meability(kapp)decreases with increasing pore pressure and is significantly suppressed by confining pressure.The apparent permeab-ility coefficient follows a decreasing trend of permeability coefficient of He>permeability coefficient of Ar≥permeability coeffi-cient of N2>permeability coefficient of CH4>permeability coefficient of CO2,with the inert gas(He)exhibiting the highest per-meability and the adsorptive gas(CO2)the lowest.Even after correcting for the slippage effect,the"intrinsic permeability"meas-ured by different gases under the same stress conditions still shows significant variation for the same sample.Both the zero-effective-stress permeability(k0)and the slippage-corrected permeability(k∞)decrease with increasing pore pressure,indicating a persistent in-fluence of adsorption-induced swelling and fracture closure.This suggests that traditional assumptions based on ideal gas behavior and Darcy's law may be insufficient to describe real gas flow in coal reservoirs.The slippage effect dominates permeability enhance-ment at low pressures,while elastic compression becomes dominant at high pressures.The interplay of these mechanisms results in a nonlinear permeability-stress relationship.The estimated fracture volume compression coefficients for both sample groups range between 0.054-0.059 MPa-1.While both samples behave similarly at high pore pressures,the sample with narrower fractures exhib-its a more pronounced slippage effect at lower pore pressures.This study elucidates the coupled mechanism between the slippage ef-fect(a fluid dynamic process)and fracture compressibility(a pore-elastic feature)in the evolution of coal permeability,providing im-portant theoretical support for the development of deep coalbed methane reservoirs.
Keywords：coalbed methane developmentcoal reservoir permeabilityslippage effectfracture compressibilityfluid dynamicspore elasticityapparent permeabilitypore pressure
Publication Date：2025-09-30
Online Publishing Date：2025-09-12(First online date of this platform, not the publication date of the document)
Pages：12( 122-133 )