Mechanism investigation and models evaluation of gas transport in dual-porosity coal seam
LIU Wei
HAN Dongyang
XU Hao
ZHOU Yujun
LI Tiannan
Abstract：Methane is an unconventional natural gas in coal seams.Coalbed methane(CBM)migration behavior constitutes a core issue in coalbed methane extraction.However,there is no consensus on the gas migration mechanism of dual-porosity coal seams.To elucidate the mechanism of gas migration and visualize the dynamic migration process of gas,two dual-porosity borehole gas seepage models were es-tablished.Specifically,the pressure gradient drives gas in the fracture,and gas in the coal matrix is driven by the pressure gradient and density gradient respectively.The finite difference method is employed to solve both models.Through the self-developed numerical simu-lation software,the gas pressure distribution,gas emission velocity,and gas accumulation emission amount were obtained.By comparing the numerical results with the field-measured data,the accuracy and disparities between the two models were investigated and discussed.The results show that:①In the initial stage of extraction,both models are predominantly governed by Darcy flow,and their gas emission velocity and cumulative gas emission quantity are substantially in consonance with the field data.In the subsequent stage,the gas within the coal matrix assumes a dominant role in gas migration.②Due to the existence of free gas,the variation range of gas pressure in bore-hole in the density gradient model is larger than that in the pressure gradient model.The diffusion behavior in coal matrix is more consist-ent with the density gradient model in coal matrix.③The gas emission rate of the borehole exhibits a positive correlation with the origin-al gas pressure,porosity,and fracture permeability coefficient,and a negative correlation with the matrix radius.The dual-porosity bore-hole gas transport model driven by the gas pressure gradient in fracture and free gas density gradient in coal matrix can reflect the physical behavior of borehole gas transport in coal reservoir more truly and accurately.
Keywords：coal seamgas transportdensity gradientdual-porosity modelgas emission rateparameter sensitivity
Publication Date：2025-02-28
Online Publishing Date：2025-08-15(First online date of this platform, not the publication date of the document)
Pages：12( 151-162 )