Laboratory investigations of gas flow behaviors in tight anthracite and evaluation of different pulse-decay methods on permeability estimation

Permeability evolution in coal is critical for the prediction of coalbed methane (CBM) production and CO₂-enhanced-CBM. The anthracite, as the highest rank coal, has ultra-tight structure and the gas flow dynamics is complicated and influenced by multi-mechanistic flow components. Gas transport in anthracite will be a nonlinear multi-mechanistic process also including non-Darcy components like gas as-/desorption, gas slippage and diffusion flow. In this study, a series of laboratory permeability measurements were conducted on an anthracite sample for helium and CO₂ depletions under both constant stress and uniaxial strain boundary conditions. The different transient pulse-decay methods were utilized to estimate the permeability and Klinkenberg correction accounting for slip effect was also used to calculate the intrinsic permeability. The helium permeability results indicate that the overall permeability under uniaxial strain condition is higher than that under constant stress condition because of larger effective stress reduction during gas depletion. At low pressure under constant stress condition, CO₂ permeability enhancement due to sorption-induced matrix shrinkage effect is significant, which can be either clearly observed from the pulse-decay pressure response curves or the data reduced by Cui et al.'s method. But within the same pressure range, there is almost no difference between Brace's method and Dicker & Smits's method. Gas slippage effect is also significant at low pressure for low permeability coal based on the obtained experimental data.