Chapter 14 - Dynamic Gas Flow in Coals and Its Evaluation

Abstract

The gas permeability of coal varies due to the comprehensive effects of effective stress increase, coal matrix shrinkage, gas slippage, and Knudsen diffusion during gas pressure depletion. In this chapter, a model was established to predict the adsorbing-gas (CO2) permeability change by coupling pore size evolution with the permeability change during gas pressure depletion. Two parameters (change ratio of permeability and total loss of flow rate) were proposed to quantitatively evaluate the dynamic process of CO2 flow in different coal ranks. Furthermore, the effects of the maceral composition and initial permeability of coal on the dynamic gas flow were observed. The total loss of flow rate is positively related to the change ratio of permeability and decreases with increasing coal rank from high volatile bituminous to semianthracite coal but increases in anthracite coal. In different coal ranks, the total loss of flow rate and change ratio of permeability are comprehensively affected by the maceral composition and initial permeability of the coal, of which the maceral composition primarily affects the total loss of flow rate and change ratio of permeability changes in bituminous and semianthracite coals. The initial permeability dominates the changes in the total loss of flow rate and change ratio of permeability as coal rank varies from semianthracite to anthracite coal. For the same rank, coals (especially some bituminous and anthracite coals) with lower and higher initial permeabilities have smaller values of total loss of flow rate and change ratio of permeability.