Control of CO2 permeability change in different rank coals during pressure depletion: An experimental study

The gas permeability of different rank coals varies because of the summative effects of increasing effective stress, gas slippage, and coal matrix shrinkage during gas pressure depletion. In this paper, the natures of CO2 permeability change were primarily investigated in a high-volatile A bituminous coal (core D2-2), a moderate volatile bituminous coal (core Si), and an anthracite coal (core P11-2-1). Under a 4.3 MPa confining stress condition, as the gas pressure declines, the CO2 permeability of core D2-2 gradually decreases and then has a slight increase at mean gas pressures of less than approximately 0.8 MPa, the CO2 permeability of core SI initially decreases but subsequently increases above a mean gas pressure of approximately 1.3 MPa, and the CO2 permeability of core P11-2-1 continuously increases, especially at mean gas pressures of less than approximately 1.8 MPa. These pressure-depletion observations on CO2 permeability are considered to be the result of three effects: (a) increasing effective stress decreases CO2 permeability; (b) increased gas slippage increases CO2 permeability exponentially, becoming significant at mean gas pressures of less than approximately 0.8 MPa for the three cores; and (c) a positive effect on CO2 permeability from matrix shrinkage occurs at mean pressures of less than approximately 1.3 and 1.8 MPa for cores Si and P11-2-1, respectively, whereas the CO2 permeability of core D2-2 is negatively affected by matrix shrinkage at all tested pressures. Additionally, it is found that the three effects on the CO2 permeability depend upon the permeability of the coal and gas pressure.