Abstract
The influences of coal matrix expansion/contraction and effective stress on the permeability of coal with different thermal maturities are different during the CH4-CO2 replacement process due to different mechanical properties and gas adsorption capacities. To accurately predict the variation law of coal permeability during the CH4-CO2 replacement process, it is critical to understand how the matrix expansion/contraction and effective stress affect the permeability of coal at different thermal maturities during the CH4-CO2 replacement. In this study, the permeability of two coal specimens with anthracite and high-rank bituminous coal during the CH4-CO2 replacement process under different confining and injection pressures was tested using a CBM replacement testing machine. The results demonstrate that with decreasing gas injection pressure, the permeability of the two coal specimens exhibited a U-shaped correlation under different confining pressures. Under the same gas injection pressure, with increasing effective stress, the permeability presented a negative exponential decrease and the permeability of the anthracite decreased more significantly. Moreover, under the same confining pressure, with increasing gas injection pressure, the decreasing permeability agreed with Langmuir curve and the permeability of high-rank bituminous coal was more significantly reduced.
Highlights
Coal has a greater adsorption capacity for CO2 compared to CH4 under the same conditions [1, 2]
Extensive studies have been made aiming at understanding how the matrix expansion/contraction and effective stress impact the permeability during the replacement
Many studies have shown that as the amount of swelling of coal adsorbed with CO2 is greater than that of CH4 under the same conditions [6–8], the permeability of coal specimens decreases during the CH4-CO2 replacement process [5, 9–11]
Summary
Coal has a greater adsorption capacity for CO2 compared to CH4 under the same conditions [1, 2]. Many studies have shown that as the amount of swelling of coal adsorbed with CO2 is greater than that of CH4 under the same conditions [6–8], the permeability of coal specimens decreases during the CH4-CO2 replacement process [5, 9–11]. To investigate the amount of swelling of coal after absorbing gas, a number of approaches including probe test methods [12–14], stress detectors [15–17], and fiber optic cameras [18–21] have been adopted by some researchers. Data from these studies show that the amount of swelling and adsorption capacity are in close agreement with the results of the Langmuir adsorption isotherm.
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