Sub- and super-critical carbon dioxide flow behavior in naturally fractured black coal: An experimental study

A proper understanding of super-critical carbon dioxide (CO2) flow behavior in coal is essential, as CO2 normally exists in its super-critical state in deep coal seams and studies are lacking. The main objective of this study is to distinguish the permeability behavior of coal for sub-critical and super-critical CO2 flows. Therefore, a series of triaxial experiments was conducted on naturally fractured black coal specimens. Permeability tests were carried out for 15, 20 and 25 MPa confinements at 33.5 °C temperature. Three test scenarios were conducted to investigate, (1) variation of the permeability behavior of coal with CO2 phase condition, (2) the swelling effect on sub- and super-critical CO2 permeability patterns, and (3) the potential of nitrogen (N2) to reverse CO2-induced swelling. According to the test results, the permeability of super-critical CO2 is significantly lower than sub-critical CO2 due to the higher viscosity and swelling associated with super-critical CO2. Moreover, at super-critical state there is a higher decline of CO2 permeability with increasing injecting pressure due to the higher increments in the associated viscosity and swelling. Although CO2 adsorption-induced swelling causes permeability of both CO2 and N2 to be reduced at low injection pressures the poro-elastic effect becomes more dominant and may cause CO2 permeability to increase for higher injecting pressures, because CO2 flow behavior may transfer from super-critical to sub-critical after the swelling due to the decline of downstream pressure development. Moreover, N2 has the potential to reverse some swelling effects due to CO2 adsorption, and this recovery rate is higher at lower injecting pressures and higher confining pressures.