The effects of sub-critical and super-critical carbon dioxide adsorption-induced coal matrix swelling on the permeability of naturally fractured black coal

Swelling of the coal matrix with the adsorption of CO2 is one of the leading problems for CO2 sequestration in deep coal seams as it causes coal seam permeability to be significantly reduced. The main objective of this study was to investigate the effect of coal mass swelling on the permeability of naturally fractured black coal. A series of permeability tests were conducted using a newly developed tri-axial apparatus on 38 mm by 76 mm naturally fractured black coal specimens. These tests were carried out for CO2 and N2 injections at 2–20 MPa injection pressures under 10 to 24 MPa confining pressures at 33 °C. Each coal specimen was then allowed to swell under sub-critical and super-critical CO2 adsorption and the corresponding effects on CO2 and N2 permeabilities were examined. Results indicate that the permeability of naturally fractured black coal is significantly reduced due to matrix swelling, which starts as quickly as within 1 h of CO2 injection. A further reduction is then observed, and the maximum swelling rate occurs within the first 3–4 h of CO2 adsorption. The amount of coal matrix swelling due to CO2 adsorption clearly depends on the phase condition of the CO2, and super-critical CO2 adsorption-induced swelling is about two times higher than that induced by sub-critical CO2 adsorption. Interestingly, although a fractured coal specimen which has already fully swelled under sub-critical CO2 adsorption can swell significantly more under super-critical CO2 adsorption, after swelling under super-critical CO2 adsorption, no further swelling effect occurs under any CO2 pressure or phase condition. Moreover, the swelling process continues longer under super-critical CO2 adsorption. It is concluded that super-critical CO2 adsorption can induce more matrix swelling than sub-critical CO2 adsorption under the same adsorption pressure.

► Sub- and super-critical CO2 induced coal swelling effects on its permeability. ► Triaxial experiments for naturally fractured black coal. ► Permeability significantly reduces due to immediate swelling. ► Permeability reduction due to super critical CO2 is about two times higher. ► Super critical CO2 makes a stronger and longer swelling effect.