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

Abstract

Swelling of the coal matrix with the adsorption of CO 2 is one of the leading problems for CO 2 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 CO 2 and N 2 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 CO 2 adsorption and the corresponding effects on CO 2 and N 2 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 CO 2 injection. A further reduction is then observed, and the maximum swelling rate occurs within the first 3–4 h of CO 2 adsorption. The amount of coal matrix swelling due to CO 2 adsorption clearly depends on the phase condition of the CO 2, and super-critical CO 2 adsorption-induced swelling is about two times higher than that induced by sub-critical CO 2 adsorption. Interestingly, although a fractured coal specimen which has already fully swelled under sub-critical CO 2 adsorption can swell significantly more under super-critical CO 2 adsorption, after swelling under super-critical CO 2 adsorption, no further swelling effect occurs under any CO 2 pressure or phase condition. Moreover, the swelling process continues longer under super-critical CO 2 adsorption. It is concluded that super-critical CO 2 adsorption can induce more matrix swelling than sub-critical CO 2 adsorption under the same adsorption pressure.