The influence of CO₂ saturation time on the coal gas flow: Fractured bituminous coal

The permeability reduction due to the coal matrix swelling induced by the adsorption of CO₂ is the major concern during enhanced coalbed methane recovery (ECBM) as well as CO₂ sequestration in coal reservoir. Many problems arise in the context of this process regarding the coal permeability variations, and very few studies have been made with consideration of long-term CO₂ saturations on gas flow behaviour alterations. The main objective of this study is therefore to examine the influence of both subcritical and supercritical CO₂ flooding durations on N₂ and CO₂ permeability variation of naturally fractured low volatile bituminous coal under various tri-axial conditions. Five CO₂ flooding durations, up to 65 h, both subcritical (6 MPa) and supercritical (14, 20 MPa), were introduced to the sample under three confinements, respectively. Results show that for 11 MPa and 17 MPa confining environment, most permeability reduction occurs during the first CO₂ flooding process, and supercritical CO₂ flooding causes significant higher N₂ permeability reduction (from 14.38% to 50.18%) than subcritical CO₂ (from 4.11% to 11.25%). The permeability reduction rate decreases with time but this reduction continues even after a total of 153 h flooding. Longer permeability reduction process is observed upon supercritical CO₂ adsorption due to the much higher viscosity of supercritical CO₂. Under deep underground condition (>1 km), CO₂ exhibits a N₂-like flow behaviour because of the dominant poroelastic effect. The influence of CO₂ flooding on permeability alterations has been significantly compromised with much smaller permeability reduction after 20 MPa CO₂ flooding compared with 14 MPa CO₂ flooding (7.33% and 38.75% for 7 MPa CO₂ injection pressure, respectively), along with no apparent permeability reduction for the further CO₂ flooding scenarios.