A macro-scale experimental study of sub- and super-critical CO₂ flow behaviour in Victorian brown coal

In deep coal seams carbon dioxide (CO₂) exists in its super-critical state, which emphasizes the importance of studying super-critical CO₂ flow behaviour in coal, especially for field applications, such as CO₂ sequestration and enhanced coal bed methane recovery. Although there has been some research on the subject, the studies have been conducted on only certain types of coal (e.g., naturally fractured black coal) using small coal samples, which makes it difficult to verify the applicability of adsorption theories at a higher scale to estimate field CO₂ storage capacity. The main objective of this study is therefore to determine the permeability behaviour of coal for sub-critical and super-critical CO₂ flows using large coal specimens (203 mm in diameter and 1000 mm in length). A series of core flooding experiments was conducted on brown coal specimens collected from the Latrobe Valley basin, Victoria, using an advanced core flooding apparatus, for a range of injection pressures (6-10 MPa) at 11 MPa axial stress and 38ºC temperature. According to the test results, CO₂ permeability in coal may reduce with increasing injection pressure due to the phase transition of CO₂ from sub-critical to super-critical during pressure increment. N₂ injection into a coal mass permeated with CO₂ shows lower permeability values compared to N₂ injection into a fresh coal mass, because in the former case, the coal mass structure has already been critically reformed during the CO₂ flood. Although the pressure development trends for first N₂, CO₂ and second N₂ injections along the sample are similar, there may be a noticeable pressure reduction, especially closer to the injection point during the second N₂ injection. This is due to the coal structure re-arrangement during the CO₂ flood, where the pressure development is less in the regions, when CO₂ is in the super-critical state. Such CO₂ and N₂ migration patterns through coal seams after injection are highly important for field applications.