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

Abstract

Abstract In deep coal seams carbon dioxide (CO 2 ) exists in its super-critical state, which emphasizes the importance of studying super-critical CO 2 flow behaviour in coal, especially for field applications, such as CO 2 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 CO 2 storage capacity. The main objective of this study is therefore to determine the permeability behaviour of coal for sub-critical and super-critical CO 2 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 38 °C temperature. According to the test results, CO 2 permeability in coal may reduce with increasing injection pressure due to the phase transition of CO 2 from sub-critical to super-critical during pressure increment. N 2 injection into a coal mass permeated with CO 2 shows lower permeability values compared to N 2 injection into a fresh coal mass, because in the former case, the coal mass structure has already been critically reformed during the CO 2 flood. Although the pressure development trends for first N 2 , CO 2 and second N 2 injections along the sample are similar, there may be a noticeable pressure reduction, especially closer to the injection point during the second N 2 injection. This is due to the coal structure re-arrangement during the CO 2 flood, where the pressure development is less in the regions, when CO 2 is in the super-critical state. Such CO 2 and N 2 migration patterns through coal seams after injection are highly important for field applications.