Swelling of moist coal in carbon dioxide and methane

Determining the feasibility of injecting CO2 into coal seams for enhanced coalbed methane (ECBM) recovery as well as providing long-term carbon sequestration is an active area of research. It is now well known that coal swells in the presence of water and gases, which in turn may affect its in-seam permeability. If the swelling of the coal matrix by each component can be quantified, it may be possible to make better predictions about the suitability of particular seams for ECBM and carbon sequestration. Despite numerous studies where coal swelling has been measured in gases or water, there is relatively little information relating to how swelling of coals by gases is affected by water.In this paper we report on the gas-induced swelling behaviour of four moist Australian coals. Blocks of coal, nominally 30 × 9 × 9 mm, were cut parallel and perpendicular to the bedding plane from larger lumps. Samples were moisture-equilibrated at 97% relative humidity before being exposed to CO2 or CH4 at pressures up to 16 MPa and a temperature of 55 °C. Swelling of each sample was measured directly using digital cameras to monitor the change in length of the block as a function of pressure.Results show that swelling was greater in CO2 than CH4, with lower rank coals swelling more than high rank material. The presence of moisture significantly reduced the amount of additional swelling by the gas compared to dry coals; however, the degree to which the swelling of the coals was affected by moisture depended on the rank of the coal. It was also found that, proportionally, CH4-induced swelling was more affected by the presence of moisture than CO2-induced swelling. Although moist coals swelled less in CO2 or CH4 than dry coals, if the swelling due to moisture is included, the total swelling is more than that induced by the corresponding gas in the dry coal.

Research Highlights
► The effect of moisture on the CO2- and CH4-induced swelling of coal was measured for a range of Australian coals.
► Swelling was reduced in the presence of moisture with low rank coals being affected more than high rank coals.
► Proportionally, the presence of moisture reduced CH4-induced swelling more than CO2.
► The total swelling in moist coals i.e. the swelling initially induced by the presence of the moisture plus the swelling induced by the gas is more than that induced by the dry gas alone.
► Although swelling of coal seams during CO2 injection may be significant, it could be offset by the combined shrinkage caused by the displacement of the in situ methane and water from the seam.