Swelling of Coals by Supercritical Gases and Its Relationship to Sorption

If CO2 can be sequestered in coal seams while simultaneously displacing coalbed methane [enhanced coalbed methane (ECBM)], some of the sequestration costs can be recovered through the production of methane. One potential difficulty with ECBM is that CO2 is known to swell coal, which may reduce its permeability. Coals also swell in other gases, although not to the same extent. Here, we report on the swelling of sub-bituminous and bituminous coals in CO2, CH4, N-2, CF4, ethane, and various noble gases. Helium and Ne induced negligible swelling; all other gases swelled the coals to varying degrees. The maximum swelling was proportional to the critical temperature of the gas, except for CF4, which is attributed to its greater size, preventing it from penetrating the coal as completely as the other gases. This indicates that swelling of these coals by all of these gases has a similar basic mechanism; CO2 is only different in the extent to which it swells coal. All coals swelled more in the direction perpendicular to the bedding plane than parallel to it, with the ratio of the swelling in each direction independent of pressure or gas type. Gas sorption and swelling in coal were found to be related according to a simple quadratic polynomial expression. The same relationship held for all of the coals and all gases investigated here. This means that swelling can be accurately predicted from the condensed volume of the gas adsorbed, regardless of the type of coal or gas.