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Abstract: Swelling of coal, especially in the presence of CO2, may reduce the permeability of coal seams thus affecting the viability of enhanced coalbed methane production or CO2 sequestration operations. In this paper we report laboratory measurements of swelling in four Australian bituminous and sub-bituminous coals in CO2, methane and various mixtures of the two gases. Measurements were made on unconstrained monolithic samples at 55°C and at pressures up to about 15MPa. Volumetric swelling at 15MPa ranged from about 1.9% to 5.5% in CO2 and 1.0% to 2.5% in CH4 depending on the rank of coal and the proportion of CO2 in the gas mixture. Swelling measurements made at a constant pressure of 15MPa but with varying gas composition showed that even CO2 with a high affinity for coal could be completely displaced by helium, which does not adsorb, causing the coal sample to shrink to its original dimensions. Similarly, CH4 was displaced by injecting CO2 into the system which then caused the coal to swell to the same level as if it had been exposed to pure CO2. The results of these experiments show that there is no enhanced swelling in mixed gases above that would be observed in the pure CO2 at the same total pressure. As well, the swelling behaviour of coal in mixed gases was determined solely by the partial pressure of the sorbing gas so that weakly or non-sorbing gases are effective at displacing strongly adsorbed gases. [Copyright &y& Elsevier]

The gas-induced swelling behavior of coal is important when considering CO2 sequestration into coal seams or enhanced coalbed methane applications, but coals may also swell in the presence of moisture, or shrink on drying. In this paper we examine the moisture-swelling properties of coals from Australia and elsewhere. Results on the moisture uptake and corresponding swelling measurements are presented for 15 coals of various ranks (sub-bituminous and bituminous) at 22°C and atmospheric pressure. Measurements were made by exposing sample blocks of coal (nominally 30 × 10 × 10 mm) to relative humidities ranging from 0% to 97%. A selection of coals was also fully saturated in water. Moisture uptake at 97% relative humidity (RH) ranged from about 2.5% to more than 16% db. Maximum linear strain associated with the moisture sorption (measured at 97% RH) varied from about 0.2% to 1.3%, with lower rank coals showing the most swelling. In all cases, swelling was greater in the direction perpendicular to the bedding plane. These results correspond to volumetric swelling of about 0.5% to around 5%. Although exhibiting significant expansion, all of the samples returned to their original dimensions upon drying. Volumetric moisture sorption and the amount of swelling induced were found to be strongly correlated by a single linear expression that held for all of the coals examined. It was further found that the volume of the water adsorbed was linearly related to the pore space within the coal, however, at 97% relative humidity, only about 60% of the available pore space is occupied by water. Exposure to liquid water allowed the pores to completely fill; although for the lowest rank coals, the volume of water absorbed appeared to be slightly more than the corresponding pore volume. Despite the additional water uptake, immersion in water did not produce further swelling beyond that induced at 97% relative humidity. [ABSTRACT FROM AUTHOR]