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Characterizing pore structure of coal under CO2 sequestration conditions

High pressure supercritical CO2 (HP-ScCO2) geochemical reactor was designed to study the interaction between coal and HP-ScCO2 in order to simulate the CO2 geo-sequestration into and/or enhancement of coalbed methane recovery from deep coal, focusing on the characterization of the pore structure of coal and its changes. Four coal samples with different coal rank were chosen for the HP-ScCO2 tests using the geochemical reactor under around 40 degrees C and 9.8 MPa for 72 hours. The coal samples with and without the ScCO2-H2O treatment were further investigated using the mercury porosimetry, providing the mercury intrusion data for characterization of the pore structure of coals. Fractal analysis was used to distinguish inter- and intraparticle pores at lower mercury intrusion pressure and to define the initial pressure associated coal compressibility. The fractal dimension phenomena corresponding to three pressure ranges were observed associated with three different mercury intrusion processes. These fractal dimension phenomena can be described by means of the fractal dimensions. The fractal dimensions in relatively low pressure are not change much, mainly resulted from the accumulation mode of particle samples in the penetrometer and the roughness of samples which caused by crushing and grinding process. In the higher pressure range, the fractal dimensions decreases with increasing pressure as the coal rank increased, which probably related to the hardness of coal. In general, CO2 sequestration process makes all the samples become easier to be compressed than the raw samples. Moreover, coal rank and ash content may play more important role in maintaining the pore structure. After reacted with HP-ScCO2, the higher rank samples exhibit larger pore structure changes than the lower rank ones.