VARIATIONS IN PORE CHARACTERISTICS FOR SUBBITUMINOUS COALS OF THE WILCOX GROUP, GULF COASTAL PLAIN, in RELATION TO COALBED GAS

The mechanisms of adsorption and storage of gas have been widely studied, particularly in relation to the porosity of coal. Previous studies indicate that coal pore characteristics have an important effect upon methane gas transport through the coal matrix. In this study, we examine porosity and other characteristics (including moisture content, depth of coal, vitrinite reflectance, maceral composition) of three subbituminous coal core samples collected from the Paleocene-Eocene Wilcox Group of south Texas and north Louisiana. The goal of the study is to determine whether variations in these characteristics are related to coalbed desorbed gas content.

Gases of Wilcox Group coal beds in these areas are reported to be primarily microbially-generated through the bacterial reduction of CO₂, although mixtures of biogenic gas with thermogenic gas are present in some areas (Warwick et al., 2008, doi:10.1016/j.coal.2008.05.009). Porosity measurements include micropore (0-2 nm) volumes and micropore size distribution, determined by CO₂ adsorption, and surface areas, mesopore (2-50 nm) volumes and mesopore size distribution determined by nitrogen adsorption. The coal samples analyzed for porosity have a wide range of desorbed gas contents (dry ash free basis), from near zero gas content (Zavala County, TX); to 46.9 SCF/ton (Ouachita Parish, LA); to 233 SCF/ton (Caldwell Parish, LA). Preliminary results indicate that mesopore volumes for the 60-mesh (250 µm) splits range from a minimum of 0.010 cm³/g (sample with near zero gas content, Zavala County, Texas) to 0.044 cm³/g (sample with the highest gas content, Caldwell Parish, Louisiana). Micropore volumes for the 60-mesh splits range from 0.040 to 0.057 cm³/g; the relationship between micropore volumes and gas content will be evaluated in future work. Comparisons are also made between 16-mesh (1190 µm) and 60-mesh splits of each coal sample to determine whether split sizes affect coal porosity measurements. Mesopore and micropore volumes for the 60-mesh splits were found to be higher (about 2-200 % higher) than those for the 16-mesh splits, for 5 of 6 splits analyzed. Future work includes evaluating geologic factors such as the salinity of formation waters, proximity to salt domes, faults, and cleats and fractures in coal, in relationship to coalbed gas content.