INVESTIGATION OF PORE CHARACTERISTICS IN THE WILCOX GROUP COAL, GULF COAST REGION: RELATIONSHIPS TO BIOGENIC GAS

Coal samples from three cores of the Paleocene-Eocene Wilcox Group of north Louisiana and south Texas were analyzed for pore characteristics and examined in relation to desorbed gas contents. Previous work suggested that gases contained in the Wilcox coal beds of north Louisiana are primarily microbially-generated via bacterial CO₂ reduction. The core samples analyzed are from the Ouachita Parish, LA (13 samples), Caldwell Parish, LA (1 sample), and Zavala County, TX (1 sample). The desorbed gas contents (dry, ash-free basis) for the samples range from 1.9 standard cubic feet per ton (scf/t) for the Zavala County sample to 233 scf/t for the Caldwell Parish sample. Porosity measurements were made on 16 mesh (1.19 mm) splits of the coal samples. Measurements include (1) micropore (0-2 nm) volumes and micropore size distribution determined by CO₂adsorption, and (2) surface areas, mesopore (2-50 nm) volumes and mesopore size distribution determined by nitrogen adsorption. Cleats and microfractures (size, orientation) will also be examined to determine if there is a relationship to the desorbed gas content of the coal.

For the Ouachita Parish core, mean mesopore sizes (13.76 to 23.6 nm) showed the strongest correlation with desorbed gas contents (40.9 to 66.6 scf/t) of the coal (r = 0.60), of all variables. The carbon/ash yield ratio of the coal samples was found to be positively correlated to the micropore surface area (r = 0.94) and micropore volume (r = 0.94) and negatively correlated to the mesopore surface area (r = -0.73) and mesopore volume (r = -0.65). Preliminary results suggest that maceral composition is not correlated to (1) micropore or mesopore surface areas or volumes, or (2) desorbed gas content, as noted previously.

Data for the Ouachita Parish and Zavala County core samples suggest that average mesopore sizes may be correlated to desorbed gas volumes (r = 0.81). The data for the one high gas sample from Caldwell Parish does not follow the trend of the other two cores; work is in progress to analyze additional samples from this core. Preliminary results suggest that although the porosity of coal may have some influence on the amount of methane generated and stored in coal, there are other controlling geologic factors (such as depth of coal, salinity of groundwater, proximity to salt structures) that are more important.