VARIATIONS IN PORE CHARACTERISTICS OF HIGH VOLATILE BITUMINOUS COALS: IMPLICATIONS FOR COALBED GAS CONTENT

The Seelyville Coal Member (Linton Formation, Pennsylvanian) in Indiana was studied in order to: 1) determine variations in pore characteristics for a single location and compare them with variations from different locations; 2) evaluate the influence of mineral matter and maceral composition on mesopore and micropore characteristics; and 3) discuss implications of these variations on coalbed gas contents . The coal is high volatile bituminous with Ro ranging from 0.57 percent to 0.60 percent. BET surface areas of the coal samples studied range from 1.8 to 22.9 m²/g, BJH adsorption mesopore volumes range from 0.0041 to 0.0339 cm³/g, and micropore volumes range from 0.0315 to 0.0540 cm³/g. The highest surface areas and mesopore volumes are recorded for shallow coals, whereas the lowest values occur in the deepest coals. Micropore volumes, however, are not depth dependent. Increasing ash content is related to a decrease in micropore volume. In contrast, an increase in ash content is related to either a decrease or increase in surface area and mesopore volume, depending upon mineralogical composition. No obvious relationships were noted between the total vitrinite content and pore characteristics, but after splitting vitrinite into individual macerals, collotelinite correlates positively with both meso- and micropore volume, whereas collodetrinite correlates negatively with these volumes. In some cases, variations in gas contents within a coal seam at a single location are attributed to variability in the distribution of pore populations, whereas at other locations, gas content and mesopore/micropores distributions appear to be unrelated. This is interpreted to be the result of variable contributions from free and adsorbed gas. Therefore, both accurate determination of gas-in-place and the calculation of accurate gas saturations require desorbing the whole coal thickness.