Coalbed Methane Characteristics of Gates Formation Coals, Northeastern British Columbia: Effect of Maceral Composition

The majority of research reported on methane adsorption characteristics of coal seams has focused on vitrinite-rich coals. However, western Canadian coals are more inertinite-rich than those of the western United States and are shown to differ in gas adsorption characteristics. The influence of maceral composition upon gas adsorption characteristics of medium-volatile coal samples from the middle Cretaceous Gates Formation of northeastern British Columbia was investigated. Lithotype (coal facies) samples were analyzed for surface area, maceral and mineral composition, and methane adsorption; standard coal analyses were also performed (proximate, low-temperature ash, and equilibrium moisture). The vitrinite content of the samples analyzed ranges from 18 to 95% (vol. %, mineral matter free); the ash yield varies from 4.4 to 33.7% (wt. %). Both maceral composition and mineral matter content have an important influence on adsorption characteristics as indicated by carbon dioxide surface areas and methane adsorption isotherms. On a mineral matter-free basis, the amount of methane adsorbed generally increases with vitrinite enrichment. The lowest methane adsorption occurs in the sample with the highest inertinite content. Carbon dioxide surface areas of the lithotypes range from 87 to 176 m2/g on a raw-coal basis, and from 99 to 184 m2/g on a mineral matter-free basis. Surface area generally decreases with increased mineral matter content and increases with increased vitrinite content. The increase in adsorption of both methane and carbon dioxide with increased vitrinite concentration is interpreted as resulting from differences in the pore size distribution of vitrinite and inertinite: vitrinite is predominantly microporous whereas inertinite is meso-to macroporous. The monolayer volumes of carbon dioxide (as calculated from the DubininRadushkevich equation) are higher than those of methane (as determined from the Langmuir equation), but are correlated. The methane adsorption isotherms and surface area data indicate that the maceral compositional variations in coal are at least as significant as coal rank in determining the potential volume of adsorbed methane and thus the coalbed methane potential of a deposit.