COALBED METHANE CHARACTERISTICS OF GATES FORMATION COALS, NORTHEASTERN BRITISH-COLUMBIA - EFFECT OF MACERAL COMPOSITION

The majority of research reported on methane adsorption characteristic s of coal seams has focused on vitrinite-rich coals. However, western Canadian coals are more inertinite-rich than those of the western Unit ed States and are shown to differ in gas adsorption characteristics. T he influence of maceral composition upon gas adsorption characteristic s of medium-volatile coal samples from the middle Cretaceous Gates For mation of northeastern British Columbia was investigated. Lithotype (c oal facies) samples were analyzed for surface area, maceral and minera l composition, and methane adsorption; standard coal analyses were als o 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 i mportant influence on adsorption characteristics as indicated by carbo n 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 th e sample with the highest inertinite content. Carbon dioxide surface a reas 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 g enerally 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 i nterpreted as resulting from differences in the pore size distribution of vitrinite and inertinite: vitrinite is predominantly microporous w hereas inertinite is meso- to macroporous. The monolayer volumes of ca rbon dioxide (as calculated from the DubininRadushkevich equation) are higher than those of methane (as determined from the Langmuir equatio n), but are correlated. The methane adsorption isotherms and surface a rea data indicate that the maceral compositional variations in coal ar e at least as significant as coal rank in determining the potential vo lume of adsorbed methane and thus the coalbed methane potential of a d eposit.