MODELING EARLY METHANE GENERATION IN COAL

Thermogenic methane yields can be estimated indirectly from the averag e elemental composition of coals of different rank or inferred from th e results of coal pyrolysis experiments. Unfortunately, most published studies have been insufficiently detailed to estimate gas contents in lignite, subbituminous coal and high-volatile B bituminous coal. In a ddition, we note that the theoretical coalbed methane generation curve s of Juntgen and Karweil and other commonly quoted papers overestimate methane yield because they do not consider hydrogen loss from coal in the form of water. In order to place better constraints on the econom ic potential of methane in low-rank coals, anhydrous, sealed-tube pyro lysis experiments were carried out on a Paleocene lignite from North D akota. Experiments were conducted at heating rates of 10 degrees C/h a nd 10 degrees C/day between temperatures of 100 and 454 degrees C. Wit h increasing final pyrolysis temperature, mean random huminite/vitrini te reflectance values increased from 0.31 to 1.61%, atomic WC values o f the extracted coal decreased from 0.88 to less than 0.56, and methan e yields increased to a maximum of 46 mL/g initial lignite, or approxi mately 1560 cf/ton (dry, asl;-free basis) (cf = cubic feet). Based on these results, coalification to high-volatile A bituminous-rank or hig her (R(o) greater than or equal to 0.8%, atomic H/C less than or equal to 0.72, and NAI [log(n-C-16/n-C-30)] greater than or equal to 0.03) appears required to achieve a modest in situ economic threshold of 300 cf/ton methane. Pyrolysis yields were used to model early methane gen eration with a series of parallel, first-order reactions with activati on energies between 41 and 54 kcal/mol and a single frequency factor o f 9.88 x 10(11) s(-1). Extrapolation of these parameters and a modifie d version of the EASY%Ro vitrinite reflectance model to geologic heati ng rates suggests that T > 120 degrees C and R(o) greater than or equa l to 0.9% are required to exceed the 300 cf/ton threshold. We conclude that while methane concentrations greater than 300 cf/ton may be foun d in high-volatile B bituminous and lower rank coals, in most cases th ey must be attributed to migrated gas or to near-surface (less than or equal to 3000 ft) microbial activity.