Modeling Early Methane Generation in Coal

Thermogenic methane yields can be estimated indirectly from the average elemental composition of coals of different rank or inferred from the 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 addition, we note that the theoretical coalbed methane generation curves 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 economic potential of methane in low-rank coals, anhydrous, sealed-tube pyrolysis experiments were carried out on a Paleocene lignite from North Dakota. Experiments were conducted at heating rates of 10 °C/h and 10 °C/day between temperatures of 100 and 454 °C. With increasing final pyrolysis temperature, mean random huminite/vitrinite reflectance values increased from 0.31 to 1.61%, atomic H/C values of the extracted coal decreased from 0.88 to less than 0.56, and methane yields increased to a maximum of 46 mL/g initial lignite, or approximately 1560 cf/ton (dry, ash-free basis) (cf = cubic feet). Based on these results, coalification to high-volatile A bituminous rank or higher (R o ≥ 0.8%, atomic H/C ≤ 0.72, and NAI [log(n-C16/n-C30)] ≥ 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 generation with a series of parallel, first-order reactions with activation energies between 41 and 54 kcal/mol and a single frequency factor of 9.88 × 1011 s-1. Extrapolation of these parameters and a modified version of the EASY%Ro vitrinite reflectance model to geologic heating rates suggests that T > 120 °C and R o ≥ 0.9% are required to exceed the 300 cf/ton threshold. We conclude that while methane concentrations greater than 300 cf/ton may be found in high-volatile B bituminous and lower rank coals, in most cases they must be attributed to migrated gas or to near-surface (≤3000 ft) microbial activity.