The use of coal fields as subsurface bioreactors for producing sustainable methane gas from terrestrial sources of biomass is described. Microbial presence is determined for a target coal formation, and tracers are injected to determine permeability, porosity, volume, and minimum and a maximum material injection rates. At least one injection well and at least one circulation well effective for generating an injection rate between the minimum and maximum injection rates are provided for injecting a solution of biodegradable materials into the coal seam. A chosen quantity of biodegradable materials is allowed to be digested, fermented and converted by microbial action within the coal seam. Methane gas is extracted through producing and injecting wells, although pumping will enhance gas recovery.
STATEMENT REGARDING FEDERAL RIGHTS
 This invention was made with government support under Contract No. RPSEA 07122-14 awarded by the Research Partnership to Secure Energy for America. The government has certain rights in the invention.