Microbially Enhanced Coalbed Methane: Limitations and Possible Benefits

Andrew R. Scott, W. R. Kaiser

Biogenic gases represent a significant fraction of the world's coalbed methane resources. Microbially Enhanced Coalbed Methane (MECoM) would imitate and enhance the natural processes of secondary biogenic gas generation through the introduction of nutrients and/or anaerobic bacterial consortia into coalbed methane wells to stimulate production. The purpose of this presentation is to conceptually outline the limitations and possible benefits of MECoM and to encourage additional MECoM research. MECoM has the potential to stimulate coalbed production by generating additional methane, removing pore-plugging waxes, and increasing permeability. MECoM conversion of only one-hundredth of one percent (1/10,000) of U.S. coal resources into methane would increase gas reserves by 23 cf.

Only limited research has been performed to determine if in situ stimulation of coalbed methane production by bacterial consortia is possible and if the process is economically viable. Available cleat surface area and reaction rates may be limiting factors in MECoM. Bacteria are smaller than many cleat apertures, ranging in size from less than 0.3 to several microns in diameter. Preliminary calculations suggest that gas content could increase by up to 100 scf/ton or more. Unlike organic solvents, bacteria may also remove troublesome coalbed waxes from production strings without damaging the reservoir. Moreover: removal of sorbed organic compounds may increase available gas sorption sites, allowing more gas to be sorbed to the coal.

Bacteria may also have the potential to increase the permeability of low-permeability coals by enlarging cleat apertures. The permeability increase would be more pronounced in low-permeability coal beds than high-permeability ones. For example, assuming a cleat spacing of 0.423 cm, permeability would increase from 0.16 to 4.31 md if 2 microns of coal were removed from either side of the cleats; gas content would be increased up to 42 scf/ton. In addition to microbiological considerations, a detailed understanding of coalbed geometry, basin hydrodynamics, and coal chemistry is critical for successful implementation of MECoM.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995