Mario García-González¹

(1) IER University of Wyoming - Universidad Industrial de Santander, Laramie, WY

ABSTRACT: Coal-bed Methane Potential of Shallow and Deep Reservoirs

Coal-bed methane reservoirs contain much more gas than conventional reservoirs. In conventional sandstone or limestone reservoirs, gas occurs either in a free or dissolved phase, but in coal gas exists almost exclusively in a condensed, liquid-like state because of physical sorption. Furthermore, coal has high microporosity with large internal surface areas and therefore adsorbs and retains large amount of gas.

CBM reservoirs occur at various depths, but most exploration is currently focused on depth less than 6,000 ft. Shallow CBM reservoirs (<3,000 ft) contain bacteriogenic gas. The generation of gas per ton of coal tends to be low (<50scft/ton), but the thickness and extension of some coal deposits have resulted in gigantic gas-in-place accumulations such as the Powder River Basin with a resource ranging from 24 to 34 Tcf. Deep CBM reservoirs (>3000 ft) contain methane generated by cracking of kerogen and oil in coal to gas. Generation of thermogenic gas increases exponentially as vitrinite reflectance (coal rank) increases, so these deep CBM reservoirs have greater potential than the shallow CBM reservoirs. Therefore deep CBM reservoir can be attractive exploration target because of its greatest generation potential and the lowest water saturation, however fracturing and hence permeability is the limiting factor. Deep coal beds are the source of large amounts of thermogenic gases that migrates up-dip originating overpressured CBM reservoirs in stratigraphic or structural traps.

Infiltration of meteoric water flows through the permeable coal beds controls gas migration, this water flow can creates gas accumulations at permeable barriers or flush the gases out to the surface. Also the infiltration of meteoric water starts the generation of bacterial gases in shallow coal beds. Intensity of coalbed fracturing determines it permeability for water infiltration, dewatering and gas production.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado