Hydrogeologic Aspects of Coal-Bed Methane Occurrence, Powder River Basin

VAN VOAST, WAYNE A., Montana Bureau of Mines and Geology, Montana College of Mineral Science and Technology, Butte, MT

Fort Union coal in the Powder River basin has undergone extensive hydrologic study since 1972. Hydraulic-conductivity (permeability)

values from 66 field tests range from 8 md to 30 darcys; mean permeability is 400 md. Permeability is highly anisotropic, being far greatest parallel to the face cleat. Face cleat orientation in Wasatch coal near Gillette, Wyoming, has been found to be generally normal to the regional dip; if reliable elsewhere, this relationship could be valuable for gas exploration and production.

During groundwater studies in the basin, methane has been only occasionally encountered. Where present, it invariably coincides with conditions of chemically reduced groundwater containing Na and HCO3 as the principal ions. A chemical model accounting for methane and the associated water-quality conditions requires the exchange of cations Ca and Mg (to clays) for Na (to water), followed by: 4CH2O (coal) + SO4 + H2O - H2S + 3HCO3 + CH4. Exchange, reduction, and methane generation occur along groundwater flow paths, so methane would not be expected, nor has it been evident, near recharge areas. At two downgradient points along regional flow paths, the West Decker mine and a research well 25 mi to the east, energy values in gas from coal were 950 and 982 BTU/ft3, respectively, with metha e contents of 94 and 97%. Only a trace of heavier gas (C2) was present.

Mining near Decker has created a potentiometric depression over an area 5 mi wide and 15 mi long. Whether the greatly reduced hydrostatic pressures have enhanced or diminished potential for commercial gas production has not yet been shown.

AAPG Search and Discovery Article #91010©1991 AAPG Rocky Mountain Section Meeting, Billings, Montana, July 28-31, 1991 (2009)