HYDRODYNAMICS OF COALBED METHANE RESERVOIRS IN THE BLACK WARRIOR BASIN: KEY TO UNDERSTANDING RESERVOIR PERFORMANCE AND ENVIRONMENTAL ISSUES

Coalbed methane has been produced commercially in the Black Warrior basin of Alabama since 1980, and the basin remains among the world's most prolific, with 4,180 wells producing 3.4 Bcm of gas from the Pottsville Formation (Pennsylvanian) in 2004. Coal is a continuous-type unconventional gas reservoir in which coal is both the source rock and the reservoir rock. Adsorption is the dominant mechanism of gas storage, and gas is produced by reducing hydrostatic pressure through water production. Accordingly, coalbed methane is a hydrologic gas play, and basin hydrodynamics influence reservoir performance and play a central role in environmental issues.

Gas in the Pottsville Formation is produced from 5 to 25 coal seams at depths ranging from 150 to 1,200 m. Reservoir coal beds are exposed at the surface in a steeply dipping fold limb that marks the southeast margin of the Black Warrior basin. This upturned basin margin is a site of meteoric recharge that dominates the basin hydrodynamics. Fresh-water plumes containing bicarbonate waters with low TDS extend into the interior of the basin from the upturned basin margin. Northwest of the plumes, Pottsville coal beds are dominated by chloride waters with moderate to high TDS. Carbon isotope data indicate that fresh water facilitates late-stage bacterial methanogenesis and high gas content. Near the recharge zone, virgin reservoir pressures tend to be normal, whereas distal to the recharge zone, reservoirs can be extremely underpressured.

Water produced from the fresh-water plumes can be disposed of safely at the surface, whereas underground injection is used locally to dispose of highly saline water. Wells in areas with normal hydrostatic reservoir pressure tend to produce large volumes of water and can take up to four years to reach peak gas production. By contrast, wells drilled in underpressured areas distal to the fresh-water plumes typically produce little water and achieve peak gas rates during the first year of production. Environmental concerns have focused on the confinement of deep reservoir coal beds and shallow aquifers, and a broad range of hydrogeologic data demonstrates that bed-parallel flow in coal is predominant and that cross-formational flow is limited by thick intervals of marine shale.