GEOLOGIC CONTROLS ON PRODUCED FLUID COMPOSITION IN COALBED METHANE RESERVOIRS OF THE BLACK WARRIOR BASIN, ALABAMA, USA

The cradle of the modern coalbed methane industry is the Black Warrior Basin of Alabama, where fluid production now exceeds 2.6 Tcf of gas and 1.6 Bstb of water. The coalbed gas industry in this area is dependent on in-stream disposal of co-produced water, which ranges from nearly potable sodium-bicarbonate water to hypersaline sodium-chloride water. This study employed diverse geological and geochemical methods to characterize water and gas chemistry in more than 200 wells, thereby providing a robust database that facilitates effective water management.

Results reveal strong interrelationships among regional geology, water chemistry, and gas chemistry. Coalbed methane is produced from multiple coal seams in Pennsylvanian-age strata of the Pottsville Coal Interval, in which water chemistry is influenced by a structurally controlled meteoric recharge area along the southeastern margin of the basin. The most important constituents of concern in the produced water include chlorides, ammonia compounds, and organic substances. Regional mapping and statistical analysis indicate that the concentrations of most ionic compounds, metallic substances, and nonmetallic substances correlate with total dissolved solids and chlorides.

Gas is effectively produced at pipeline quality, and the only significant impurity is N₂. Geochemical analysis indicates that the gas is of mixed thermogenic-biogenic origin. Stable isotopic analysis of produced gas and calcite vein fills indicates that widespread late-stage microbial methanogenesis occurred primarily along a CO₂ reduction metabolic pathway. Organic compounds in the produced water appear to have helped sustain microbial communities. Ammonia and ammonium levels increase with total dissolved solids content and appear to have played a role in late-stage microbial methanogenesis and the generation of N₂.