--> Abstract: Origin and Consequences of Variable Gas Saturation in Coalbed Methane Reservoirs of the Black Warrior Basin, by Jack Pashin; #90078 (2008)

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Origin and Consequences of Variable Gas Saturation in Coalbed Methane Reservoirs of the Black Warrior Basin

Jack Pashin
Geological Survey of Alabama, Tuscaloosa, AL

Gas saturation is highly variable in coalbed methane reservoirs of the Black Warrior basin, and a diversity of geologic data provides insight into the origin of this variability and the consequences for production performance. Coalbed methane is produced from numerous thin (0.3 to 3 m) coal seams that are distributed through 700 to 1200 m of stratigraphic section. Gas content generally increases with depth, and gas saturation typically varies between 40 and 95 percent of Langmuir volume, even among closely spaced seams in the same well.

Exploration has been successful in coal seams that are in the thermogenic gas window. Isotopic analysis, however, demonstrates that produced methane is depleted in 13C. Analysis of gases and fracture-filling carbonate cements indicates mixing of thermogenic gases with late-stage biogenic gases that formed along a metabolic pathway associated with CO2 reduction. Bacterial methanogenesis was most intense in areas affected by fresh-water recharge and appears to be a key factor supporting locally high gas saturation.

Adsorption isotherms of coal samples in the Black Warrior Basin are steep below 5 MPa and flatten at higher pressure. In deep, normally pressured coal beds, even a small degree of undersaturation can necessitate prolonged dewatering before a large reservoir volume can reach the critical desorption pressure. Where reservoir pressure is relatively low, however, isotherm geometry indicates that significantly undersaturated reservoirs can sit close to the critical desorption pressure. Consequently, production maps and decline curves indicate that areas of underpressure favor high gas production in concert with minimal water production.

 

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas