--> Abstract: Influence of Anomalous Zones of Low Gas Content on Regional Coalbed Methane Production Trends, Black Warrior Basin, Alabama, by J. R. Levine and P. G. Malone; #90937 (1998).

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Abstract: Influence of Anomalous Zones of Low Gas Content on Regional Coalbed Methane Production Trends, Black Warrior Basin, Alabama

LEVINE, J.R., and P.G. MALONE; GeoMet, Inc., Bessemer, AL

Summary

Gas production rates of “coalbed methane” (CBM) wells in the northern part of the Black Warrior basin, Alabama vary over five orders of magnitude from nil to more than 500,000 ft3 (STP) per day (14,000 m3/day). Such variations might conceivably be related to differences in: 1) permeability, 2) net coal thickness or 3) gas content; however net coal thickness varies at most by a factor of 2, and differences in permeability are subtle for the most part. Instead, regional variation in gas production is attributable mainly to differences in gas content. Non-geologic factors, such as drilling and completion method, produce no discernible systematic effect on production.

Gas contents vary both regionally and stratigraphically, ranging from virtually nil to >500 ft3 (STP)/ton (0 to 14 m3/ton). Well documented controls on gas content are: 1) present-day depth of burial and 2) coal rank. However, in certain parts of the Warrior basin there are anomalous zones of gas-depleted coals, which occur in linear belts, ranging from 0.5 to 2.0 miles (1-3 km) wide and up to 10 miles (16 km) or more (?) in length, oriented generally NE-SW. CBM wells located within these zones produce little or no gas but can produce large volumes of water, depending on permeability. At some locations, gas-depletion is evident only in the shallower seams, while deeper seams appear to be fully gas-saturated. At other locations, the entire coal-bearing sequence is gas depleted. The transition zone between low gas and high gas coals can be quite abrupt—occurring within 50 ft (15 m) stratigraphically or <0.6 mile (1 km) laterally.

Similar areas of low gas production occur in other parts of the Black Warrior basin, suggesting that low gas anomalies may exist there as well. Similar trends of gas depletion are probably present in other basins. Therefore, CBM development should proceed prudently in unknown areas, allowing also for the possible presence of low gas coals.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah