The Source of Longwall Gob Gas and an Analysis of Factors that Influence its Migration and Production

DIAMOND, WILLIAM P., U.S. Bureau of Mines, Pittsburgh, PA

Gas content data collected from four core holes drilled at various times in the mining cycle of a longwall operating in the lower Kittanning coal bed, Pennsylvania, provides the data necessary to define the source and migration characteristics of gob gas at this site. Analysis of the data indicates that 91% of the gas produced by the mine's ventilation system and gob gas vent holes originated from the various coal beds overlying the bed being mined. Coal beds as much as 200 ft above the mined bed were found to have contributed gas to the gob. Material balance calculations indicate that 60% of the produced gas migrated laterally to the gob from beyond the strata directly overlying the panel. Most of this gas probably originated from an unmined area downdip of the mined panel. The migra ion of gas may have been aided by a long-term pressure gradient resulting from the presence of extensive old workings updip of the longwall panel, which may have acted as a pressure sink. Production rates of individual gob gas vent holes were found to vary considerably, and were observed to be influenced by several factors, including completion practices, mining rate, and position on the panel. Peak production rates ranged from approximately 3.9 to 0.7 MMCFD. The highest producing and longest lasting gob gas vent holes generally were located at the ends of the panel. Also, it appears that an increase in mining rate generally corresponded to an increase in gob gas vent hole production.

AAPG Search and Discovery Article #91005 © 1991 Eastern Section Meeting, Pittsburgh, Pennsylvania, September 8-10, 1991 (2009)