Assessment of Gas Production Potential from Strata above Longwall Mining

Abbie W. Layne, Hema J. Siriwardane, Charles W. Byrer

Pipeline-quality gas production potential from the highly fractured methane-bearing strata above longwall coal mine panels may be many times that of the gas-in-place estimate for the minable coal bed. The permeability in these strata is increased significantly due to the fractured zone generated during longwall mining activity. This paper assesses the potential production and influence of well spacing for drainage of gas from the strata above coal mine panels. Reserves were quantified by use of the predicted extent of the failure zone, the formation gas contents, and geologic cross sections from throughout the Appalachian basin. A finite element method was used to determine the size and shape of the affected stratigraphic areas and gas contents were derived.

A two-phase reservoir model was used to compute increased production rates from methane wells. Production data from actual wells above a longwall panel in the Appalachian basin were matched historically to characterize flow mechanisms and reservoir properties, such as permeability. These properties were then evaluated in parametric studies to determine the influence of well location on gas production. Results indicate that cumulative gas production can be increased by placing wells farther than 1,000 ft apart. The model appears to make reasonable predictions of the size of the multireservoir area in complex geologic conditions. The study considered the influence of an advancing mine face on gas production. Thus, the methodology presented in this paper is applicable to the optimum desi n of gas wells in highly fractured reservoirs and prediction of long-term gas production.

AAPG Search and Discovery Article #91031©1988 AAPG Eastern Section, Charleston, West Virginia, 13-16 September 1988.