Gas Production Potential from Gas-Bearing Strata Above Longwall Mining of United States Coal Beds

A. W. Layne, H. J. Siriwardane, C. W. Byrer

Pipeline-quality gas production potential from the highly fractured methane-bearing strata above longwall-mine coal 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 presentation assesses the potential production and influence of well spacing for optimum drainage of gas from the strata above coal mine panels. Potential gas reserves were quantified by use of the predicted extent of the overburden failure zone, the formation gas contents, and geologic cross sections from the United States coal basins having longwall mining operations. A finite element method was used to determine the size and shape of the af ected stratigraphic zones above mining operations.

A two-phase reservoir model was used to compute increased production rates from methane wells. Production data from actual wells above a longwall coal 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 esign of gas wells in highly fractured, multistrata reservoirs and prediction of long-term gas production from such reservoirs.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.