Abstract Pipeline quality gas production from the highly fractured methane-bearing strata above long wall coal mine panels is predicted to be many times that of the gas in-place estimate for the minable coalbed. This production is thought to come from relatively shallow gas-bearing coals, organic shales, and sandstones. In the mining process, permeability of shallower strata is increased significantly because of the fractured zone generated during long wall mining activity and, hence, gas could be produced at volumes many times that of an average conventional gas well. This paper assesses the potential production and the influence of well spacing on the drainage of gas from the strata above mining operations in southern Appalachia. In this assessment, gas reserves were quantified for a typical failure zone, formation gas content, and geologic cross section present in the Appalachian Basin. A finite element method was used to determine the size and shape of the affected stratigraphic areas for which gas contents were estimated (based on prior knowledge). A two-phase reservoir model was used to compute increased production rates from methane wells that penetrate the multiple strata and coalbed. Production data from actual wells above a longwall panel were history matched to characterize flow mechanisms and reservoir properties. 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 apart than 1,000 ft (304.8 m). The model appears to make reasonable predictions of the size of the multi-reservoir 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 location of gas wells in highly fractured reservoirs to optimize long-term gas production.
Introduction Coal and coalbed methane are abundant, complementary, fossil energy resources located in many of the regions in the continental United States with various types of coals underlying about 13 percent or 380,000 square miles (984,195 km 2) in 37 states. Mineable underground coal reserves in 25 of these states are enormous (290 × 109 tons; 236 Pg).1 Estimates of the coalbed methane resource in the United States range from 400 trillion cubic feet (Tcf) to 850 Tcf, of gas in these coal basins. The present paper involves the minable coals of the Appalachian Basin (northern, central, and southern regions) which encompasses portions of eight states. Coalbed methane estimates combined for the northern and central Appalachian Basin are nearly 109 Tcf (3.087 Tm 2) of gas and the estimate for the southern portion (Warrior) is nearly 20 Tcf (.566 Tm3). Methane production from gob wells at longwall mines can be a significant contribution to the nation's energy supply. One mining operation with 15 gob wells recently produced about 4 billion cubic feet (113.27 Mm of pipeline-quality s in just a 2-year period. The gob-gas quality ranges from 90- to 95-percent methane, and the level of production is much greater than that expected from conventional wells. Both the coal and the methane in the coal represent a tremendous dual-energy resource in the eastern United States.
During the last decade, the coal industry has undergone many changes. Mining has become more efficient and there have been periodic economic adjustments because of fluctuating marketing conditions. Today's longwall mining technology from coal panels (average length of 5,500 feet; width of 900 feet) produces coal much more efficiently than conventional roomand-pillar mining. One of the major changes in mining technology with a longwall operation is the wider mine face.
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