Coalbed Methane Production Potential in U.S. Basins

Abstract

Summary The major emphasis of the U.S. DOE's coalbed methane research has been on estimating the magnitude of the resource and developing systems for recovery. Methane resource estimates for 16 basins show that the greatest potential is in the Piceance, Northern Appalachian, Central Appalachian, potential is in the Piceance, Northern Appalachian, Central Appalachian, Powder River, and Greater Green River coal basins. Small, high-potential Powder River, and Greater Green River coal basins. Small, high-potential target areas have been selected for in-depth analysis of the resource. Industry interest is greatest in the Warrior, San Juan, Piceance, Raton Mesa, and Northern and Central Appalachian basins. Production curves for several coalbed methane wells in these basins are included. Introduction The DOE has integrated all the available geologic and coal resource data acquired over the past 8 years to determine the stratigraphic units and geographic areas where coalbed methane production potential is classified as favorable. Sixteen basins have been studied (Fig. 1). The integration of these data has removed the uncertainty about what production potential exists and where the favorable production potential exists and where the favorable geologic trends are located in the basins. Table 1 gives a summary of the resource estimates for the basins and target areas for further exploration and/or development. Known coalbed methane production activity is also presented in six of these basins with examples of well production curves. Background Coal resources in the U.S. are widespread and abundant, with various types of coals underlying about 13%, or 380,000 sq miles [984 200 km2], of the land area within the contiguous U.S. Coals are present in 37 states, with coal-bearing areas representing a substantial percentage of the total area in many of these states. Coals vary in thickness from less than 1 to >200 ft f less than 0.3 to >61 m] and in depth from near surface to >10,000 ft [ >3050 m]. By today's mining standards, nearly 90% of all coals in the U.S. are considered unminable. Methane is present in nearly all U.S. coalbeds as natural gas formed during the coal formation process and is absorbed in the structure of the coal. Higher-ranking coals (bituminous and anthracite) may have from 200 to 500 ft3 methane/ton 16.2 to 15.6 m3/Mg) of coal, whereas the lower-rank coals (subbituminous and lignite) may contain less than 200 ft3 /ton [ less than 6.2 M 3 /Mg]. Gas content usually increases significantly with depth. Sand formations adjacent to coal seams and fractures where methane has accumulated by desorption may serve as reservoirs for additional quantities of released gas. Most of the available coalbed methane data are from mining areas in the eastern U.S. where the coalbeds are well defined and mining is extensive. Estimates of coalbed methane in the western U.S. are less accurate because the distribution of the coal and methane content has not been determined in many beds more than 3,000 ft [914 m] deep. The eastern coals are mostly bituminous or higher in rank, and western coals are generally of subbituminous to bituminous rank. Since 1978, the DOE has supported the assessment of gas potential in coalbeds throughout the U.S. and the recovery and use of methane associated with mining operations. Recent efforts have focused on the use of coalbed methane for regional economic gas self-sufficiency, energy parks, self-help, and cogeneration or small-power potential. The following major resource data have been acquired: coal rank, specific gas content, reservoir properties, coal thickness, overburden characteristics, well test data, and pre- and poststimulation data. This information provides insight into the volume of methane in place and allows for prediction of the potential of the various coalbed reservoirs. Existing geologic potential of the various coalbed reservoirs. Existing geologic data and limited reservoir property data in many of the basins permit the development of a rational approach for identifying areas with a high potential for the production of coalbed methane. Current Industry Activity The current level of activity and interest in coalbed methane development is focused on six basins. These include three eastern basins with shallow coals (Northern Appalachian, Central Appalachian, and Warrior) and three western basins with deep coals (San Juan, Raton Mesa, and Piceance) (Fig. 2). The remaining basins listed here have areas designated for high-potential coalbed methane development. The Appalachian basin is divided into three coal basins: the Northern, Central, and Warrior (southern). U.S. coalbed methane development began as early as the 1930's in the Northern Appalachian coal basin in the Pittsburgh coal seam. Production activity currently is highest in the Warrior basin in Alabama. Interest in development of coalbed methane reserves continues for the Northern and Central Appalachian basins. Most of the coal in the Appalachian basin is Mississippian and Pennsylvanian in age and is located at <2,000 ft [<610 m]. JPT P. 821