Evaluation of Gas-Bearing Coal Seams

Abstract

Summary This paper discusses test activities and results from four selected wells used to acquire coalbed methane data in the Illinois, Piceance, Arkoma, and Green River basins. Techniques used to evaluate the coalbed gas resource are well logging, conventional and sidewall coring, gas desorption of the coal cores, and drillstem testing. Experience gained by employing these techniques is presented. Introduction Methane recovery from coal beds is one of several unconventional gas resources currently under evaluation by the U.S. DOE and industry. The methane gas associated with both minable and unminable coal seams represents a sizable resource. Approximately 700 to 800 Tcf of gas is locked in these beds, of which an estimated 300 Tcf is believed to be contained in minable bituminous coal beds. No universally accepted estimate exists for the recoverable portion.An integral feature of the joint Methane Recovery From Coalbeds Project (MRCP) of DOE's Morgantown Energy Technology Center (METC) and industry is to construct a data base which will provide essential information on quantity and quality of coalbed methane, its distribution, and production characteristics. This can be accomplished by evaluating each coal bed encountered in the candidate wells using conventional reservoir assessment methods.Methane, the major component of natural gas, is generated during geologic formation of coal and frequently is trapped in the coal and associated strata. As might be expected, the distribution of existing coal resource data is influenced more by local economics than by the actual concentration of total coal deposits. The most accessible deposits have been mapped and defined in preference to the deeper, less economical minable and unminable coal seams. Fig. 1 outlines the bituminous and subbituminous coal fields located in the conterminous U.S. TRW Inc.'s Energy Systems Group, as integrating contractor for the MRCP, has prepared plans in conjunction with METC to examine 380,000 sq miles of coal-bearing rocks estimated to contain 700 Tcf of methane. The distribution of the gas content in the coals to be tested is unknown. The initial evaluation efforts have concentrated on 80,000 sq miles of this area where the probabilities of finding, producing, and using the methane are high. A cooperative drilling, coring, logging, and well testing program was initiated to extend the existing data base.The basins selected for immediate testing are listed in Table 1. The selections were guided by these criteria:physical and chemical characteristics of coali.e., fixed carbon, percent volatiles, percent sulfur, etc. (higher-rank coals generally contain more methane),bed depth: east greater than/equal to 1400 ft, west greater than/equal to 700 to 800 ft (deeper coals are more likely to have retained the methane),total effective coal thickness: east greater than/equal to 20 ft for high rank, west greater than/equal to 30 ft (higher total production per well possible on basis of multibed completion),individual seam thickness: minimum of 5 to 10 ft for shallow beds, with multiple coal beds considered as one if separated by permeable rock units (minimize need for multiple fracturing), andareal extent: continguous basin (allow extrapolation of results to a wider area). Based upon the criteria, portions of major basins were selected as the targets for initial resource delineation activities. A total of 80 wells will be evaluated for coalbed methane content; 32 wells have been drilled and tested as of Aug. 1, 1980. JPT P. 195^