HR: 17:30h
AN: H32G-07    [PDF]
TI: Characterizing Hydrologic Properties of Coal Beds From the Powder River Basin, Southeastern Montana, by Analysis of Geophysical Well Logs
AU: * Morin, R H
EM: rhmorin@usgs.gov
AF: U.S. Geological Survey, Mail Stop 403, Federal Center, Denver, CO 80225 United States
AB: As part of a study designed to evaluate the potential for coal-bed methane development in the Powder River Basin of southeastern Montana, six wells were drilled through Paleocene coal beds of the Fort Union Formation along a 31-km transect within the Tongue River drainage basin. Sets of geophysical logs were recorded in these wells, cores were collected, and a complementary aquifer test was conducted at one site across an isolated 4.6-m thick coal seam. Natural gamma and electrical resistivity logs were highly effective in distinguishing individual coal beds. Sonic velocities computed from full-waveform logs were combined with rock densities determined from cores to estimate aquifer storage and results are in reasonable agreement with the value derived from the aquifer test. Televiewer logs, both acoustic and optical, provided magnetically oriented images of the borehole walls. Inspection of these digital projections and comparison with coal cores infer a face cleat orientation of approximately N$33\deg$E, in close alignment with regional lineament patterns and the northeast trend of the nearby Tongue River. Further analysis of these televiewer data reveals two primary sets of shallow-dipping features, presumably bedding planes, that are interspersed throughout the rocks and that strike east-west and north-south. These orthogonal directions are aligned with the principal axes of the contemporary stress field in this physiographic province as inferred from a nearby 1984 earthquake. The aquifer test consisted of pumping one of these wells for nine hours and monitoring water levels in three nearby observation wells. The drawdown ellipse generated during this test delineates a horizontal anisotropy ratio of 2.1, with the principal transmissivity tensor aligned N$88\deg$E and oblique to cleat orientation. Thus, interpretation of this aquifer test within the context of local structural and tectonic conditions indicates that transmissivity anisotropy in coals at this site is controlled by bedding configuration and perhaps a mechanical response to contemporary stresses rather than solely by cleat geometry.
DE: 1829 Groundwater hydrology
DE: 5104 Fracture and flow
DE: 5194 Instruments and techniques
DE: 8020 Mechanics
SC: Hydrology [H]
MN: 2003 Fall Meeting