HR: 17:45h
AN: H32G-08    [PDF]
TI: Transmissivity Anisotropy in a Bounded Coal-Bed Aquifer in the Powder River Basin, Southeastern Montana Determined by Aquifer Test Analysis
AU: * Weeks, E P
EM: epweeks@usgs.gov
AF: US Geological Survey, Box 25046, MS 413, Lakewood, CO 80225
AB: Extensive installation of well fields to develop coal-bed methane in the Powder River Basin has resulted in large environmental impacts due to discharge of the co-produced water to drainage channels and reservoirs, and to surface disruptions caused by construction of drill pads and gas and water pipelines. These impacts can be reduced by optimizing well spacing based on knowledge of hydraulic properties and boundaries of the coal bed aquifers, including the commonly observed transmissivity anisotropy oriented with face cleat structure. A multiple-well aquifer test was conducted of a 7.6-m thick coal bed located in southeast Montana to determine the magnitude and orientation of the transmissivity tensor, the storage coefficient, and possible aquifer boundaries. Results of the test appear ideal for determining aquifer anisotropy, as the slopes of the semilog time-drawdown curves are nearly identical and the zero-drawdown intercepts are significantly different. The maximum transmissivity, trending N88$\deg$ E, is 20 m$^{2}$/d and the minimum transmissivity 9 m$^{2}$/d, defining an anisotropy ratio of 2.1. The effective or geometric mean transmissivity, the value that would be determined from a single-well test, is 13 m$^{2}$/d. The corresponding value of hydraulic conductivity is about 2 m/d, which is somewhat larger than most published values for Powder River Basin coal-bed aquifers. Specific storage of the unit is 4X10$^{-6}$/m, which is smaller than most published values for Powder River Basin coal beds. The principal anisotropy tensor direction is not aligned with the face cleats, which is dominantly oriented N33$\deg$ E, as determined from geophysical well logs, but instead is oriented with another fracture set and with dominant east-west tectonic compression. The aquifer test results also indicate the presence of an impermeable or low-permeability boundary. Image well theory, adapted through scale transformation for application to an anisotropic aquifer, indicates that the boundary is likely located about 400 m to the northeast of the production well, and has a strike to the ENE. Results of this test demonstrate that multiple-well aquifer tests can provide hydrologic information not otherwise available to implement optimum spacing and orientation of coal bed methane development wells.
DE: 1829 Groundwater hydrology
SC: Hydrology [H]
MN: 2003 Fall Meeting