Hydrologic properties of coal-beds in the Powder River Basin, Montana. II. Aquifer test analysis

A multiple well aquifer test to determine anisotropic transmissivity was conducted on a coal-bed in the Powder River Basin, southeastern Montana, as part of a multidisciplinary investigation to determine hydrologic conditions of coal-beds in the area. For the test, three wells were drilled equidistant from and at different angles to a production well tapping the Flowers–Goodale coal seam, a 7.6-m thick seam confined at a depth of about 110m. The test was conducted by air-lift pumping for 9h, and water levels were monitored in the three observation wells using pressure transducers. Drawdown data collected early in the test were affected by interporosity flow between the coal fracture network and the matrix, but later data were suitable to determine aquifer anisotropy, as the slopes of the late-time semilog time-drawdown curves are nearly identical, and the zero-drawdown intercepts are different. The maximum transmissivity, trending N87°E, is 14.9m2/d, and the minimum transmissivity 6.8m2/d, giving an anisotropy ratio of 2.2:1. Combined specific storage of the fractures and matrix is 2×10−5/m, and of the fracture network alone 5×10−6/m. The principal direction of the anisotropy tensor is not aligned with the face cleats, but instead is aligned with another fracture set and with dominant east–west tectonic compression. Results of the test indicate that the Flowers–Goodale coal-bed is more permeable than many coals in the Powder River Basin, but the anisotropy ratio and specific storage are similar to those found for other coal-beds in the basin.