HR: 15:25h
AN: H23I-08 INVITED     [Abstracts]
TI: Coal Bed Aquifer Tests: a Case Study
AU: * Weeks, E P
EM: epweeks@usgs.gov
AF: U. S. Geological Survey, Box 25046, MS 413, Lakewood, CO 80225 United States
AB: Coal bed methane development is proceeding at a rapid pace in the USA and in several other countries. Development of coal bed methane requires the simultaneous co-production of water in a manner that maximizes the amount of drawdown while minimizing the amount of water pumped. Determination of optimal well spacing and production rates to achieve such drawdowns requires knowledge of the hydraulic properties of the coal aquifer. Natural closely spaced fractures, termed cleats, develop during coal formation as an orthogonal fracture network that creates anisotropic transmissivity. Water held in the matrix porosity of the coal is released slowly to the cleat system during pumping, resulting in coal beds behaving as dual-porosity aquifers. Knowledge of the magnitude and orientation of the principal axes of the transmissivity tensor, as well as of the late-time dual-porosity storage coefficient, are needed to optimally design well fields for the exploitation of coal bed methane. An aquifer test with three observation wells was conducted to determine these properties for a 7.6- m thick coal bed located in the Powder River Basin, southeast Montana. The test results exhibit all the features that would be expected for a test on an anisotropic dual-porosity medium. However, the test was initially misinterpreted, providing a cautionary tale. The initial interpretation assumed a single-porosity aquifer, and the late-time break in slope was assumed to represent the effects of a hidden boundary. Despite their apparent plausibility, the results of the analysis raised several red flags. An attempt to determine the location of the hidden boundary failed, the indicated specific storage was implausibly small, and the analysis of recovery data provided transmissivity values that were in disagreement with those determined from the drawdown analysis. Reanalysis of the test using type curves developed for a dual-porosity aquifer resulted in a transmissivity value that is about 25% smaller and a storage coefficient about 4 times larger (and of more plausible magnitude), than was determined from the single-porosity aquifer analysis. Values for the anisotropy ratio and orientation of the major transmissivity axis are nearly identical for the two analyses, however. A post-mortem analysis of the recovery data indicates that a reliable value for transmissivity will be obtained only if very late recovery data are available. In summary, multiple-well aquifer tests on coal aquifers can provide very useful information for the design of coal-bed methane well fields. However, as for all aquifer tests, misinterpretation will result if all aspects of the prevailing hydrogeologic conditions are not fully taken into account.
DE: 1028 Composition of meteorites (3662, 6240)
DE: 1859 Rocks: physical properties
SC: Hydrology [H]
MN: Fall Meeting 2005