HR: 0800h
AN: H21E-1077    [Abstracts]
TI: Pressure and Displacement Responses During Slug Tests in Deformable Fractures
AU: * Svenson, E
EM: esvenso@clemson.edu
AF: Clemson University, 340 Brackett Hall, Clemson, SC 29634 United States
AU: Schweisinger, T
EM: tschwei@clemson.edu
AF: Clemson University, 340 Brackett Hall, Clemson, SC 29634 United States
AU: Murdoch, L
EM: lmurdoc@clemson.edu
AF: Clemson University, 340 Brackett Hall, Clemson, SC 29634 United States
AB: Slug tests are a common method of estimating the transmissivity of aquifers, but limited work has been done on how to interpret these tests in fractured media. We have conducted suites of air-slug tests using straddle packers and a highly sensitive extensometer to evaluate the distribution of aquifer properties in fractured biotite gneiss at a site in Clemson, SC. Pressure was measured as a function of time, and the extensometer allowed us to also measure transient changes in fracture aperture during the tests. Results show pressure signals similar to that predicted by the Cooper, Bredehoeft, Papadapolous (1967) solution, but the pressure drops slightly faster than predicted by that solution during about half of the tests. Moreover, the slug-out pressure responses commonly differ from the slug-in responses at the same depth. Fracture apertures open by 1 to 20 microns during slug-in tests, depending on the applied air pressure, and the maximum aperture lags behind the maximum pressure by up to several tens of seconds. The field results were modeled using a hydrid-finite difference numerical code designed to simulate coupled flow and deformation of a single, idealized fracture. The model predicts the field data remarkably well. The compiled results of 25 slug tests along the length of a single well show that there are three conductive intervals (K ­™ 10-3 cm/s) separated by relatively low conductivity material (K<10-5 cm/s). Hydraulic conductivity estimates for slug-in tests are as much as 1.25 times greater than estimates from slug-out tests at the same depth. These differences are consistent with pressure-induced aperture changes at the wellbore predicted by the model. Transient changes in fracture aperture measured with the extensometer can be used to improve the estimate of specific storage during slug tests. Pressure and displacement data were inverted to estimate the locations of permeable zones intersecting the fractures. These results indicate that steeply dipping fractures occur within several meters of the borehole at most depths.
DE: 1829 Groundwater hydrology
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting