HR: 0830h
AN: H21D-0861    [PDF]
TI: Multilevel, Multiwell Slug Tests as a Means to Determine Hydraulic Properties of Aquifer Heterogeneities: Preliminary Tests at a Geologically, Well-Constrained Test Site
AU: * Johnson, B
EM: johnson@geo.tamu.edu
AF: Department of Geology \& Geophysics, Texas A\&M University, MS 3115, College Station, TX 77843-3115 United States
AB: Efforts are on-going to develop a high resolution, aquifer characterization procedure based upon crosswell hydraulic tomography using multiwell (interference) slug test data from an array of multilevel wells with pressure measuring ports and flow ports. This poster presents preliminary results of a series of slug tests in a multilevel well network in a faulted, siliciclastic aquifer/aquitard system in central Texas. Emphasis is on presenting the testing methodology and showing how known, low permeability, stratigraphic and structural (faults) heterogeneities are manifest within measured transient pressure histories and their spatial patterns. The aquifer is a sandstone-dominated deposit with a few discontinuous mudstone interbeds that grades upward into interbedded sandstone and mudstone strata representing the confining aquitard. At the test site a 20 m displacement normal fault partially offsets the aquifer/aquitard system and impedes cross-fault flow. Prior work provides a highly constrained geological model of the site. Mulitlevel monitoring systems in eight boreholes provide 94 hydraulically isolated, pressure measurement zones distributed in a region 40 m by 20 m by 70 m encompassing the fault and adjacent aquifer/aquitard strata. In addition, 25 zones have "injection/withdrawal" ports, which can be used to perform the localized, slug tests. Both falling-head and rising-head slug tests are performed, using initial slug magnitudes of 20 to 40 m. Using pressure transducers with 20 Pa (0.003 psi) resolution, a test series for a single slugged interval typically provides usable pressure histories for 30 to 40 zones located from 3 m to 27 m away. Normalized peak amplitudes of head changes range from 0.1 to 0.0003. The effects of low permeability strata and faults are most easily seen in the spatial patterns of the arrival time and amplitude of the maximum head change. Examples are presented to demonstrate these effects. Having multiple zones within and across a flow impedance structure appears critical to inferring properties of the zone. Somewhat unexpectedly, in addition to the later arriving, diffusive pressure wave response, a prominent, early-time, poroelastic response is measured in selected zones of low permeability.
DE: 1829 Groundwater hydrology
SC: Hydrology [H]
MN: 2003 Fall Meeting