HR: 0830h
AN: H21D-0860    [PDF]
TI: Finite Element Modeling of Transient Head Field Associated with Partially Penetrating, Slug Tests in a Heterogeneous Aquifer with Low Permeability, Stratigraphic Zones and Faults
AU: * Cheng, J
EM: chengj@geo.tamu.edu
AF: Department of Geology \& Geophysics, Texas A\&M University, MS 3115, College Station, TX 77843-3115 United States
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
AU: Everett, M
EM: everett@geo.tamu.edu
AF: Department of Geology \& Geophysics, Texas A\&M University, MS 3115, College Station, TX 77843-3115 United States
AB: Preliminary field work shows slug interference tests using an array of multilevel active and monitoring wells have potential of permitting enhanced aquifer characterization. Analysis of these test data, however, ultimately will rely on numerical geophysical inverse models. In order to gain insight as well as to provide synthetic data sets, we use a 3-D finite element analysis (code:FEHM-LANL) to explore the effect of idealized, low permeability, stratigraphical and structural (faults) heterogeneities on the transient head field associated with a slug test in a packer-isolated interval of an open borehole. The borehole and packers are modeled explicitly; wellbore storage is selected to match values of field tests. The homogeneous model exhibits excellent agreement with that of the semi-analytical model of Liu and Butler (1995). Models are axisymmetric with a centrally located slugged interval within a homogenous, isotropic, confined aquifer with embedded, horizontal or vertical zones of lower permeability that represent low permeability strata or faults, respectively. Either one or two horizontal layers are located opposite the borehole packers, which is a common situation at the field site; layer thickness (0.15-0.75 m), permeability contrast (up to 4 orders of magnitude contrast) and lateral continuity of layers are varied between models. The effect of a "hole" in a layer also is assessed. Fault models explore effects of thickness (0.05-0.75 m) and permeability contrast as well as additional effects associated with the offset of low permeability strata. Results of models are represented most clearly by contour maps of time of arrival and normalized amplitude of peak head perturbation, but transient head histories at selected locations provide additional insight. Synthesis of the models is on-going but a few points can be made at present. Spatial patterns are distinctive and allow easy discrimination between stratigraphic and structural impedance features. Time delays and amplitude reduction increase nonlinearly with increasing permeability contrast. The capacity to discriminate the effect of layer thickness decreases as permeability contrast increases.
DE: 1829 Groundwater hydrology
SC: Hydrology [H]
MN: 2003 Fall Meeting