CHARACTERIZING HETEROGENEITY IN A FAULTED AQUIFER USING PUMP TESTS AND WESTBAY MULTILEVEL MONITORING WELLS

We present preliminary results of a study using pump tests and multiple Westbay multilevel monitoring wells to characterize the permeability field in a 15m x 30m x 70m volume of a faulted, heterogeneous, siliciclastic aquifer in central Texas. Emphasis is on a normal fault zone within the study volume that partially offsets the aquifer and impedes cross-fault flow. Prior work provides a high resolution geological model of the site based on 1050 m of continuous core and geophysical logs from 11 boreholes (3-10 m spacing). Eight boreholes have Westbay multilevel monitoring systems with a total of 94 measurement zones. Each of multilevel wells (MLW) has between 8 to 15 measurement zones with lengths varying from 0.5 to 7 m. Hydraulic packers of 1.1 m length separate measurement zones. Packers are positioned at all significant fault piercements and laterally extensive, clay-rich strata, in order to reestablish natural flow conditions within hydrostratigraphic intervals.

Pump tests utilized two, uncased, fully penetrating irrigation wells (150 and 200 gpm pumping rates) that straddle the fault and are in close proximity (7 to 65 m) to the MLW. Thirty MOSDAX pressure transducers (resolution 0.008 psi) were available for the tests, hence three pump tests using three different transducer configurations were necessary to construct a complete composite test for each well. Pumping and recovery transient pressure histories were data logged using a sampling rate ramped from 2 to 300 seconds. Pumping intervals ranged from 8 to 30 hr with recovery monitored for at least 3x of the pumping interval.

Drawdown and recovery histories exhibit significant variability from zone to zone in a single MLW as well as between adjacent MLW and clearly demonstrate the power of using multilevel monitoring wells. The variabilities qualitatively correlate with known local stratigraphic features and faults. The spatial pattern of drawdown reflects the geological heterogeneities as well as the vertical variation of stratigraphy. The drawdown pattern also shows the effect of the large seepage face that develops in the open pumping well. Analysis of the data using standard well analyses and numerical inverse FE models are on-going.