Kansas Geological Survey, Open-file Report 98-20
Part of the Well Tests for Site Characterization Project
|James J. Butler, Jr., John M. Healey
Kansas Geological Survey
1930 Constant Avenue, Campus West
University of Kansas
Lawrence, KS 66047
Vitaly A. Zlotnik and Brian R. Zurbuchen
Department of Geosciences
University of Nebraska at Lincoln
Lincoln, NE 68588
KGS Open-File Report 98-20
Prepared for presentation at
The American Geophysical Union
Spring Meeting in Boston, Massachusetts
May 29, 1998
The dipole flow test (DFT) is a single-well hydraulic test for characterizing variations in hydraulic conductivity along the screened (open) interval of a well. This approach, which was first proposed by Kabala (1993), involves use of the three-packer tool depicted in Figure 1. A pump in the central pipe of the middle packer transfers water from the upper chamber to the lower one, setting up a recirculatory pattern in the adjacent formation. Zlotnik and Ledder (1994, 1996) and Zlotnik and Zurbuchen (1998) developed theory, equipment, and field methodology for the DFT. They found that the radial component of hydraulic conductivity (Kr) can be estimated from the total head change () in the upper and lower chambers at steady state using the following approximate formula:
Figure 1. Schematic of the dipole flow test.
The vertical variation in Kr along the well screen can be estimated through a series of dipole flow tests between which the tool is moved a short distance along the well screen. Figure 2 is an example of Kr profiles that were produced by DFT surveys. As Zlotnik and Zurbuchen (1998) emphasize, the DFT has several important advantages for field applications. These include 1) no water is added or removed from the well during a test program, 2) the scale of the region influenced by the test can be readily defined and controlled, 3) Kr estimates can be obtained using a simple formula (eqn. (1)) that does not require optimization methodology and which is applicable adjacent to and at a distance from boundaries in the vertical plane, and 4) relatively low flow rates can be used in high K media so that the well losses associated with other methods are avoided.
Figure 2. Dipole depth versus hydraulic conductivity--10/97 dipole flow tests at Gems4N and Gems4s. A larger version of this figure is available.
The purpose of this presentation is to discuss an intensive program of field testing directed at assessing the practical utility of the DFT. The research site at which this work was performed will first be described, after which a series of practical considerations for the DFT will be discussed.
Figure 3. Location map for Geohydrologic Experimental and Monitoring Site (GEMS). A larger version of this figure is available.
Figure 4. GEMS stratigraphy and natural gamma logs. A larger version of this figure is available.
The dipole flow tests were performed with a tool (Fig. 1) designed and fabricated at the University of Nebraska (Zlotnik and Zurbuchen, 1998). Pressure transducers were placed in the upper and lower chambers (transducers labelled UT and LT on Fig. 1, respectively) to measure . A control transducer (CT) was placed above the tool to detect short circuiting through fittings in the upper packer or along a near-well disturbed zone.
In the following sections, a series of practical considerations for the DFT will be discussed.
The analysis method (eqn. (1)) proposed by Zlotnik and Ledder (1996) uses the steady-state drawdown to estimate Kr. Zlotnik and Ledder (1994) show that the time required to reach steady-state conditions (ts) can be defined as:
ts > 10SsL2/Kz
For the confined aquifer at GEMS (relatively low specific storage (Ss) and high Kz), one would expect a ts on the order of seconds. Figure 5 displays a record of head change versus time for one of the more permeable test intervals. As shown, the time to steady state for all three pumping rates was a matter of a few seconds. Note that the pumping rate is given in frequency (Hz), which corresponds to the setting on the Grundfos Redi-Flo2 pump used in the dipole tool. The relationship between pump frequency and flow rate depends on back pressure. In this case, a frequency of 100 Hz corresponds to a pumping rate of 14.3 m3/day (2.6 gpm). Figure 6 displays a similar record for a less permeable section. Again, ts is on the order of a few seconds. In this case, the control transducer indicates that there is some water transfer from the overlying water column in the well, either through the fittings of the upper packer or along a near well disturbed zone. However, as shown in the figure, the impact of that transfer on the steady-state drawdown was negligible. Note that further tests indicated that movement through the packer fittings was the most likely mechanism for the water transfer.
Figure 5. 10/30/97 Gems4N. Dipole center at 17.74 m. A larger version of this figure is available.
Figure 6. 10/30/97 Gems4N. Dipole center at 14.69 m. A larger version of this figure is available.
Figure 7. Dependence on flow rate. Hydraulic conductivity versus depth--Gems4N 10/30/97 test. A larger version of this figure is available.
Figure 8. Dependence on well development. Hydraulic conductivity versus depth--Gems4S 10/97 test. A larger version of this figure is available.
Gems4S was drilled using the hollow-stem auger method; no significant problems were encountered in the drilling and installation process. Gems4N was also drilled using the same technique. At this well, however, a miscalculation by the driller resulted in significant amounts of clay and silt from the upper section (see Fig. 4) being smeared across the sand and gravel interval. Figure 9 displays the results of DFT surveys performed at Gems4N after varying degrees of well development. The steps in the well development were similar to those used at Gems4S, except that three stages of intensive development were required to diminish the impact of the driller-induced smearing. In this case, well development significantly altered both the magnitude and pattern of the Kr estimates. Clearly, well installation and development is an important component of a DFT survey.
Figure 9. Dependence on well development. Hydraulic conductivity versus depth--Gems4N 9/97-10/97 tests. A larger version of this figure is available.
In the preceding paragraphs, the focus was on diminishing the impact of a near-well zone of relatively low K. In the case of an artificial filter pack, however, the near-well K may be considerably greater than that of the formation. Kabala and Xiang (1992) present results of numerical simulations that indicate that a near-well zone of relatively high K can introduce error into DFT estimates. This error, which is primarily a function of the width and relative K of the filter pack, and tool geometry, can be significantly reduced through tool design and use of appropriate well installation procedures. In unconsolidated formations, natural filter packs should be utilized whenever possible. Note that natural filter packs were used at both Gems4S and Gems4N.
Figure 10. Hydraulic conductivity and natural gamma profiles--Gem4S. A larger version of this figure is available.
Figure 11. Hydraulic conductivity and natural gamma profiles--Gem4N. A larger version of this figure is available.
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