CHARACTERIZATION OF FLOWPATHS FROM BOREHOLES

The everting liner mechanism is well suited to the measurement of flow rates from the hole while the everting blank liner is being installed. This method is described in detail under methods, FLUTe hydraulic conductivity profiler. The basis of the technique is that the sealing liner descends; it displaces the borehole water into the formation. The descending liner also seals the hole and therefore, it covers the flow paths from the hole. The descent rate is controlled by how easily the water flows from the hole. Hence, by recording the descent rate of the liner and the level of excess head inside the liner, one can deduce the effective transmissivity of the remaining hole. As each significant flow path is sealed by the liner descent, the remaining transmissivity decreases. That decrease is the transmissivity of the portion of the hole just covered. The end result is a plot of the hole transmissivity with depth. Relatively tight hole measurements are only more difficult because of the low descent rate in the hole and the long time required to measure the hole. However, experience has shown that the flow from the hole is often dominated by a few relatively free flowing fractures. A typical measurement of the entire hole only takes about 2 hours.

Another technique for assessing the formation conductivity is to use the recovery rate of the sampling system for each port to deduce the conductivity of that portion of the hole defined by the spacer length (see Water FLUTe system for the description of the sampling system geometry). This recovery rate is only useful if it is dominated by the formation conductivity and not by the tubing size and length. For that reason, the conductivity assessment via the recovery rate is generally limited to conductivity values below 10E-5 cm/sec. If larger tubing is used (possible in holes of greater than 6" diam.), the upper bound on possible conductivity measurements rises quickly, because of the radius to the fourth power dependence of flow in tubes.

The liner descent rate method described above has beeen performed in over 120 boreholes. The monitoring transducers and recording system have been built and attached to a linear capstan. The software has been perfected to deduce the borehole transmissivity with depth. The results are described in the 2004 NGWA paper and the 2006 GSA paper.

FLUTe has a U.S. patent on this method and hardware, with numerous foreign patents pending.

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