A Rock Borehole Packer System for Identifying Hydraulically Active Fractures Under Natural Gradient Flow
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To improve capabilities for understanding and predicting contaminant migration in fractured rock there is need for better field methods to identify the fractures that have active groundwater flow. Current methods have limitations, for example, borehole geophysical imaging, such as acoustic and optical televiewing, identifies fractures appearing on borehole walls but cannot sense groundwater flow. Borehole hydraulic tests determine the transmissivity of fractured zones under conditions altered by the presence of the borehole and its testing and not under natural flow conditions. The natural flow conditions are important because they govern contaminant transport in the whole flow system. Furthermore, conventional tracer tests are used to identify flow in fractures, but these too are typically done under imposed rather than ambient (natural) hydraulic conditions. High resolution fluid temperature logging in lined boreholes can identify some of the hydraulically active fractures, but this method lacks the sensitivity needed to indicate ambient flow in each individual fracture. This thesis presents a new method aimed at determining whether or not any particular fracture targeted for borehole measurement has substantial ambient flow. This method involves a device lowered into an open hole to a target zone where a packer is inflated. This packer has a water-flow-sensitive dyed cotton fabric wrapped around its exterior so that when the packer is inflated, it not only seals the borehole but presses the cotton fabric against the borehole wall. This set-up causes the exact location of hydraulically active fractures at the borehole wall to show up as imprints marked on the fabric. When viewed under black light, individual fracture markings can be seen, and the distribution of the hydraulically active fractures is identified. For this new method, a prototype system was developed for use in 10cm diameter wells and was tested first in a conventional slotted well screen in the laboratory and then in a simulated fracture (slotted) PVC pipe installed in a sandy aquifer where groundwater flow rates are well understood. From a large number of fabric/dye combinations tested in the laboratory, it was found that cotton dyed with a particular food grade additive provides the best fracture markings by far. The prototype system uses the double-acting packer system originally developed by Solinst Canada, and this novel packer design provides ease of use and flexibility for configuring multiple packers on a single pipe. This prototype system is now ready for the first field trials in a fractured dolostone borehole in Guelph, ON. While the ability of the device to identify active fractures as effectively as it has in the slotted casing trials may be reduced by the interaction of the dye with the porous rock matrix, it is anticipated that this new system for identifying hydraulically active fractures under resealed borehole flow conditions (resealing brings flow back to ambient conditions) will be useful in its own right in fractured rock investigations. This device also represents the first step in the creation of a more elaborate device to measure both the groundwater flux and the contaminant flux within plumes in fractured rock.