Unsteady State Tracer Dilution Method for Characterizing Water Contributing Fractures in Crystalline Bedrock Wells

Evaluating contaminants impacting wells in fractured crystalline bedrock requires knowledge of the individual fractures that contribute water to the wells. This typically involves using a sequence of methods including downhole geophysics, flow meters, and straddle packers. These methods are expensive, time consuming, and can be logistically difficult to implement. This study demonstrates an unsteady state tracer method as a cost effective alternative for gathering fracture information in bedrock wells. The method entails: introducing tracer dye throughout the wellbore by trolling and mechanical mixing; packing off and draining the upper portion of the well; releasing the packer to induce a slug test and fracture flow into the well which dilutes out the dye within the well; monitoring the dilution front movement by profiling the tracer concentration to locate water contributing fractures, calculate their transmissivity values, determine their hydraulic heads, determine ambient flow conditions, and calculate contaminant concentration within the fractures. This method was tested on a large physical model well and a bedrock well. The model well was used to test the theory underlying the method and to perfect method logistics. The approach located the fracture and also generated transmissivity values that were in excellent agreement with those calculated by slug testing. For the bedrock well tested two major active fractures were located. Fracture location and ambient well conditions matched results from conventional methods. Estimates of transmissivity values by the tracer method were within an order of magnitude of those calculated using heat-pulse flow meter data from a previous study.