Identifying downhole fracture characteristics using in-situ fluorescence monitoring : the results and interpretation of a large-scale radially divergent tracer experiment conducted in a dolomite aquifer
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In several field studies it was concluded that highly transmissive features transmit the majority of solute mass in horizontally-fractured bedrock aquifers. The purpose of this investigation is to develop a new technique for determining the location and relative role of hydraulically connected fracture features with respect to solute transport. To explore this, a radial-divergent tracer experiment was conducted in a four-borehole array completed through a horizontally-fractured dolomite of Silurian age in Smithville, Ontario, Canada. The injection interval included several hydraulically-identified features located in the Upper Eramosa member of the Lockport formation. 496 L of Lissamine FF (a conservative fluorescent dye tracer at a concentration of 200 mg/L) was injected at a rate of 23.25 L/min +/- 1 %. The arrival of fluorescent tracer was detected in a series of open monitoring wells located in a down gradient direction using a submersible fluorometer equipped with a pressure transducer. Correlating the fluorescence signals at depth with hydraulically-identified features provided an in-situ measure that identified the vertical intersection and relative role of each feature with respect to mass transport. FRAC3DVS a discrete fracture finite element model was used to simulate the tracer experiment. Based on the results of the tracer experiment and numerical simulations, it is concluded that the highly transmissive features identified using hydraulic techniques do not always carry all of the mass in transport. The majority of mass transport, however, followed at least one of the largest features in every borehole, just not every large feature. Results of this experiment suggest it is imperative that a distinction between large fracture features that transmit and do not transmit mass be made.