SINGLE-WELL THERMAL TRACER TESTS USING DISTRIBUTED TEMPERATURE SENSING

Recent work on the Cambrian-Ordovician Aquifer System in the upper Midwest has revealed laterally extensive preferential flow zones in sandstone units that were previously characterized as homogenous and isotropic. Differences in recharge elevations and/or pumping stresses between aquifer units can produce vertical head gradients and ambient flow in multi-aquifer wells. Distributed temperature sensing (DTS) is a powerful new technology that allows for the rapid profiling of temperature using fiber optic cables. In two naturally flowing multi-aquifer wells near Madison, WI, single-well thermal tracer tests involving the injection of heated water were monitored with DTS. The results elucidate the ambient flow regimes of the wells in great detail, revealing fracture-dominated flow in some sandstone intervals and intergranular flow in others. Comparison of DTS data with down-hole video and other borehole geophysical logs shows diverging flow in both wells to be emanating from a cluster of bedding-plane fractures in the Wonewoc Formation. In addition to being stationary and synoptic, DTS is sensitive to a wider range of flow rates than traditional heat-pulse and spinner flowmeter techniques. As the cost of DTS continues to decrease, this method could become a standard tool in site investigations.