BOREHOLE FLOW CHARACTERIZATION USING DTS TO MONITOR DISCRETE IN-WELL HEAT TRACER TESTS

Heat tracer tests in open wells or boreholes provide valuable information about the adjacent formations, including the presence of permeable features such as fractures, the presence of aquitards, and whether flow is upward or downward. Heat tracer tests in wells are complicated due to the challenges of adding heat to the well and then measuring the movement of heat in the well while minimizing disturbance of the water column.

Distributed Temperature Sensing (DTS) technology makes use of the physical behavior of light in optical fiber to measure temperatures along a fiber optic cable. DTS systems have been deployed in a wide variety of hydrological settings, and recently have been used in downhole environments to measure ambient temperature profiles and responses to induced temperature perturbations. DTS monitoring of borehole temperatures allows rapid collection of temperature data along the full borehole length while causing no disturbance to the water column during measurement.

We have developed a simple, electric heater that can be used to induce heat pulses into the water column at discrete intervals. The heater consists of a 2000 watt cartridge heater, shrouded in a perforated steel housing, and attached to electrical cable. The heater can be raised or lowered in the well to target specific zones of interest. The heating element is approximately 0.25 meter in length allowing for heat pulses to be induced at discrete intervals. The combination of discrete heat pulses induced via electric heater with DTS measurement of temperature response in the well allows for detailed characterization of the flow regime within the well.

We used DTS monitoring and the downhole electrical heater to profile the open portion of several bedrock wells. Using this system we were able to identify important flow characteristics in the wells, including borehole flow direction, velocity, and locations of inflows and outflows. The results indicate that DTS monitoring of induced heat pulses is an effective method for detailed borehole flow regime characterization.