Fibre Optic Distributed Temperature Sensing for Borehole Flow Monitoring
Subsurface environments are critical not only as water resources but also for energy use including geothermal, CO2 sequestration and oil & gas operations. These sensitive environments are directly affected by anthropogenic activities, hence monitoring is essential. Temperature offers insight into a variety of physical properties and can be used for flow monitoring and flux quantification in the subsurface. Traditionally, point sensors have been used to collect temperature data; however, significant limitations exist.
Fibre optic distributed temperature sensing (DTS) has the ability to collect high temperature resolution data fully distributed in space and continuously in time. Using active or passive DTS measurements enables groundwater flow measurements in both shallow and deep boreholes. DTS can provide data for models for 3D imaging of temperature changes at the aquifer scale, provide a tool to characterise subsurface heterogeneity into boreholes and localise inflows. Ambient groundwater flow, or flow under natural gradient conditions in fractured rock aquifers is an important component of both contaminant and heat transport. Contaminant transport in fractured media is controlled by the fracture network and resulting flow system.
Gaining an understanding of the natural flow system is needed for site conceptual model development including for predicting contaminant plume migration. In addition, natural gradient flow is particularly challenging to measure due to the very low flow rates involved. Monitoring fluid movement in such a difficult environment requires high resolution techniques that are capable of sensing individual fractures or fractured zones.
A fine temperature resolution is required for measuring very low or rapid flow rates and localised flow. The ULTIMA™ DTS can be combined with active heating techniques to create the thermal differential needed for identifying natural gradient flow distributions and borehole flow heterogeneity. The active approach does not rely on natural temperature signal; instead, a heat pulse is output at a constant rate along the measurement cable and the thermal response monitored using DTS. By creating a thermal disequilibrium, a variety of processes can be characterized using the temperature rise during heating and temperature decay during cooling.
Active DTS methods can be used to measure groundwater flux distributions in fractured rock aquifers by utilizing boreholes sealed with flexible liners.