Active Distributed Temperature Sensing for high-resolution fluid-flow monitoring in boreholes

The ability to measure the flow rates of fluids in the subsurface is critical if we are to assess and successfully manage aquifers for drinking water, geothermal energy systems, shale gas deposits, and coal gasification projects. Here, it is important to understand the interlinked relationship between fluid flows, permeabilities, and geological structure. This can be attempted through observations made in boreholes. When a borehole is installed, flow up or down the borehole may occur naturally between rock at different depths. The nearby operation of any of the above projects may disturb the fluids in the rock, disrupting the flow in the borehole. If the borehole is used to extract fluids in these engineering applications, then any variability in the flow inside the borehole indicates where the most permeable depths are. Current methods of flow measurement inside boreholes are usually made at a single location. In order to establish what is happening along the entire borehole, a probe must be repeatedly lowered, and another measurement made. This process is tedious, and when the flow is changing over time, it can be impossible to adequately determine how this is happening at all depths. On the other hand, new distributed sensors allow measurements to be made with continuous spatial coverage. Distributed Temperature Sensing (DTS) gives continuous measurements of temperature along fibre optic cables. A fibre optic cable acts as a long (100s of metres to kilometres) thermometer from which temperature measurements can be obtained up to every 12 cm. Such a cable installed in a borehole can give a highly detailed log of temperature along its entire length in just a few seconds. This is useful in itself, but exact quantification of the flows by just passively measuring the temperature is not usually possible. We believe a new method, using heated fibre optic cables and DTS, will be able to measure flow rates.

With the proposed method, a cable installed centrally and running to the base of a borehole is heated uniformly by passing a current through the protective materials surrounding the optical fibre. The temperature of the cable, measured using DTS, will increase, and the increase in temperature should depend on how fast the fluid is flowing past it. Faster flows should remove heat more efficiently, lowering the cable temperature. Such a system would potentially be able to measure flows every 12 cm, and be able to detect changes occurring in the flow every few seconds.

The method will be tested in a controlled way using a borehole constructed in a lab from PVC tubing. This would allow access inside and allow us to visually inspect the flow (using dyes) and equipment during testing. A prototype heated 'Active' DTS (A-DTS) system is to be installed in the tube. From a storage tank, water will be pumped through the tube at varying rates, mimicking flow inside a borehole. This will allow is to determine how the temperature of the cable changes in different flow conditions. We will then adjust the heating power of the cable, as the temperature changes due to different flows may be more readily detectable when using higher or lower powers. Finally, the temperature effect at inflow/outflow locations (as would happen where a rock is fractured) will be investigated using inflow/outflow ports in the centre of the artificial borehole. The exact set-ups and the underlying physics will be tested using advanced numerical model techniques.

The development of A-DTS for monitoring fluid flow in boreholes will lead to an improved capability for the monitoring and detection of fluid flow in aquifers and reservoirs. This should result in a better capability for groundwater hydrogeologists and reservoir engineers to manage the natural resource represented by groundwater for water supply, heat for geothermal energy, and oil and gas for conventional energy supply chains. Moreover, risks associated with unconventional potential future uses of subsurface resources (e.g. shale gas recovery) will be more easily assessable with an improved understanding and monitoring capability of subsurface fluid flow regimes. Thus, the successful development of A-DTS will clearly benefit professionals working in industries like water utilities, and the energy sector.
However, also regulatory bodies like the Environment Agency can employ the new A-DTS technology to detect long-term changes in fluid flow regimes that might be associated for example with saline intrusion in coastal aquifer systems, or contaminant flow from landfills. A-DTS can be expected to allow a timely detection of changes in flow paths and the occurrence of anomalous flow which can then be acted upon more swiftly than was possible before. Mitigation of the risk of aquifer contamination might thus be an important aspect of the application of A-DTS. This outcome will benefit society as a whole both in terms of the quality of the water supply, improving health of the general public, but also by preventing costly contaminant remediation in aquifers, hence potentially improving the wealth of the nation.
Representative stakeholders from the group identified above have written Letters of Support included in this proposal confirming the potential benefits they expect to see from the development of A-DTS. Towards the end of the project we will convene a workshop to engage with the stakeholders and discuss the scope of taking the development of A-DTS beyond the proof-of-concept stage towards field implementation. The general public will be engaged with via a website dedicated to the project as well as via ongoing outreach activities from the host and partner institutions.
The PDRA to be employed on the proof-of-concept project proposed here, will benefit significantly from the multi-faceted training he will receive at the interface between state-of-the-art industry involvement (through project partner Silixia) and international academic excellence (represented by the involvement of Visiting Scientist Prof John Selker and visit to international conference). The PDRA's development of skills in numerical modelling, laboratory work, and data processing (an important aspect of working with DTS data) will be of great benefit in his/her future career path, possibly within the areas of industry in which the stakeholders for this project have been identified.