Principle of European Downhole Temperature Measurement Optical Cables

Downhole temperature measurement in Europe using optical cables relies on distributed temperature sensing (DTS) technology, which measures temperature along the length of a fiber-optic cable via Raman backscatter.

Core Principle

European downhole DTS systems use fiber-optic cables installed in boreholes to continuously monitor temperature along their length. A laser pulse is sent through the optical fiber, and the light interacts with the molecular structure of the glass. Some of this light is backscattered, with a portion undergoing Raman scattering, which is sensitive to temperature changes. The system measures the ratio of temperature-dependent anti-Stokes light to temperature-independent Stokes light, allowing precise determination of temperature at each point along the fiber. The location of the temperature measurement is determined by the time delay between the emission of the laser pulse and detection of the backscattered light, following the optical time-domain reflectometry (OTDR) principle .

Measurement Techniques

1. Passive DTS: Measures ambient downhole temperatures without altering the environment. It is used to monitor natural thermal gradients, groundwater flow, or seasonal temperature variations .
2. Active DTS (A-DTS): Involves heating the fiber (e.g., via a copper core) and measuring the temperature response and dissipation rate. This method provides information on thermal conductivity, heat transport, and groundwater flow in the surrounding rock .

Installation and Spatial Resolution

Optical cables are typically grouted behind casing or attached to borehole walls to ensure thermal contact and stability. European systems achieve high spatial resolution, often 25 cm to 1 m, and temperature accuracy within ±0.01–1°C over distances exceeding 30 km. The laser pulse frequency is adjusted based on the sensing distance to avoid overlapping signals from consecutive pulses .

Applications

• Geothermal energy and heat storage: Monitoring heat injection and extraction in shallow and deep geothermal wells .
• Hydrogeology: Mapping groundwater flow, thermal advection, and subsurface thermal properties .
• Oil and gas industry: Reservoir characterization, well integrity monitoring, and optimization of production .
• Environmental monitoring: Detecting thermal impacts on aquifers and surrounding infrastructure .

Summary

European downhole DTS systems exploit Raman scattering in optical fibers to provide continuous, high-resolution temperature profiles along boreholes. By combining passive and active measurement modes, these systems allow precise monitoring of subsurface thermal conditions, supporting geothermal, hydrogeological, and industrial applications with real-time, spatially dense data .