Fiber optic cables for well temperature measurement are designed for harsh downhole conditions, offering distributed temperature sensing (DTS) with high accuracy, long-range monitoring, and robust mechanical protection.

Cable Construction and Design

Permanent downhole fiber optic cables use a tube-in-tube design, where optical fibers are housed in a hermetically sealed stainless steel or INCONEL® tube (Fiber in Metal Tube, FIMT) to protect against chemical ingress, high pressure, and elevated temperatures . The FIMT may contain a mix of single-mode and multi-mode fibers to support both temperature and acoustic sensing. Filling gels inside the tube provide additional protection against microbending and chemical effects. The FIMT is supported by an extruded polymer layer (belt) and an outer tube that acts as the primary mechanical shield, with optional armor or encapsulation for extra protection during deployment .

Fiber Types and Sensing

• Distributed Temperature Sensing (DTS) uses multi-mode fibers to generate a continuous temperature profile along the cable length .
• Distributed Acoustic Sensing (DAS) uses single-mode fibers for vibration and acoustic monitoring, which can complement temperature measurements .
• DTS systems, such as Optromix DTS 500/1000 Series or Yokogawa DTSX, can measure temperatures over distances ranging from 16 km to 100 km, with high accuracy (up to 0.01 °C) and real-time monitoring .

Operational Specifications

• Temperature Range: Designed to withstand elevated downhole temperatures, often exceeding 150°C, depending on the cable materials and deployment conditions .
• Pressure Resistance: Outer tube and encapsulation provide protection against high-pressure environments typical in wells .
• Chemical Resistance: Hermetically sealed fibers and protective gels prevent degradation from corrosive fluids .
• Mechanical Protection: Controlled prestressing during manufacturing ensures fibers remain strain-free, minimizing optical degradation or breakage during deployment .

Measurement Principles

DTS systems operate using the Raman scatter principle, where a laser pulse is sent along the fiber and the backscattered light is analyzed. The ratio of Stokes to anti-Stokes signals determines the temperature at each point along the fiber, enabling continuous distributed temperature measurement over long distances .

Customization and Deployment

Cables can be customized for fiber type, quantity, armor, encapsulation, and filler materials to match specific well conditions. Deployment requires careful planning to ensure strain-free installation and reliable long-term operation .

Summary

For Guatemalan wells, fiber optic cables for temperature measurement should feature:

• Tube-in-tube construction with FIMT
• Multi-mode fibers for DTS, optionally combined with single-mode for DAS
• High temperature, pressure, and chemical resistance
• Long-range monitoring capability (up to 100 km)
• Real-time, continuous temperature profiling with high accuracy
• Mechanical protection and prestressing to prevent fiber damage These specifications ensure robust, accurate, and reliable temperature monitoring in challenging downhole environments, supporting well productivity and safety.