Dimensions of Czech downhole temperature measurement optical cable

Czech downhole optical cables for temperature measurement typically use a tube-in-tube design with a fiber-in-metal-tube (FIMT) core, supported by an extruded polymer layer and outer protective sheath, with dimensions customizable for specific well conditions.

Core Design

The fiber-in-metal-tube (FIMT) is the central element of the cable, containing either single-mode or multi-mode optical fibers. The FIMT is hermetically sealed in stainless steel or INCONEL® tubing, providing protection against high pressure, elevated temperatures, and corrosive fluids. The inner metal tube diameter is typically in the range of 1/4 inch (6.35 mm), which is standard for permanent downhole installations, though smaller or larger diameters may be used depending on the application .

Supporting and Protective Layers

The FIMT is centralized within an outer tube by an extruded polymer layer called the "belt," which provides mechanical buffering and prevents microbending of the fibers. The outer tube acts as the primary pressure barrier and mechanical shield. Additional encapsulation or armor layers can be added for abrasion resistance and handling during deployment. The overall cable diameter, including outer layers, typically ranges from 6 mm to 12 mm, depending on the number of fibers, metal tube thickness, and protective layers .

Fiber and Sensing Capabilities

Each optical fiber within the FIMT can provide temperature measurements every 0.5 meters along the wellbore, enabling distributed temperature sensing (DTS). The number of fibers and their type (single-mode or multi-mode) can be customized to meet specific sensing or telemetry requirements . Filling gels inside the FIMT further protect the fibers from chemical ingress and mechanical stress.

Customization and Variants

Czech manufacturers, such as R&M, offer multi-loose tube (MLT) constructions and various jacketing materials, including FRLSZH for indoor use or PE for outdoor installations. The cable dimensions and construction are tailored to the well environment, including temperature, pressure, and chemical exposure . Special low-profile sensor housings may also be used, with diameters as small as 0.375 inches (9.5 mm) and lengths around 3 inches (76 mm) for compact installations .

Summary

• Inner FIMT diameter: ~1/4 inch (6.35 mm)
• Outer cable diameter: 6–12 mm depending on layers and fiber count
• Fiber spacing: 0.5 m for temperature measurement
• Outer jacket: FRLSZH or PE, with optional armor
• Customizable: Number of fibers, metal tube thickness, encapsulation, and overall cable length These dimensions ensure the cable can withstand high-pressure, high-temperature, and corrosive downhole conditions while providing reliable distributed temperature sensing for oil, gas, and geothermal wells.