Dimensions of French Downhole Temperature Measurement Optical Cables

French downhole temperature measurement optical cables typically use a tube-in-tube design with inner fiber-in-metal tubes (FIMT) and outer protective layers, with diameters ranging from 1/4” to 3/8” for the inner tube and overall cable diameters up to 0.75” depending on the application.

Cable Construction

French downhole optical cables, such as TEF or NovaPT-D systems, are designed for high-temperature, high-pressure, and corrosive environments. The typical design is a tube-in-tube configuration:

• Inner Tube (FIMT): Contains the optical fiber(s), which can be single-mode (SMF) or multi-mode (MMF). The inner tube is usually made of stainless steel or other corrosion-resistant alloys and can have diameters around 1/4” (6.35 mm) for standard installations .
• Outer Tube: Provides mechanical protection and chemical resistance. The inner tube is centralized and supported by an extruded material layer called a “belt”, which maintains alignment and absorbs mechanical stress .
• Outer Encapsulation: Additional layers may include polymer coatings or metal sheaths to improve abrasion resistance and handling during installation .

Key Dimensions

• Inner Fiber Tube Diameter: Typically 0.25” (6.35 mm), sometimes slightly larger depending on fiber count and alloy thickness .
• Outer Cable Diameter: Can range from 0.5” to 0.75” (12.7–19 mm) for permanent downhole installations, such as cemented behind casing or strapped to production tubing .
• Sensor Connection Tail: For systems like NovaPT-D, the fiber cable connects to a 1/4” diameter tail of the sensor housing, suitable for high-pressure ratings above 10,000 psi .

Design Considerations

• Fiber Count: Varies depending on the number of measurement points; multiple fibers can be included in the FIMT for temperature, pressure, and acoustic sensing .
• Metal Tube Thickness: Selected to withstand well pressures and corrosive fluids; alloys like SS 316L or Alloy 718 are common for high-temperature applications above 300°C .
• Customization: Cable dimensions, fiber type, and encapsulation can be tailored to specific well conditions, including offshore or geothermal wells .

Applications

These cables are used for Distributed Temperature Sensing (DTS), Distributed Acoustic Sensing (DAS), and Distributed Temperature Strain Sensing (DTSS), providing continuous monitoring along the wellbore . The robust design ensures long-term stability, high reliability, and resistance to hydrogen ingress in harsh downhole environments . In summary, French downhole optical cables are engineered with inner fiber-in-metal tubes of ~1/4” diameter, supported within an outer protective tube, with total cable diameters up to 0.75”, and are highly customizable for temperature, pressure, and environmental requirements in oil, gas, and geothermal wells .