Numerous land and undersea oil industries use temperature sensing to provide a high level of safety and functionality in severe environmental conditions. For instance, silica-based optical fibers are widely applied as DTS and DAS systems because they allow detecting various signals and recording them along the length of the fiber optic cables.
It should be noted that due to optical fibers, well and pipeline engineers can employ sensing systems based on fiber optic technology to measure in real-time an entire wellbore or pipeline span. Moreover, fiber cable can act perfectly under severe environmental conditions such as high temperatures and pressures or ionizing radiation and chemicals.
Therefore, optical fibers must have mechanical reliability and high longevity to provide a transmission of optical power with a minimum signal attenuation or loss because in most wells the temperature level can reach almost 400 °C, while pressures can increase up to 2000 psi standard atmosphere (atm).
Usually, the core and cladding of most optical fibers are made of silica that offers such benefits as high optical transmission, superior thermal stability, and mechanical reliability. Herewith, the fiber optic cable also includes a polymer coating that enables to protect the optical fiber mechanically and decrease signal losses. Thus, the coating plays a crucial role here because it provides the mechanical stability of fibers, particularly, when the fiber cables are applied in severe environmental conditions.
A team of researchers tested properties of optical fibers by putting them in various types of high-temperature/pressure liquids that included distilled water, seawater, isopropyl alcohol, and paraffin oil to determine the suitability of fiber optic cables in undersea and downhole applications where environmental conditions are regarded as difficult to operate in.
The thing is that different coating types of optical fibers are also important because they can offer different benefits and have different limitations in harsh environments. The most frequent disadvantage of fiber optic cables is considered to be an attenuation or loss of mechanical strength. Additionally, these limitations determine the longevity of optical fibers, thus, higher temperatures lead to a shorter lifetime of different fiber coatings operated under the same conditions.
Numerous factors influence the survivability of fiber cables under severe environmental conditions, they include the type of coating, environment, temperature, pressure, and usage time. The results of the research reveal that the carbon coating of optical fibers can maintain 200 °C temperature and protect the fiber from full dissolution in water compared to standard fiber optic cables.
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