A Fully Distributed Fibre Optic Sensor for the Detection of Liquid Hydrocarbons

Abstract

Here we demonstrate a distributed fibre optic sensor for the detection of liquid hydrocarbons. Two commercially available low-density ethylene-octene polyolefin elastomers with different properties are used to coat fibre optic cables, where the swelling of the coating upon absorption of hydrocarbons imparts strain on the fibre. By frequency domain analysis of the backscatter, the location of oil on the fibre is determined with high precision. The elastomer with the highest flexural modulus, hardness and tensile strength (Engage 8150) imposes more than three times more strain on the fibre compared to the softer elastomer (Engage 8180), despite the Engage 8180 expanding more than three times more in volume compared to the Engage 8150. A response occurs within a few seconds following oil interaction, and a maximum response of 700 microstrain (με) is achieved with a response time of 1.5 minutes (t63) when a dry fibre is exposed to oil, whereas a maximum response of 400 με is achieved when the fibre is exposed to oil when submerged in water. The results demonstrate that optical fibres coated in this way are able to detect and locate small quantities of oil with high degree of spatial resolution. The technique could be scaled to fibre covering long distances, and makes the sensor interesting for pipeline leak detection both on -and offshore, as well as for oil production monitoring purposes downhole.