Detection of water flow movement in oil field by nanocoated polycarbonate optical fibre sensor

Abstract

The use of chemical tracers for oil reservoir monitoring is important to provide assurance and maximise production from mature fields. Nonetheless, this practice requires a huge amount of chemical, thus special consideration in regards of the technical, cost, and environmental aspects is necessary. Henceforth, a more efficient, real-time data acquisition technique to monitor the reservoir must be established. In recent times, fibre optic sensing technology has been touted as a reliable option to improve conventional method for reservoir monitoring. However, the use of conventional glass optical fibre is not favourable as it is fragile, hard to handle and vulnerable to hydrogen attack. Hence in this scenario, the use of polymer optical fibre is beneficial. Nonetheless, the polymer selection is crucial as it must withstand high operating temperature conditions. Therefore, this study focuses on the development of a robust polycarbonate-based polymer optical fibre (PCPOF) sensor to detect oil movement in porous media. Polycarbonate (PC) was chosen as the core as it possesses excellent thermal and optical properties and they are important criteria in fabrication of POF for high temperature condition. In the first stage of the study, the extrusion temperatures were varied from 250 to 310 °C and drawing speeds was manipulated from 5 to 20 rpm during the PC core fibre fabrication in order to obtain the most desired properties in terms of its diameter, pore-free structure and sufficient mechanical strength. It was discovered that the best extrusion temperature was at 290 °C in which at this temperature, the polymer was in a completely molten and exhibited excellent light propagation properties. Meanwhile, a drawing speed of 15 rpm produced a comparable fibre diameter of ~569 µm with a mechanical strength of 76.68 MPa. Subsequently, in the second stage, two sets of PC core fibres were fabricated with individual cladding materials based on polycarbonate/polymethyl methacrylate (PC/PMMA) blend and polymethyl pentane (PMP). Prior to the cladding layer fabrication, it was initially casted into thin films for its optical and thermal characterisation, and it was found that the ideal PC/PMMA ratio for cladding application was 80/20. At this composition, the PC/PMMA cladding exhibited a transparency of ~99 % and a glass transition temperature of 124.91 °C. On the other hand, the transparency of PMP was ~95 % and its transition temperature was 228.45 °C. Next, the cladding layer on the PC core was then fabricated via dip-coating technique. The prepared PC core with different claddings were evaluated to determine their power output. It was found that PMP cladding layer enhanced the performance of the PC core with an increment in power output by 60 % as compared to PC/PMMA cladded PC core which exhibited an increase of only 28.8 %. Due to excellent light propagation properties, the PMP cladded PCPOF was utilised to fabricate the sensor in the last phase. In the third stage, the sensor was fabricated via a multistage method involving plane-by-plane inscription, partial removal of cladding and TiO2 deposition to enhance its sensing ability. The complete PCPOF sensor was tested at temperatures of 60 °C, 90 °C and 120 °C and its sensitivity was evaluated. Three central Bragg wavelengths of 750 nm, 800 nm and 850 nm were successfully inscribed on the PCPOF. Moving forward, the best etching time was found to be at 9 min by remaining ~1.47 µm of cladding thickness. Further increment of etching time may result in damage towards the fibre core. Then, TiO2 was uniformly distributed on the sensing region via a double dip coating technique to ensure all sensing regions were covered with nanoparticles. On average, it was discovered that the sensitivity of sensor was more than ~100 nm/RIU in different test surrounding temperatures. The sensor also was able to operate at a surrounding temperature of 120 °C. In the oil displacement test, the sensor managed to detect oil/water flow inside the porous media by shifting its central wavelength when in contact with different liquid. These results show that utilising fibre optic sensor to detect oil movement within a porous media is feasible and can be further developed to create a functional alternative technology to continuously monitor reservoir to better understand reservoir fluid flow pathways.