Cross sensitivity analysis of optical fibre-based sensing for high pressure, high temperature measurement in oil and gas applications.

Abstract

Measurements of physical parameters like pressure, temperature, flow rate, vibrations and displacement in harsh environment are often desired in the oil and gas industry. Pressure and temperature are the most important parameters among them. Changes in pressure and temperature have a major role in many downhole processes and they have enabled the performance of producing wells, water-injection profiles, fracture jobs, carbon dioxide (CO2) injection, and enhanced oil recovery to be successfully monitored over the years. However, measurements of these parameters were mostly carried out by logging tools that comprise several conventional electronic gauges. Temperature and pressure in deep and ultra-deep wells could reach beyond 300 °C and 20000 kPa. Fibre optic sensor developments have created new interest in oil and gas production monitoring and control. Fibre optic sensors currently deployed in the oil and gas industry can only operate up to 150 °C and even the most robust sensors are limited below 300 °C. In this research, the effect of cross sensitivity on multi-parameter optical fibre sensor for oil and gas applications in harsh environments is studied and analysed. Mathematical models of the cross sensitivity function of fibre Bragg grating (FBG) and its physical mechanism has been developed. This analysis was based on the fact that FBG sensor was in the state in which temperature and pressure were simultaneously functioning. The main objective of this research is to investigate the cross sensitivity effect of fibre optic sensing technology for oil and gas sensing applications by designing sensing structures and developing theoretical equations for the system model of the optical fibre sensor capable of addressing this issue. Different techniques have been proposed to tackle this cross sensitivity challenge. Firstly, the use of dual sensing structure that investigate and analysed the spectral characterization of a Fabry-Perot interferometer (FPI) sensor and the interaction with physical parameters. Preliminary results of this configuration are used to eliminate the issue of temperature-pressure cross-sensitivity and effectively improve the resolution of the sensor system. Secondly, the concept of a metal coated hybrid sensing system is proposed. The theoretical design and analysis of a metal coated hybrid sensing system of FBG and Extrinsic Fabry-Perot Interferometer (EFPI) cavity for high pressure high temperature (HPHT) measurement in oil and gas applications is reported. The FBG and EFPI are used to measure temperature and pressure respectively. This configuration is able to solve the problem of cross sensitivity. Lastly, a novel thin-film fibre optic extrinsic FPI sensor with an ultra-high pressure and temperature sensitivities are analysed theoretically using 16-layer graphene film deposited on silicon carbide (SiC) substrate as the diaphragm. The performance parameters of the proposed sensor are investigated in terms of sensitivity at the operating wavelength of 1550 nm. It was observed in the numerical study that the sensitivity can be greatly increased by using multilayer graphene on SiC substrate. This sensor is expected to have potential for monitoring oil and gas applications in harsh environments.