Promoteur : Christophe Caucheteur
Sensing in downhole environment can be considered as one of the toughest working conditions on earth. Despite the high pressure and high temperature working conditions, the presence of hydrogen gas or mechanical shock and vibration should also be taken into account when offering a suitable solution. The market is still dominated by crystalline quartz resonators that suffer from a high cross-sensitivity when dealing with simultaneous temperature and pressure monitoring. In case of sensing in a rapid temperature transient, a pressure error of around 25 bar is normally foreseen with quartz based sensor.
Fiber optic sensing technologies have demonstrated their adaptability, reliability and accuracy in sensing different critical parameters in downhole applications. For example, fiber optic has long operating life time as it is completely passive. Fiber optic benefits from its low attenuation which allows the possibility to have a measurement over a long distance from the field side. In this PhD thesis, the pressure and temperature monitoring is accomplished with a fiber Bragg grating (FBG) written by a femtosecond laser pulse in the so-called Butterfly micro-structured fiber (MSF). This study and the sensor development were partly carried out in the framework of the “Advanced Optical Sensor for Pressure Monitoring (AOS4PM)” project targeting an accurate pressure measurement in Ultra-high temperature and Ultra-high pressure downhole working conditions. The unique sensing ability of the Butterfly MSF can offer a negligible cross-sensitive between temperature and pressure.
To begin with, we first optimized the fiber handling procedure for the Butterfly MSF. In the step of FBG inscription in the Butterfly MSF, we then optimized the laser inscription parameters when inscribing in standard single-mode fiber (SMF) due to its simple cross-section structure as a comparison to the Butterfly MSF. In the end, the inscription laser power was selected to have acceptable grating stability at elevated temperature and to have low birefringent grating. We have studied the sensing ability of our MS-FBG sensor by presenting different characterization results both on purely fiber level and with sensor packaging. The main purpose of this characterization was to mimic the working conditions in downhole environments including the sensor sensitivity to temperature and to pressure, temperature and pressure stability and the induced pressure error in a rapid temperature transient, etc. To improve the FBG stability at elevated temperature, we empirically obtained a suitable accelerated aging recipe and can offer a great stability with less than 0.3 °C in temperature variation and with less than 0.42 bar in pressure variation over 7 days at 280 °C. We also developed a dedicated packaging. Our sensor packaging not only offers a mechanical protection to the fiber but also transfers the well pressure into its sensing element. Packaged MS-FBG sensor was also validated in a laboratory environment first before deploying in the field. Results are in good agreement to the results obtained on the fiber level. Finally, the packaged MS-FBG sensor was brought to the field and was verified in a test well before a real deployment.