Real-time monitoring of pressure and temperature of oil well using a carbon-coated and bellow-packaged optical fiber sensor

A carbon-coated and bellow-packaged optical fiber sensor for high pressure and high temperature monitoring in downhole applications is developed and successfully field-applied in an oil well. Carbon-coating and bellow-packing techniques are introduced to enable the sensor to operate under the extreme conditions in downhole. Carbon-coating is aimed at preventing oil and gas from penetrating through the silica capillary that houses the sensor and also reduce the hydrogen-induced loss and capillary corrosion. The packaging with a bellows structure is designed for both efficient pressure transfer and effective isolation of external corrosive contents from direct contact with the optical fiber probe. We developed a carbon coating technique based on a CO2 laser system and implemented a bellows – diaphragm combination using laser welding. By both measurements in lab and a long-term field test in an oil well, the coated and bellow-packed sensor has shown good pressure linearity of more than 0.99999 with 0.03% F.S. repeatability and 2.9 psi resolution – demonstrating its great potential for long-term downhole dynamic monitoring.