Cost-effective oil and gas production is becoming more important than ever. The availability of downhole information is seen to be the key to increasing oil-recovery efficiency, currently estimated to be 35% on average for North Sea oil-wells.
Fibre-optic sensors have the potential to measure many downhole parameters. However, high-temperature, high-pressure (HTHP) fluids are shown to have adverse effects on fibre pressure-sensors and cables. All unprotected, silica-fibre, pressure sensors drift in HTHP fluids. Furthermore, optical-fibre cables fail in HTHP fluids.
Diffusion of molecular water from HTHP fluids into silica fibres is shown to be the root cause of drift and damage to fibre sensors and cables. Ingress of molecular water into the fibre causes expansion of the silica and results in highly stressed regions of the fibre.
A carbon and polyimide coating process is identified as a suitable hermetic coating for fibre-optic cables in downhole conditions. However, this coating is shown to be inadequate for protecting optical-fibre pressure sensors.
A novel coating technology is developed, in which the sensor is sleeved in a silica capillary, which is filled with a liquid metal. The packaging technique improves the stability of side-hole-fibre pressure sensors to better than 0.1psi per month at temperatures up to 300°C.
The pressure sensor developed in this thesis (the SD-series sensor) is tested in a field trial at Chevron's Coalinga test facility in California.
The SD-series pressure sensor out-performs any existing downhole pressure gauge in the world and is to be made commercially-available to the global oil industry in the year 2000.
Citation
Clowes, John Redvers (2000) Fibre optic pressure sensor for downhole monitoring in the oil industry University of Southampton, Optoelectronic Research Center, Doctoral Thesis , 174pp.
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