Silica Fiber Monitors Temperature in Oil Wells

Susanna Contini Hennink

The fuel industry has traditionally used submersible electric pumps to deliver oil from underground reservoirs to ground level. These devices are affected by changes in temperature and pressure, which can slow production and shorten the pump's life. To keep a check on this, electronic pressure and temperature gauges that provide a single point of measurement are usually installed when the well is completed. But these gauges have their own drawbacks: They are prone to failure over time and under high temperatures, and they cannot be replaced.

The pure silica fiber is fed through a 1/4-in.-diameter stainless steel line to monitor temperature changes in oil wells.

A full temperature profile

British Petroleum is trying a new approach. The company is using a fiber optic temperature monitoring system as an alternative to electronic gauges in an oil well on Furzey Island in southern England. Sensor Highway Ltd. developed the technology to provide permanent fiber optic sensors for the oil and gas industries, and is developing fiber optic pressure sensors as well as temperature systems.

According to British Petroleum's senior production engineer John Davies, the temperature monitoring system continuously provides a full temperature profile over the whole well. Once the company proves the reliability and accuracy of the pressure sensor, the electronic systems may be history.

An important benefit of the Sensor Highway system is that it allows pressure and temperature sensors to be replaced without major reconstruction. The fiber is made of pure silica and coated with aluminum or gold for abrasion protection so temperatures in excess of 700 °C can be tolerated. The fiber is installed in a 1/4-in.-diameter stainless steel line, which is usually strapped to the outside of the completion tubing. This allows the fiber system to be retrieved easily and replaced if necessary.

Data are collected from an optoelectronics unit above ground. The system uses a diode-pumped solid-state laser and Raman scattering techniques to measure temperature at 1-m intervals along the entire length of the fiber.

Davies said work also is being done on a telemetry system and a flow data application: "Once these systems have been perfected, it should be possible to carry out on-line production logging of a well without the need for a shutdown. It will give us a continuous picture of oil- and water-producing zones rather than merely a snapshot."