Fiber Optic Sensor Performs Under Pressure

Daniel C. McCarthy

Driven by a $2 million contract with the US Department of Energy, researchers at Virginia Polytechnic Institute have produced a self-calibrated interferometric/intensity-based sensor for continuous monitoring of oil reservoir conditions and their changes during production. It will allow oil well operators to design recovery processes for optimal efficiency.

The fiber optic sensor prototype can monitor down-hole characteristics such as pressure, temperature, acoustic properties and fluid flow. By some estimates, current US tertiary oil recovery processes leave two-thirds or more of reservoirs untapped.

Despite the potential benefits of subsurface monitoring, the use of sensors is rare in oil drilling applications. This is largely because of the short lifetimes -- and costly replacement -- of sensors caused by tremendous temperatures and pressures they endure beneath the earth. Virginia Tech's sensor has undergone lab testing to withstand temperatures of 800 °C and pressures of 6000 psi.

The device works on a hybrid approach combining the high sensitivity of Fabry-Perot interferometry with the simple signal processing of intensity-based sensing, explained Russell G. May, an assistant professor and project researcher at Virginia Tech. The sensor's optical design and signal processing render a cross-sensitivity to temperature of 0.009 percent per degree Celsius, he said.

Two innovations mark the work. The first is the sensor's ability to self-calibrate by splitting the optical signal into two channels and adjusting their coherence. The high-coherence channel measures optical interference in the Fabry-Perot cavity, while the low-coherence channel provides information on connector or bend losses in the fiber that could be interpreted as a change in the sensor probe. "Losses are a common mode in both channels, so they get ratioed out," May said. "All that's left is the desired interference signal from the Fabry-Perot cavity."

The other innovation is the sensor probe, which has an established length and material properties to ensure that the output of the interference pattern is proportional to an increase in strain, a characteristic uncommon in interferometric sensors.

May concedes that this costs individual sensors some adaptability. "There's a trade-off between the dynamic range and resolution. But we're working on this."