What is distributed sensing?
Distributed sensing utilises optical fibre itself as the sensor, effectively creating thousands of sensor points along the fibre, all acquired continuously and instantaneously at the same time by sensing at the speed of light. Even though an optical signal is being processed, acoustic and temperature changes can be measured with fibre optic sensing.
Employing fibre-based sensors is particularly practical for elongated structures such as pipelines or oil wells.
How does distributed sensing work?
A very defined laser signal is sent down the optical fibre. As the laser light hits the fibre material it is continuously scattered back towards its origin where a detector picks it up. When there are any environmental changes in the vicinity of the fibre, such as temperature increase or a sudden pressure drop, the pattern of that backscatter is affected. Using the known physical properties of the fibre and laser, the backscattered signal can be analysed to reveal details about the magnitude of the event and its position along the length of the fibre.
How does a fibre get into an oil well?
There are different ways to measure events in an oil well. In principle, two approaches can be considered – temporary well intervention or permanent fibre installation.
During temporary well intervention, e.g. wireline operations, sensors are lowered into the borehole to collect and transmit data. Since this approach interrupts the normal production flow, it does not allow for uninterrupted continuous data acquisition. Permanent fibre installation is required for such purposes.
Fibres can be integrated into well casings, the walls of an oil well, and connected to the Xmas tree, a structure composed of valves and connectors, at the top of the well. The Xmas tree is the key junction between downhole and topside. So-called dry-mate connectors allow for the optical system to be pre-assembled at the topside before deployment underwater. Wet-mate connectors can also be used for topside assembly but are designed to be installed subsea. In addition, fibre optic penetrators are used to bridge fibre environments with different physical or chemical properties.
Fibres require protection against breaking and other damage. A simple approach is to place one or more fibres in a solid gel-filled stainless-steel tube. This serves essentially as a shield to outside stressors. Alternatively, fibres can be armoured with concentrically arranged steel or copper strands surrounding the buffer material that envelops the fibre.
For our software development, Q-DOS™, it does not matter how the fibre is installed, since it will deal with any relevant downhole data.
Distributed sensing provides operators with a wealth of information. It is key to maximise the insights generated from the data to enable real-time intervention and long-term predictive modelling.