The unique physical attributes and performance characteristics of optical fibers has been well-known for many years, and indeed the transmission properties for data communications was well-known long before full commercialisation was possible for the telecommunication industry. In today's perspective, it is difficult to believe that fiber optic data transmission systems were indeed "solutions looking for problems", from the early 1970's. Today we think of high bandwidths, data communications on fibers as extremely routine for the telecom industry.
Similarly for oilfield sensing solutions in the subsea and downhole oil and gas industry, a large number of developers have been supplying potential solutions to sensing problems, but the problems of integrating these technologies with existing subsea and downhole facilities are not insignificant.
The paper will describe some potential solutions to the subject of integrating downhole and subsea fiber optic-based sensors with other subsea and downhole facilities, and identify the necessary interfaces, both physical, electrical and optical.
Some illustrative examples will be presented, and also an appreciation of the present state-of-the-art in fiber sensing for such applications.
Fibre-optic sensors have been deployed in multiple wells for measurement of pressure and temperature. See Fig. 1 for an example from an installation in Brunei of the ABB DOGS system (for measurement of temperature and pressure). Optical sensors for other parameters are also emerging, e.g. downhole multiphase flowmeters. In the picturewe show installation of the ABB DOGS system in a test well in Brunei.
Pressure sensors are essentially point sensors (although several can be located on the same fiber).
Temperature sensors can either be point sensors (with several located on the same fibre) or distributed (measuring the temperature profile along the fibre).
As such sensors are being developed in ever deeper installations, there are some hurdles to overcome, e.g.:
The optical fibre much be brought through the tubing hanger connector. Such connectors are uncommon for conventional subsea trees, and non existent for subsea horizontal trees (although some development work is ongoing)
The allowable attenuation in the fibre is quite small. In a practical subsea system, multiple fibre optic connectors are necessary in the signal chain, and the attenuation in these connectors soon consumes the available signal budget.
A large part (50% or so) of downhole sensing systems do not survive installation and initial operation.
A Raman scanner works in principle by sending a brief laser light pulse down an optical fiber (see figure 2).
At all points along the fiber, minute portions of this light is scattered and reflected due to various mechanisms, one of which is the Raman effect. The Raman effect will cause the reflected light to have a different color than the incident light.
By measuring the color of the reflected light, and the time at which it was reflected, the temperature at each location along the fiber can be measured with high precision (=Distributed Temperature Sensing, DTS).
Another similar effect (although with more complex physics and equipment) is the Brillouin effect, which also can be used for Distributed Temperature Sensing.
Both multimode and singlemode fibres can be used for DTS systems. There is a tradeoff between accuracy, type of fibre and distance (further complicated by the number of connectors in the signal path).