Downhole fiber-optic systems work by using a small laser that fires off a pulse of light through hair-thin cables made of silica glass. In a fraction of a second, the light travels through the cable and loops back to an optical receiver, delivering a mass of signals and noise that are then sorted out by advanced electronics. For some applications, this process is repeated up to 16,000 times a second—much faster pulse rates are possible, but the top rate is limited by the fiber’s length. And aside from the cable, all the action is taking place in the interrogator unit, a box resembling a computer server that sits on the surface, not far from the wellhead.
The roots of the technology inside the interrogator units can be traced back to 15 years ago during the North American telecommunications boom that suddenly went bust in the early 2000s. Before the bubble burst, billions of dollars were poured into developing smaller lasers, sensitive optical electronics, and reliable interpretation software. This massive investment laid the foundation for the technology’s entry into the oil and gas industry.
“That was a very rich period of development of fiber-optic components,” said Vince Handerek, director of research and development at Fotech Solutions, a 6-year-old company specializing in distributed acoustic sensing. “That led to high-performance components and much lower costs than there had been previously.”
Although the boom is long gone, the technology continues to develop. Many of the fiber-optic cables being pumped into wells today will likely outlast the interrogator units they were installed with—and this is a good thing. It means that as advancements are made, or new applications are sought out by the operator, new interrogator units can simply be plugged into the end of the cable that is already installed downhole.
Compared with the development of the computer hardware, the laser technology inside the interrogator units has improved at a slower pace. Handerek said the lasers have proven to be the most difficult part of the interrogator system for technology makers to develop, partly because most lasers are designed to be used in relatively benign laboratory environments.
But out in the oilfield, temperatures can be extreme, humidity high, and instruments tend to get knocked around a bit. Drilling rigs and production platforms provide constant sources of vibrations, from dropped objects to the nonstop operation of rotating machinery. Even during delivery to the field, the interrogator units are subject to wear and tear. Developing lasers that will produce repeatable results despite these conditions, “is actually quite a tall order; you are asking a lot,” said Handerek. “That you can now buy lasers that are sufficiently stable in these environments, I think is a very important fact.”
