It has been more than 50 years since the first patent was filed that considered the use of fiber optics as a way to measure environmental events. U.S. Patent 03327584, granted in 1967, describes a fiber bundle that would simultaneously illuminate a surface and also capture the reflected light. In the early 1980s, fiber-optic acoustic systems for sensor arrays were used for lightweight wide aperture array (LWWAA) for Virginia Class submarines, towed arrays, and various surveillance systems.
The use of fiber-optic sensing for in-well/down-well monitoring in the oil and gas industry has been ongoing for the past 20 years. In the 2000s, distributed temperature sensing was used, followed by the use of distributed acoustic sensing in the 2010s. Although the technology is recognized as providing value in many instances, it is far from being deployed on every well. However, there is promise that fiber sensing will become standard in certain applications.
For the past decade, distributed fiber-optic sensing has been used to detect and prevent onshore oil and gas pipeline leaks. As an example, in 2016, OptaSense won a contract to supply a pipeline leak detection and security solution for the Trans-Anatolian Natural Gas Pipeline (TANAP). The solution will monitor more than 1850 kilometers of pipeline as well as perimeter security for all facilities. Currently there are more than 15,000 kilometers of pipeline being monitoring by fiber sensing technology around the world.
There are three types of fiber-sensing networks.
Point senor networks—In a point sensor network, each sensor is discrete and must be backhauled individually. Point sensors are usually used in shorter-length deployments. Understanding where the point sensors are along a given path is critical to being able to properly interpret data received from the environment.
Quasi-distributed sensor networks—One version of quasi-distributed sensing includes the use of several fiber Bragg gratings (FBG), which are embedded into the fiber. The refractive index of the fiber core is modified such that certain wavelengths of light pass through while others are reflected back toward the source. Each FBG can reflect a specific wavelength, making each one identifiable along the fiber pathway. In other words, FBGs are like inline wavelength filters that reflect specific wavelengths back to the source, and multiple FBGs can be employed into a single fiber path. As with point sensor networks, understanding where the FBGs are in relation to what is being detected is key to proper interpretation of the data.
Distributed sensor networks—In a distributed sensor network, the number of sensors along an optical fiber is distributed, and the numbers vary based on the length of the system, the spatial resolution of the sensors, and the interrogator box used. Typically the spatial resolution of each sensor is 1 to 10 meters. Distributed sensing is accomplished by sending a pulse of light down a fiber and interpreting the backscattered light from that pulse. By looking at Rayleigh, Brillouin, and Raman backscatter, it is possible to detect acoustic, strain/temperature, and temperature, respectively. Distributed sensing can replace the often-cumbersome and costly integration of hundreds or thousands of separate sensors into a single continuous solution.
There are three main types of distributed sensing applications.
- With distributed acoustic sensing (DAS), “virtual” microphones are distributed along a fiber. The number of microphones is based on a combination of spatial resolution, distance, and pulse width. Depending on the vendor, each interrogator has a typical range of 30 to 50 kilometers. Multiple interrogators can be networked together, which gives a single operator thousands of kilometers to monitor.
- In distributed temperature sensing (DTS) applications, “virtual” thermometers are distributed along a fiber. DTS can have a range of 10 to 100 kilometers, spatial resolution of 1 to 5 meters, measurement time from 2 to 30 minutes, and temperature measurement accuracy from <0.5 degrees to <5.5 degrees Celsius. Keep in mind that range, spatial resolution, measurement time, and temperature accuracy are interdependent.
- Distributed strain sensing (DSS) includes “virtual” strain gauges distributed along a fiber. Using a Brillouin-based system, it is possible with some solutions to measure strain at a range of more than 65 kilometers, spatial resolution of approximately 1 meter, and a strain resolution of less than 10 microstrains.
As noted above, in a distributed sensor network, backscattered light can be broken down into three components: Rayleigh, Brillouin, and Raman.
Rayleigh backscatter is used primarily for distributed acoustic applications. Acoustic signals or sound waves that impact the fiber cause small changes in the refractive index. These changes can be detected with Rayleigh backscatter when using a coherent optical time-domain reflectometer (COTDR). Standard singlemode optical fiber is typically used.
Brillouin backscatter is used for strain and/or temperature measurements. When the fiber is under strain a Brillouin frequency shift can be detected and analyzed. The Brillouin optical time domain reflectometer (BOTDR) is used, or for enhanced detection, a Brillouin optical time domain analyzer (BOTDA) can be used. Standard singlemode optical fiber is typically used.
Raman backscatter is primarily used for temperature detection applications. In order to detect temperature changes, the anti-stokes Raman peak, which is temperature-dependent, and the stokes Raman peak, which is almost temperature-independent, are compared. The temperature is determined based on the delta between the two. For shorter distances, standard multimode fiber is typically used.
