Distributed Sensing Technology

Distributed sensing technology is related to large-scale continuous measurement on temperature, acoustic/vibration/seismic, pressure, and strain. Raman distributed temperature sensing (DTS) is more matured technology that has been used in Oil/Gas downhole and reservoir enviornment for about two decades, which is based on optical time-domain reflectometry technology. Brillouin backscattering based optical frequnecy-domain reflectometry (OFDR)technology with single-mode optical fibers can be used for both strain and temperatrue measurement. Distributed Acoustic Sensing (DAS)relies on light which is Raleigh backscattered from small variations in the refractive index of the fiber. The backscattered light has the same frequency as the transmitted light. In DAS, the optical fiber cable becomes the sensing element which allows acoustic frequency of the localized strain signals to be detected over large distances. In addition, distributed pressure sensing (DPS) and distributed chemical sensing (DCS) technolgies are also under development specifically for oil field wellbore, and reservoir production process monitoring and efficency optimization.

Optical Time-domian Reflectometry-based Distributed Temperature Sensing (DTS)

Distributed temperature sensing (DTS) is a temperature measurement technique that uses optical fibre as a temperature sensor, and exploits Raman scattering within the fibre to determine temperature. The technique is described in Dakin, J. P. et al.: "Distributed Optical Fibre Raman Temperature Sensor using a semiconductor light source and detector"; Electronics Letters 21, (1985), pp. 569-570, and UK Patent Application GB 2140554A. A pulse of probe light, typically a high power pulse from a laser, is launched into the fibre and propagates therein. The light undergoes scattering within the fibre from which several signals result. Rayleigh backscattering gives back-propagating light at the original probe wavelength. Raman scattering produces light at two Raman-shifted wavelengths, the Stokes and anti-Stokes signals, the amplitude of which is temperature-dependent. This scattering is generated in both the forward and backward directions. The backscattered Raman signals are detected as they emerge from the launch end of the fibre. The time between launch and detection is proportional to the distance traveled by the light in the fibre, so that the instantaneous Raman amplitude can be related to the position along the fibre of the originating scattering event. A distributed profile of temperature along the fibre is thus obtained. The anti-Stokes signal is more sensitive to temperature changes than the Stokes component, so the former is generally measured, and further improvement is often achieved by measuring both and calculating the ratio of the anti-Stokes to Stokes signals. The temperature can then be calculated as a function of the ratio between the Stokes and the anti-Stokes intensity by 

T-cal

For single-end DTS system, a fiber cable can be distributed for miles that can give customer near 1600 sensing points every mile. While double-ended systems are commonly employed in the Oil field or downholes, in which the fibre is deployed in a loop, and measurements made from both ends of the fibre. A double-ended DTS system is described in P. di Vita, U. Rossi, "The backscattering technique: its field of applica3C-97-0E-46-58-DAlity in fibre diagnostics and attenuation measurements"; Optical and Quantum Electronics 11 (1980), pp. 17-22. Comparison of the two measurements can be made to take account of losses in the fibre, which, unlike the temperature effects, appear opposite in sense when viewed from opposite ends of the fibre. With this dual-end DTS system, the hydrogen darkness on the fiber attenuaiton can be corrected for accuracte temperature measurement. It should be pointed out that the intensity of the Raman scatter is weaker than rayleigh and Brillouin signals and so it is normally necessary to average for many seconds or even minutes in order to get reasonable results. Therefore Raman based DTS systems are only suitable for measuring slowly varying temperatures.

Raman_effect

Optical Frequency-domian Reflectometry-based Distributed Temperature/Strain Sensing (DTSS)

Brillouin backscattering method from a single-mode fiber is alternative to Raman DTS technology, which is based on Optical Frequency Domain Reflectometry (OFDR) method to analyze local environment-induced frequency shift. In principle, Brillouin scatter occurs due to the interaction between the light and acoustic phonons travelling in the fibre. As the light is scattered by a moving phonon its frequency is shifted by the Doppler effect by around 10 GHz. Light is generated at both at above (anti-Stokes shift) and below (Stokes shift) the original optical frequency. The intensity and frequency shifts of the two components are dependent on both temperature and strain and by measuring the shifts, absolute values of the two parameters can be calculated using a Distributed Temperature and Strain Sensing (DTSS) system. Brillouin scatter is much weaker than Rayleigh scatter and so the reflections from a number of pulses must be summed together to enable the measurements to be made. Therefore the maximum frequency at which changes can be measured using Brillouin scatter is typically a few 10ís of Hz. The measured frequency shift provides information on the local temperature and strian.

Coherent Rayleigh backscattering-based Distributed Acoustic Sensing (DAS) for distributed seismicity detection

Distributed Acoustic Sensing relies on light which is coherent Rayleigh backscattered from small variations in the refractive index of the fiber. The backscattered light has the same frequency as the transmitted light. Rayleigh scattering based distributed acoustic sensing (DAS) systems use fiber optic cables to provide distributed strain sensing. In DAS, the optical fiber cable becomes the sensing element which allows acoustic frequency of the localized strain signals to be detected over large distances. In Rayleigh scatter based distributed fibre optic sensing, a coherent laser pulse is sent along an optic fiber, and scattering sites within the fiber cause the fiber to act as a distributed interferometer with a gauge length approximately equal to the pulse length. The intensity of the reflected light is measured as a function of time after transmission of the laser pulse. When the pulse has had time to travel the full length of the fiber and back, the next laser pulse can be sent along the fiber. Changes in the reflected intensity of successive pulses from the same region of fibre are caused by changes in the optical path length of that section of fibre. This type of system is very sensitive to both strain and temperature variations of the fibre and measurements can be made simultaneously at all sections of the fibre. The sensitivity and speed of Rayleigh based sensing allows distributed acoustic monitoring over distances of up to 100 km from each laser source. With suitable analysis software, continuous monitoring of pipelines for unwanted interference, as well as leaks or flow irregularities is possible. Roads, borders, perimeters etc. can be monitored for unusual activity with the position of the activity being determined to within approximately 10 metres. Due to the a3C-97-0E-46-58-DAlity of the optic fibre to operate in harsh environments, the technology can also be used in oil well monitoring applications, such as "Leaks and Integrity" and "Flow Profile", allowing real-time information on the state of the well to be determined.

Existing DAS technology utilizes fiber in the metal tube (FIMT) cable that is normally used for distributed temperature sensing. One challenge lies in the FIMT cable package is louse tube package that cannot couple external strain field into the fiber core because of strain-free package requirement, which is required by distributed acoustic or seismic sensing. The other challenge lies in the optical fiber is amorphous silicon dioxide material that will change optical refractive index under high-pressure and high-temperature downhole environment. This leads to travelling speed of light in the fiber core is no longer constant so that existing DTS and DAS instruments cannot give a correct depth or distance, which may be a few ten meters even a couple of hundred meters errors. As a fiber optical laser radar technology, this doesn't provide accurate distance or range data, which are critical issues facing Oil/Gas industry for distributed seismic profiling, downhole leak detection, and many other production related application where the accurate location or depth is critical for repair.