Laser Focus World - January 2015

Wavelength (nm)

Induced attenuation (db/km)

1550 1450 1350 1250 1150 1050 950

948 1080 1130

1240

1530

1380

1580

Interstitial H2 Interstitial H2

DTS
window

1170

1196

Si-OH Si-H
Si-OH
H2 + Si-OH

850 750 650

25

20

15

10

5

0

FIGURE 2. Fiber spectral attenuation due to hydrogen-induced losses in downhole sensing environments.

of flight, the temperatures and/or strains are correlated to the spatial position in the fiber.

The light propagating in the sensing fiber and the resulting backscattering is very responsive to environmental changes and, therefore, optical fiber sensors are extremely sensitive and can provide very fine spatial resolution along the fiber length. For example, today’s typical DTS systems can determine temperatures with a spatial resolution of 1 m (some high-end systems can achieve 20–30 cm resolution) and an accuracy of ±0.5 to 1°C with a resolution of 0.01°C.

This is important because oil well operators will optimize and change the operation of the well based on small tempera- ture changes on the order of a few degrees Celsius. Additionally, due to the low loss of optical fibers, distributed sensing over tens of kilometers (as many as 50 km) is possible depending on the type of sensing and environment. The long lengths are essential since oil wells can be several kilometers deep and some double-ended sensing deployments require the fiber to loop back, necessitating fiber lengths up to 10 km.

Fibers for downhole sensing applications

All three of the distributed sensing technologies are used by the oil industry, but the most widely deployed is Raman-based DTS, especially for downhole temperature measurements. To present the most comprehensive example, we focus on optical fibers for realizing an operational Raman-DTS system for high-temperature enhanced oil recovery wells.

This requires specialty optical fibers in which both the glass design and the fiber coating are capable of operating at high temperatures (up to 300°C) and in typical oil well environments for long periods of time. At such high temperatures and in the presence of hydrogen, most optical fibers will degrade rapidly, becoming highly attenuated (dark). Therefore, the overall fiber design and the manner in which the fiber is manufactured become important considerations for high-temperature downhole applications.

Also, the SNR is critical in DTS, and it is imperative the Raman scattering Stokes and Anti-Stokes signals are not compromised by interference from absorption peaks related to interstitial hydrogen and reactive hydrogen bonds. Figure 2

summarizes the absorption peaks attributed to interstitial hydrogen and reactive hydrogen bonds (Si-O-H and Si-H). 4

Molecular hydrogen can diffuse into the fiber and occu- py interstitial sites that have specific absorption peaks. Once removed from the hydrogen enviroment, the dissolved hy- drogen can diffuse out and hence these losses are reversible. Absorption peaks can also form when hydrogen reacts with the Si or Si-O in the fiber, creating Si-H or Si-O-H bonds. This often happens at defect sites in the fiber where the oxygen at- oms are not fully bonded. In Ge-doped fibers, there are broad induced losses as hydrogen reacts with Ge-defect sites at both short (<900 nm) and long wavelengths (>1400nm), as well as losses associated with Ge-OH formation (1410 nm). 5 The induced attenuation from the –H and –OH bonds are irreversible and permanent.

The typical operating window for Raman DTS is also shown in Figure 2. The key wavelengths for Raman DTS are the signal wavelength (1064 nm) and the wavelengths of the Anti-Stokes (1014 nm) and Stokes (1114 nm) processes. Sensing fibers must exhibit minimal losses at these wavelengths in the downhole environment. The challenge is designing an optical fiber that has very low losses in the presence of hydrogen partial pressures of 1–2 atm at temperatures of up to 300°C or more.