Examples of blistered fber coating
protect the glass and often lack the chemical resistance required
in downhole environments. Figure 4 gives examples of under-cured, poorly optimized polyimide coating where blistering is
prevalent after baking at 400°C.
Another critical test is the chemical resistance of the coating.
In the oil industry, the fiber can be exposed to several types
of solvents. One such solvent is isopropyl alcohol (IPA), sometimes used when the fiber is pumped into oil wells. Figure 5
shows before and after microscope images of optimized polyimide fibers during a 400°C bake and exposed to 50°C IPA.
Figure 5 demonstrates a properly designed fiber coating can
withstand both elevated temperatures and exposure to IPA.
Testing for specific
as it relates to the final application is important. Not all fibers
that pass high-temperature bake tests perform well upon exposure to a specific
chemical, such as IPA.
Therefore, both sets of
tests should be performed, as shown in Figure 5. Additional
testing for chemical resistance against other solvents is pru-
dent. As shown in Figure 5, the fiber coating after exposure
to 400°C shows no signs of deterioration or damage. Both
coating material selection and draw process optimization are
critical to achieve a robust coating for harsh environments.
Meeting the challenges
Traditional measurement devices are significantly challenged
by the high temperatures and harsh environments found in
many oil wells—especially those using enhanced oil recov-
ery technologies such as CSS or SAGD. Distributed sensing
methods using optical fibers have become valuable tools
providing critical process feedback for monitoring the entire wellbore.
Advanced sensing technologies such as DTS have required the
development of specially designed fibers to withstand the harsh
downhole environments. These specialty fibers have been com-
mercially available for several years, but today’s fibers perform
much better than previous generations and can provide many
years of service even in hydrogen-rich environments at tempera-
tures up to 300°C. Fiber optic sensors will become more preva-
lent in downhole applications as the demand for improved mea-
surement and monitoring capabilities continues.
1. R.C. Ferguson, et al., “Storing CO2 with enhanced oil recovery,” Energy
Procedia 1, 1989–1996 (2009).
2. J. Alvarez and S. Han, “Current overview of cyclic steam injection
process,” J. Pet. Sci. Res. 2( 3), 116–127 (July 2013).
3. S.Q. Tunio, et al., “Comparison of different enhanced oil recovery
techniques for better oil productivity,” Int. J. Appl. Sci. Technol. 1( 5),
143–153 (September 2011).
4. O. Humbach, et al., “Analysis of OH absorption bands in synthetic silica,”
J. Non-Cryst. Solids 203, 19–26 (1996).
5. A. Mendez and T.F. Morse, “Hermetic optical fibers: Carbon-coated
fibers,” Chapter 14 in Specialty Optical Fibers Handbook, Lemaire and
Lindholm (Academic Press, 2007), 453.
George Oulundsen is fiber product line manager at Nufern. Daniel
Hennessey is technical manager and Mike Conroy is test engineer
in the Fiber R&D Group at Nufern, 7 Airport Park Rd., East Granby, CT;
email: firstname.lastname@example.org; www.nufern.com.
FIGURE 4. Examples of coating
blistering on optical fiber due to poorly
optimized polyimide coating process.
FIGURE 5. Microscopic examination of fiber coating for downhole
sensing applications: during high-temperature baking (a) and
exposed to 50°C IPA (b).