Environmental Engineering Reference
In-Depth Information
be as tolerant. Table
14.1
shows different coating materials and their suggested
working temperature. Most fibers deployed for DTS and DAS applications are
coated with polyimide for well monitoring purposes. Metal coatings are more ther-
mally resistant than polymer coating; however, the coating process is more tedious
and costly.
Table 14.1
Fiber optics
coating material selection for
high temperature
Max. working
temperature (ᄚC)
Coating material
UV—cured acrylate
100
UV—cured dual
acrylate
150
Polyimide
400
Copper with polyimide
400
Aluminum
450
Copper alloy
600
Gold
>700
Disadvantages of Fiber Optic Sensors
Placement of fiber optic cables in the wellbore brings with it operational problems.
Sometimes they may get washed out of hole. Existence of free hydrogen atoms
(mainly in gaseous form such as hydrogen sulfide) can affect the performance of the
fibers. This phenomenon is called “hydrogen darkening” which occurs with the
physical process of hydrogen intrusion into the fiber. As a result the optical glass
fiber degrades causing optical distortion. In this darkening process, the hydrogen
atoms react with the silica glass (SiO
2
) compound and form hydroxyl (OH) which
interferes with the passage of light through the glass. One way to prevent such issue
is carbon coating of the fiber together with other coating materials (see Table
14.1
).
Another approach is to use a purer silica as the fiber core. There are some other
partial cures such as using gel to absorb hydrogen, etc.
Data storage is another issue that needs to be addressed. Generally speaking,
DTS and DAS systems required huge electronic storage capacity if they are record-
ing at a high resolution. For example to record 6 h of high resolution DTS (1,000
samples per second recording) requires a 1 TB hard drive capacity.
Types of Fiber Optics
Fiber optics may be categorized into two types, single mode and multi-mode. DTS
fibers are multi-mode fibers and DAS fibers are single mode. Multi-mode fibers are
thicker than single mode fibers (up to ten times greater core diameter) and are good
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