Hydrogen darkening
Encyclopedia
Hydrogen darkening is a physical degradation of the optical properties of glass
. Free hydrogen atoms are able to bind to the SiO2 silica glass compound forming hydroxyl
(OH) - a chemical compound that interferes with the passage of light through the glass.
The problem is particularly relevant to fiber optic cables — particularly in oil and gas wells where fiber optic cables are used for distributed temperature sensing
. Hydrogen can be present due to the cracking
of hydrocarbon
s in the well. The darkening of the fiber can distort the DTS reading and possibly render the DTS system inoperable due to the optical loss budget being exceeded.
To prevent this, coatings such as carbon
are applied to the fiber, and hydrogen capturing gels are used to buffer the fiber, and other proprietary techniques may be used to prevent hydrogen atoms from reaching the glass fiber via the cable sheath
.
Glass
Glass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...
. Free hydrogen atoms are able to bind to the SiO2 silica glass compound forming hydroxyl
Hydroxyl
A hydroxyl is a chemical group containing an oxygen atom covalently bonded with a hydrogen atom. In inorganic chemistry, the hydroxyl group is known as the hydroxide ion, and scientists and reference works generally use these different terms though they refer to the same chemical structure in...
(OH) - a chemical compound that interferes with the passage of light through the glass.
The problem is particularly relevant to fiber optic cables — particularly in oil and gas wells where fiber optic cables are used for distributed temperature sensing
Distributed temperature sensing
Distributed temperature sensing systems are optoelectronic devices which measure temperatures by means of optical fibres functioning as linear sensors. Temperatures are recorded along the optical sensor cable, thus not at points, but as a continuous profile. A high accuracy of temperature...
. Hydrogen can be present due to the cracking
Cracking
Cracking may refer to:* Cracking, the formation of a fracture or partial fracture in a solid material* Fluid catalytic cracking, a catalytic process widely used in oil refineries for cracking large hydrocarbon molecules into smaller molecules...
of hydrocarbon
Hydrocarbon
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons from which one hydrogen atom has been removed are functional groups, called hydrocarbyls....
s in the well. The darkening of the fiber can distort the DTS reading and possibly render the DTS system inoperable due to the optical loss budget being exceeded.
To prevent this, coatings such as carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
are applied to the fiber, and hydrogen capturing gels are used to buffer the fiber, and other proprietary techniques may be used to prevent hydrogen atoms from reaching the glass fiber via the cable sheath
Sheath
Sheath may refer to:* Scabbard, a sheath for holding a sword, knife, or other large blade, as well as guns, such as rifles.* The outer covering of a cable* Condom, a kind of contraception* Debye sheath, a layer of a plasma in physics...
.