One of the earliest problems that plagued downhole fiber-optic technology is known as hydrogen darkening. As hydrocarbons flow through the wellbore and over the fiber-optic cable, some of the free hydrogen atoms find their way into the silica glass material, causing a distortion or darkening that makes it difficult, or impossible, for the surface interrogator to take accurate measurements. The higher the well’s temperature is, the faster the darkening occurs. “Hydrogen darkening is like death and taxes; it will occur sooner or later,” said Mikko Jaaskelainen, senior technology manager at Pinnacle, Halliburton’s fiber-optic and laser unit. “You either buy time by putting in mitigation barriers, or you learn how to live with it, and we can do both.”
The mitigation barriers include the use of chemically resistant materials in the fiber and carbon coatings around it to slow down the rate at which hydrogen intrudes into the cable. In addition to improved glass chemistry, Halliburton has developed a dual-laser technology by using a wavelength designed to operate away from hydrogen peaks (areas with a high absorption of hydrogen), thus allowing the fiber to be used as the hydrogen darkening occurs. These improvements have helped Halliburton install fiber-optic systems in wells undergoing steam-assisted gravity drainage, where temperatures can reach as high as 575°F.