U-shaped fiber optical cable assembly for use in a heated well and methods for installing and using the assembly

A U-shaped fiber optical cable assembly (11, 21) is arranged in a heated well (1) such that a nose section (13, 23) comprising the bent U-shaped cable section (11C, 21C) is located near the toe (1 A) of the well where the ambient well temperature is lower than the temperature of an intermediate section of the well which is heated by steam injection, electrical heating and/or influx of heated hydrocarbon fluids from a heated section of the surrounding formation to a temperature above 200 degrees Celsius, thereby inhibiting the risk of hydrogen darkening of the bent U-shaped cable section. It is preferred to make the nose section of a glass solder, to arrange the U-shaped fiber optical cable assembly in an aluminium guide tube (22) sealed at its lower end with end cap (31), and to use a heat resistant fiber optical cable to further inhibit the risk of hydrogen darkening of the assembly.