Optical Fiber Sensors in Upstream Oil & Gas
Authors: Brown George A. , Hartog Arthur 
Published: June 2014

Technology Today Series articles are general, descriptive representations that summarize the state of the art in an area of technology by describing recent developments for readers who are not specialists in the topics discussed. Written by individuals recognized as experts in the area, these articles provide key references to more definitive work and present specific details only to illustrate the technology. Purpose: to inform the general readership of recent advances in various areas of petroleum engineering.

Introduction
Optical fibers have, in the past few years, begun to gain acceptance as permanently installed downhole sensors, although trials were carried out as long ago as 1990.1 Optical-fiber sensors are fully passive, totally immune to electromagnetic radiation, and small enough that, in certain cases, they can be installed deep in a well after the completion itself has been run. The strong E&P industry interest in optical sensors derives mainly from the expectation of a higher reliability than conventional gauges because of the absence of active electronics in the downhole device.
The technology that these devices exploit was developed mainly for the telecommunications industry during the past 35 years.2 The optical attenuation of the best fibers is well below 0.2 dB/km (equivalent to a signal reduction by a factor of 10 for every 50-km of propagation path), and a usable information transmission rate is well in excess of 1 Tbit/s baud rate. Thus, optical fibers are a well-developed, high-bandwidth, low-loss transmission medium. The telecommunications industry has standardized on a fiber diameter of125 µm that is sufficiently small to be flexible, yet retaining reasonable strength.
A conventional optical fiber consists of a central core (typical diameter of5 to 50 µm) surrounded by a cladding of slightly lower refractive index. In general, both regions are made from silica, modified by the addition of materials to tailor the refractive index profile and the dispersion of the structure thus formed. A coating is applied to the fiber (typically to an outer diameter of 250 µm) to protect the surface from scratches, which will weaken the fiber, and to buffer it from microbending that would cause light to be lost. For oilfield use, higher levels of fiber protection for temperature, chemical attack, and mechanical abrasion and crushing are required. In increasing order of effectiveness, coatings have been manufactured from ultraviolet curable acrylates, silicone rubber, fluorated polymers, polyimides, and metal. In general, a metallic tube, similar to a 1/4-in. diameter controlline, further protects the coated fiber.
Commercially available sensors can be categorized into those that aim to replace conventional permanently installed devices (such as pressure gauges and flow meters) and replaceable, distributed measurements [such as a distributed temperature system (DTS)].

Pressure Sensors
There are several ways in which an optical fiber can be turned into a pressure sensor. Two methods currently in use are fiber Bragg gratings (FBG)and Fabry-Perot interferometry.
An FBG3 is a device consisting of a short piece of fiber, which has a periodic modulation of the refractive index longitudinally along the fiber axis. Primarily, FBGs are narrow-band reflectors that transmit all wavelengths with little loss, except for a narrow range centered on a wavelength that matches the pitch of the index modulation. Reflection bandwidths as low as 1 GHz can be manufactured, and FBGs are used as filtering devices in the telecom industry. More sophisticated FBGs are used as band-pass filters or as dispersion-compensating devices. For sensor applications, FBGs work by stretching the fiber in which the grating is inscribed, thus altering the wavelength of the reflected light.
The Fabry-Perot technique4 uses the interference pattern setup by the reflection of laser light from two reflectors, such as the cleaved ends of fibers placed in close proximity (approximately 50 µm) and sealed together in a 350-µm diameter silica tube. The gap between the mirrored fiber ends varies mechanically by fluid pressure acting externally on the tube causing the interference pattern set up between the mirrored ends of the fiber to change with the imposed external pressure.
Unfortunately, neither of these systems yet provides the pressure resolution and accuracy available in the current generation of quartz electrical gauges, as shown in Table 1. Neither has their reliability yet proved superior, although only a few gauges have been installed and it is probably too early to make a direct comparison with electrical equivalents. Installation requires that the gauge be in a mandrel, allowing transmission of reservoir pressure to the sensor through a bellows. The mandrel is installed on the production tubing in a fashion similar to electrical-pressure-gauge mandrels. Thus, the whole downhole instrumentation must be installed with the completion.