Trevor W. MacDougall

Large-diameter waveguide Bragg grating components and their application in downhole oil and gas sensing

A new class of Bragg grating components based on large diameter cylindrical waveguides has been commercially released. Unique properties of the waveguide including grating fabrication, low loss splicing to optical fibers, and specialized machining for optimization in sensor applications are reported. The waveguide structure enables packaging of Bragg gratings that overcomes attachment, mechanical creep, and hermeticity problems commonly associated with fiber Bragg gratings. This enables exceptionally robust Bragg grating sensor transducers well suited for the high temperature, corrosive downhole environment of oil and gas. Sensor transducers have been demonstrated showing no measurable drift or error after over 4-year aging tests at 150°C. More than 50 pressure/temperature installations have been successfully completed and are operational, delivering real-time data cumulatively over 500,000 operation hours. These systems integrate a range of support components specific to in-well oil and gas applications, such as downhole cables, interconnects, and platform instruments. This optical sensing platform, coupled with other optical techniques, has been extended beyond optical pressure/temperature measurements to distributed temperature measurements, multiphase flow, and in-well seismic sensing. These systems have been successfully deployed in multi-zonal, multi-parameter system architectures. This sensing technology is integrated with in-well controls, data acquisition and interpretation, and reservoir modeling. This systems approach is required to close the value loop of intelligent completions in oil and gas production.

Wavelength-Modulated Sensors

Wavelength-modulated sensors use changes in wavelength to detect the sensing function. Fluorescence and phosphorescence emit a characteristic wavelength of light if perturbed in the proper way. For instance, a dye in the presence of an analyte can give off a characteristic excitation spectrum. Chapter 15 describes fluorescent sensors for chemical sensing and Chapter 11 for temperature sensing. The emitting spectrum provides a qualitative sensing function, but the intensity of the spectrum (usually ratiometric) is required for qualitative sensing measurements. Bragg gratings are truly wavelength-modulated sensors. The parameter being monitored is a direct function of the wavelength shift associated with the Bragg resonance condition. Once again it is important to mention that the fundamental parameter measured is light intensity. However, the wavelength shift is a direct effect of the associated environmental perturbation and is independent of the light source intensity. Gauge lengths can be as small as 0.01 mm. A distinct advantage of Bragg grating devices is their ability to be used as quasi-distributive sensors. They can be used in wavelength-division multiplexing schemes without additional wavelength-encoding filters.

Rotation Rate Sensors (Gyroscopes)

The major advantages of a fiber optic gyroscope over mechanical devices include: no moving parts, no warm-up time, unlimited shelf life, minimal maintenance, large dynamic range, and small size. Fiber optic gyroscopes are used broadly in inertial navigation systems, and tactical and strategic missiles. New applications are emerging, such as in oil well drilling. The scope of applications is quite broad with a wide range of specifications. Figure 17.1 graphically shows the various applications in relation to the required dynamic range and sensitivity. As an example, aircraft navigation gyroscopes have requirements of 0.1 to 0.001 deg/hr. The earths rotation rate, ΩE = 15 deg/hr. Therefore, aircraft navigation needs expressed in earths rotation rate are l0-2 to 10-4ΩE. Geophysical applications require detection of rotation rates at 10-6ΩE. All optical rotation sensors are based on the Sagnac effect, which is described in Chapter 4.

Magnetic and Electric Field Sensors

The main application for magnetic and electric field fiber optic sensors has been the electric power industry. The ability to dielectrically isolate equipment and personnel from high power provides an attractive feature in terms of safety and, ultimately, cost for fiber optic systems versus those using conventional technology. In terms of performance, the theoretical bandwidth of the physical processes occurring within the optical techniques is much faster than the corresponding conducting devices. Also, effects such as hysteresis and saturation (dynamic range), which are common issues with magnetic materials, are, for all practical purposes, absent from the optical components. The size and weight of these devices are also an advantage over iron-corebased designs. All of these characteristics have inspired the vast amount of research and engineering devoted to the development and commercialization of optical-based products. Several sensing approaches have been successfully conceived using fiber optics for both magnetic and electric field sensing. For magnetic field sensing (of which current is a special case), the majority of the designs exploit the Faraday effect and are polarization based. Interferometric approaches using metallic glass and other various magnetostrictive coatings on fibers have also been demonstrated in the past. More recently, the Sagnac interferometer configuration has proven to be the architecture that has found the most commercial acceptance. For electric field and voltage sensing, polarization-based schemes have been described using electro-optical materials to utilize the Pockels and Kerr effects. Interferometric phase-modulation techniques for electric field monitoring have centered on piezoelectric fiber coatings.

Intensity-Modulated Sensors

Intensity-modulated sensors were defined in Chapter 2 as sensors that detect the variation of the intensity of light associated with the perturbing environment. The general concepts associated with intensity modulation include transmission, reflection, and microbending. However, several other mechanisms that can be used independently (intrinsically) or in conjunction with the three primary concepts include absorption, scattering, fluorescence, polarization, and optical gratings.While intensity-modulated sensors are analog in nature, they have significant usage in digital (on/off) applications for switches and counters. The transmissive sensor concept is normally associated with the interruption of a light beam in a switch configuration. However, this approach can provide a good analog sensor. Figure 3.1(a) shows the probe configuration for measurement of axial displacement. Figure 3.l(c) gives a curve of output versus distance between the probes. The curve follows a l/r2 law, where r is distance. A more sensitive transmissive approach employs radial displacement as shown in Fig. 3.l(b). The sensor shows no transmission if the probes are displaced a distance equal to one probe diameter. Approximately the first 20% of the displacement gives a linear output. The curve in Fig. 3.1(c), showing the effects of radial displacement, is for probes with a single fiber, 400-mm diameter. A modification of the transmissive concept is referred to as frustrated total internal reflection. The two opposing probes have the fibers polished at an angle to the fiber axis, which produces total internal reflection for all propagating modes, as shown in Fig. 3.2. As the fiber ends come close in proximity to one another, energy is coupled. The intensity of light coupled into the receiving fiber is shown in Fig. 3.3. This approach provides the highest sensitivity for a transmissive sensor.

Polarization-Based Sensors

As discussed in Chapter 1, the propagating modes of a single-mode optical fiber can be expressed as a combination of linearly polarized (LP) modes with the fundamental mode designated as the LP01. Also highlighted was the concept of describing the light energy as a combination of two degenerate modes possessing orthogonal linear polarizations. The polarization evolution of light in an optical fiber has been studied quite extensively. Typical eigenmode analysis can be applied, and, in most cases, linear polarization states are used as the basis vectors. However, it is important to note that any two orthogonal basis vectors are sufficient to completely describe all polarization states in the waveguide. In fact, as will be discussed later in this chapter, for Faraday rotation it is more mathematically convenient to use circular states (left and right) as the basis vectors to describe the process. Polarization-based fiber optic sensors typically involve an extrinsic birefringent component to perform the actual polarization modulation. Intrinsic types of sensors include Faraday rotation and some Bragg gratings, which are written in polarizing-maintaining (PM) type fibers. Other components required for the system including polarizers and analyzers can also be implemented in fiber and are described in further detail in Section 7.2. The sensitivity of optical components to polarization has been known and studied for a very long time. Most of the effects studied manifest themselves as operations on linear coordinate systems. Again, these are systems that possess linear eigenvectors. Detailed in Fig. 7.1 is a general component in a linear right-handed coordinate system. The angle of the light vector θ is defined as being positive when measured from the vertical coordinate y as observed while looking into the light source.