I. Introduction

Fiber optic sensors are opening up new capabilities for sensing a wide range of parameters, such as pressure, temperature, vibration, flow, acoustic fields, for downhole reservoir monitoring applications, for both retrievable and permanently installed systems. Significant interest is being directed towards this area, and several types of fiber optic sensors have been demonstrated for downhole use, including a Distributed Temperature Sensor (DTS) for temperature profiling, an optically excited resonant pressure sensor, an interferometric point sensor for pressure monitoring and Bragg grating based sensors for a range of parameters. Each of these sensor types will be briefly described in this paper. The CiDRA Corporation is developing transducers based on grating technology for a wide range of applications in the oil and gas industry not limited to downhole production monitoring, but also for seismic sensing, down-stream process monitoring, platform structural & pipeline monitoring, and other sensing requirements covered under the generic fields of exploration, production and transportation. Fiber optic sensor technology has been under development for the past 20 years and has resulted in several successful new products; fiber optic gyroscopes, temperature sensors, acoustic sensors, accelerometers and chemical probes (particularly biochemical) are examples. Applications of the technology include civil structural monitoring (e.g., smart structures), military systems (e.g., under water acoustic arrays), industrial applications (e.g., process control sensor networks), chemical sensing (distributed spectroscopy), and security monitoring (intrusion detection) to name a few. The use of this technology in the oil and gas industry, and particularly for downhole applications is driven by the inherent advantages of fiber optics over conventional sensor technologies, namely:

  • Electrically passive devices (no downhole electronics)

  • Intrinsically safe

  • Immune to EMI

  • Operate at high temperatures

  • Can be 'multiplexed' or operated in a 'distributed' mode

  • Small cross section

Of these advantages, the multiplexed or distributed sensing capabilities of fiber optic sensors is of particular pertinence for downhole applications, were there is the need to monitor a parameter, or parameters, at many spatial locations through the wellbore, or along horizontal/multi-lateral components of the well.

II. Fiber Sensor Types

Fiber optic sensors are typically distinguished by two generic forms of sensing technique; intensity-based or interferometric [I]. These two approaches differ in the overall system complexity of the sensors and the sensitivities of the technique. In general intensity based sensor concepts tend to be simpler to implement but less sensitive. Interferometric configurations generally result in more complex optical implementations, but high sensitivity. As will be discussed later, the fiber Bragg grating sensor technology under development by CiDRA are unique in the sense that they can be configured in simple arrangements for a range of parameters such as pressure and temperature, but also used to form simple high-sensitivity interferometric sensor configurations. Another important distinction made with fiber optic sensors is the ability of the sensing technique to be configured in as a point or in a distributed (or quasi-distributed) mode. point or in a distributed (or quasi-distributed) mode. The point mode of operation is analogous to conventional sensor technology, where the parameter of interest is measured at a particular point - e.g. a thermister, pressure sensor, resistive foil gag

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