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Abstract
The basic advantages of fiber optic sensors such as immunity to EMI, compact size, and fast response hold true as well for fiber optic sensors applied to the measurement of pressure. Over the years, pressure sensing has included a wide variety of technologies and application areas. The product spectrum ranges from low-cost measurements, such as tire-pressure monitors, to space shuttle applications. The discussion here will be limited to devices that incorporate fiber optic technologies to perform pressure measurement. The application sectors for fiber optic pressure sensors can mainly be divided into segments based on pressure range, temperature range, and frequency response. The basic operating principles of optical modulation for sensing applications have been addressed earlier in the text and will not be repeated here. In regards to the fiber optic-based designs, the main applications have been directed toward the following: low-pressure, in situ, biomedical, high frequency equipment monitoring, and high pressure oil/gas reservoir monitoring. In addition, as is similar to most fiber optic sensing research areas, there have been many other niche ideas that have been designed and built for a multitude of specialized requirements. The designs that have found the most widespread application have been those based on either FBG technology or Fabry-Pérot architectures. Other approaches such as microbending are described in detail in Chapter 9 of Ref. 23. Applications of fiber optic sensors in the high-pressure oil and gas reservoir monitoring market have seen the most commercial penetration. The unconventional sector of this market is characterized by temperatures approaching 300 °C, while conventional offshore installations are subjected to temperatures of 150 °C and below. The maximum pressure for these applications are approximately 1000 and 15,000 psi respectively. From a commercial standpoint, a reliable, long-term, pressure-monitoring device can bring substantial value to a producing field, which then justifies the investment in this state-of-the-art measurement system. The application of reservoir monitoring is characterized by very slow-changing pressures. Update rates on the order of one minute or more are quite acceptable. The important value comes from being able to monitor the pressure of the reservoir from its immediate turn on far into its later life. In order to do this effectively, the sensor must maintain a high level of stability, as recalibration is typically not an option. These measurements provide valuable information on the depletion of the reservoir and how best to optimize the production and yield of its resources. For example, a 1% improvement on a 10,000 barrels/day well is equivalent to an annual savings of ~$3.5 million (in 2013 dollars). This is somewhat of an extreme case, but it can be seen from this example that the right monitoring equipment combined with competent reservoir management can be well worth the investment. Distributed acoustic sensing (DAS) based on coherent Rayleigh backscatter, as well as incoherent Rayleigh and Brillouin scattering have also been implemented in pressure sensing architectures.
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CHAPTER 12
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