Evaluation of a Distributed Fiber-optic Temperature Sensor for Logging Wellbore Temperature at the Beowawe and Dixie Valley Geothermal Fields

Abstract

A distributed temperature sensor (DTS) system, utilizing Raman backscattering to measure temperatures of optical fiber, has recently been installed in production wells at the Beowawe and Dixie Valley, NV, geothermal fields. The system has the potential to reduce the cost and complexity of acquiring temperature logs. However, the optical transmission of the initial fibers installed at Beowawe degraded over several months, resulting in temperature errors. Optical transmission spectra of the failed fibers indicate hydroxide contamination via hydrogen diffusion as a possible failure mechanism. Additional fibers with coatings designed to resist hydrogen diffusion were installed and have maintained their optical transmission over several months in the 340-360 °F Beowawe wells. The same fibers installed in a 470 °F Dixie Valley well rapidly failed. Possible methods to prevent fiber degradation include encasing the fiber in metallic buffer layer that resists hydrogen diffusion. Additional methods to correct temperature errors include using additional optical sources to measure fiber losses at the operating wavelengths. Although the DTS system is expected to have one degree F accuracy, we have observed an average accuracy of five degrees. The fiber connections appear to be the uncertainty source. Using connectors with greater stability should restore accuracy.

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