A Multi-modality Fiber Optic Sensing Cable for Monitoring Enhanced Geothermal Systems

Abstract

A suite of optical sensors for monitoring pressure and temperature in enhanced geothermal systems is being developed by a multidisciplinary team of researchers from GE, Qorex LLC, AFL Telecommunications and Sandia National Labs. In the second year of this DOE-sponsored project, a fiber optic cable and key sensing subsystems have been fabricated and tested in the laboratory for temperature and pressure response and accuracy, reliability and survivability at temperatures up to 400 C and for hydrogen darkening of the fiber at high temperature and pressure. A downhole experiment of the fiber cable and some of these sensors is scheduled before the end of 2011. GE has designed and fabricated four different resonant MEMs sensors for point pressure measurements that can be optically modulated and read, and operated in EGS environments. Resonant frequencies vary between ~15 kHz and 90 kHz depending on design, and laboratory measurements yielded sensitivities of 0.9-2.2 Hz/psi. Linear performance was verified at temperatures up to 100 C. An FPGA-based optoelectronic feedback system for these sensors was also implemented and validated. Five metal and polyimide-clad single mode and multimode fibers have been qualified for use at 374°C, and an industrial supply chain confirmed. An FBG sensor for distributed pressure sensing has been designed for temperatures up to 400 C and is being fabricated. The accuracy of a Raman distributed temperature sensing instrument has been demonstrated for use with pure silica core fiber even when the fiber exhibits a nominal amount of reversible hydrogen darkening. Results of the open and closed loop pressure sensing measurements, temperature dependence, manufacturing tolerance and yield analysis of the sensors, and downhole field tests will be presented.