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Optical MEMS Pressure Sensors for Geothermal Well Monitoring
Abstract
The technology for enhanced geothermal systems (EGS), in which fractures connecting deep underground wells are deliberately formed through high pressure stimulation for energy generation, is projected to enormously expand the available reserves of geothermal energy in the U.S. EGS could provide up to 100,000 MWe within the U.S. by the next 50 years. Nevertheless, there are still many challenges to understand and develop this resource for low cost energy production. Pressure measurements, in particular, are important for determining the state of the fluid, i.e., liquid or steam, the fluid flow, and the effectiveness of the well stimulation. However, it has been especially difficult to accurately measure pressure at temperatures above ~200ºC. MEMS technology has been employed for many years for extremely accurate pressure measurements through electronic readout means. By substituting optical readout of a sensor at the end of a fiber optical cable, it is possible to employ these highly accurate sensors within the harsh environment of geothermal wells. A two year, DOE-funded project has developed MEMS-based sensors specifically for this purpose. A multidisciplinary team of researchers from GE, QOREX LLC, AFL Telecommunications and Sandia National Labs developed the MEMS optical sensor and readback electronics, fabricated a fiber optic cable for downhole testing, and has tested the sensor in a geothermal well.