| Abstract |
US Department of Energy has funded Paulsson, Inc. to develop a high temperature 3C borehole seismic source to be deployed in the same borehole as a large array of 3C fiber optic vector seismic sensors. The combination of a clamped source and clamped receivers will in combination create a borehole tool that can record single well seismic data. All components of the system will be designed to operate in a high temperature environment suitable for geothermal applications. This is not just a borehole logging system, this is a borehole seismic system which will be able to image objects 100s and even 1,000s of feet from the survey boreholes. Fiber Optic based multi sensor systems have been shown to operate efficiently and economically in boreholes recording exceptional data. We have simultaneously operated Fiber Optic Seismic Vector Sensors (FOSVS), Distributed Acoustic Sensors (DAS), Distributed Temperature Sensors (DTS), and Distributed Strain Sensors (DSS) in the same borehole. Other fiber optic-based sensors are under development and will be combined with the current borehole sensor system. Fiber Optic Sensors can currently operate to temperatures over 300°C. We have demonstrated in the laboratory that we can operate our fiber optic seismic vector sensors at 320°C for a week. If the silica-based core and cladding that make up the two inner layers of optical fiber are combined with an outer coating of gold as the final and third layer rather than polyimide used currently, operational temperatures up to 700°C have been demonstrated by other researchers. Using other metals for the coating even higher temperatures, up to 1,000°C, has been demonstrated. If pure silica core fiber is used in the sensor system, then the system is practically immune to hydrogen darkening allowing operations in geothermal reservoirs that often have a presence of hydrogen sulfide. One very large advantage with fiber sensors is that the sensors can be placed in boreholes while the instruments can be placed in a benign surface environment. Fiber optic-based sensors can thus be interrogated by lasers and instruments placed on the surface eliminating placing fragile instrument components in the high temperature borehole environment making the overall system much more robust. Using fiber optic based seismic vector sensors, data with a Moment Magnitude smaller than M-5 has been recorded in a recent survey. In this case the source-receiver distance was about 2,000 ft and the maximum frequencies recorded were above 2,000 Hz. In the laboratory we have recorded high signal to noise ratio vector seismic data from a seismic source with an estimated energy of much smaller than 2.5 micro Joules with frequencies approaching 10,000 Hz. Many times, the development of geothermal resources require drilling deviated boreholes making deployment of sensor and sensor system using wireline technology impractical. To allow deploying our optical vector seismic sensors in deviated or horizontal boreholes, which will be common during the development of EGS resources, we developed a small diameter drill pipe-based borehole deployment system which includes a very robust clamping system. This system has proved to eliminate all the tube waves traveling in the borehole greatly enhancing the borehole seismic data and simplifying the data processing. In addition to the outstanding data this system can be designed to operate at extremely high temperatures. All the components, such as seals, in the current deployment system are rated to at least 300°C but can be upgraded to all metal components with a projected temperature rating of 1,000°C. |