Development of a Distributed Optical Sensor Array for Improved Subsurface Characterization and Monitoring

Today 80% of the produced oil and gas from unconventional oil and gas reservoirs is from 20% of the hydro fracturing operations. Although geophone based micro-seismic monitoring has improved the hydraulic fracturing process, conventional micro-seismic monitoring alone is not sufficient to optimize the process. To improve the hydraulic fracturing process new instruments must be developed to record new types and larger volumes of better quality seismic data. Cost-effective solutions are also required so that society may understand the entire fracturing and water disposal process through wide spread use of cost-effective sensors that will provide improved characterization and monitoring. Electronic solutions usually record only low frequencies and only detect larger signals and have thermal limits, leading to limited applications. Electronic geophones are not capable of measuring and mapping most of the low magnitude, high frequency signals that are being produced by the fracturing process. Furthermore, the arrays deployed today are too short, which limits the positioning accuracy of the mapped events. Longer aperture antennas are required to accurately map micro seismic events.Borehole seismology can map geologic formations in outstanding detail; it has also been shown that borehole seismology can effectively monitor fluid and gas injection processes. A single channel interferometric fiber optic seismic sensor was developed in Phase I that is capable of being multiplexed into systems containing hundreds of sensors generating a large aperture antenna. This will be created by writing reflectors into a fiber whereby the fiber between reflectors becomes the sensor. This type of Distributed Optical Seismic Sensor (DOSS) combines the positive attributes of Distributed Acoustic Sensor (DAS) having a large aperture with the positive attribute of interferometric interrogation which is orders of magnitude more sensitive than DAS. The fiber will be placed inside a ¼” Inconel tubing to finalize the sensor manufacturing. An optical seismic interrogator will be designed and manufactured that can multiplex enough sensors to monitor a 5,000 ft long fracturing process. The sensor will be exposed to simulated environments in a laboratory while recording data to validate and optimize the design. Applications According to the Department of Energy, at least 2 million wells in the US have been hydraulically fractured. There is also a large backlog of wells awaiting fracturing since the price of oil significantly dropped midway through 2014. The proposed large aperture system will cost effectively monitor and map these operations. The low sensor cost and the sensitivity will make them attractive for deployment in fracturing operations. Once cost-effective solutions are available for monitoring hydraulic fracturing it is likely to be legislated that monitoring must be performed.