Development of a Low Noise Optical Interrogator for Interferometric Sensing Technologies

Electronic geophones are common sensors in the oil and gas industry for both exploration and development applications. They have great sensitivity, especially at low frequency, but they cannot meet the ever-more demanding technical requirements of seismic sensing. The need is for higher frequencies and lower magnitude events, outstanding vector fidelity, higher pressures and temperatures and deeper boreholes. Electronic geophones are not capable of measuring and mapping a vast majority of the low magnitude signals that are being produced by the hydraulic fracturing process. Furthermore, the electronic systems deployed today use sensors with limited vector fidelity which limits the accuracy of the location of the events that are mapped. Only fiber optic based sensors can achieve the stringent requirements of accurately mapping the complex pattern of hydraulic fracturing using small amplitude events. Fiber vector sensors are not as complex as their electric counterpart which makes them more robust. They are furthermore intrinsically safe since no electric power is used or generated. However, the optical interrogator used for these sensors is a complex precision instrument. Optical interferometric interrogators are not commercially available and they are not easy to design; the improved sensing comes at the cost of increased interrogator complexity. With most of the major players in the seismic industry developing their own proprietary optical interrogators using Rayleigh scattering for DAS sensors, there is no one providing commercially available interferometric interrogator units. A broadband, multi-channel low noise optical interrogator is required to monitor near-wellbore and far-field effects during hydraulic fracturing. In Phase 1 a large channel count interrogator for interferometric fiber optic seismic sensors will be designed and prototyped that is capable of being multiplexed into systems containing hundreds of channels. The interrogator will support interferometric fiber optic sensors using an active modulation process. The interrogator will be tested and verified against current interrogator technologies at Paulsson, Inc.’s facilities. A low noise, push-pull Michelson interferometer sensor will also be constructed to test the noise capabilities of the newly developed interrogator. A comparison will be made between current interrogators and this newly developed low noise interrogator. The sensor will be exposed to simulated environments in a laboratory setting while recording data to validate and optimize the design. There are currently 82,000 wells in the United States that have been fractured, and a backlog of wells awaiting fracture stimulation since the price of oil significantly dropped midway through 2014. Once solutions are available for monitoring near-wellbore and far-field effects, hydraulic fracturing it is likely to be legislated that such monitoring must be performed. The proposed low noise interrogator system will cost effectively monitor and map hydro fracture operations making the process much more attractive.