We have acquired and processed 3D vertical seismic profile (VSP) data recorded simultaneously in two wells using distributed acoustic sensing (DAS) during the acquisition of the 2012 Mars 4D ocean-bottom seismic survey in the deepwater Gulf of Mexico. The objectives of the project were to assess the quality of DAS data recorded in fiber-optic cables from the surface to the total depth, to demonstrate the efficacy of the DAS VSP technology in a deepwater environment, to derisk the use of the technology for future water injection or production monitoring without intervention, and to exploit the velocity information that 3D VSP data provide for evaluating and updating the velocity model. We evaluated the advantages of DAS VSP to reduce costs and intrusiveness, and we determined that high-quality images can be obtained from relatively noisy raw 3D DAS VSP data, as evidenced by the well 1 image, probably the best 3D VSP image we have ever seen. Our results also revealed that the direct arrival traveltimes can be used to assess the quality of an existing velocity model and to invert for an improved velocity model. We identified issues with the DAS acquisition and the processing steps to mitigate them and to handle problems specific to DAS VSP data. We described the steps for conditioning the data before migration, reverse time migration, and postmigration processing to reduce noise artifacts. We outlined a novel first-break picking procedure that works even in the absence of a strong first arrival and a velocity diagnosis method to assess and validate velocity models and velocity updates. Finally, we determined potential applications to 4D monitoring of fluid movement around producer or injector wells, identification of active salt movements, and more accurate imaging and monitoring of complex structures around the wells.
SUMMARYIn 2012, Shell acquired its first dual-well 3D DAS-VSP (Distributed Acoustic Sensing -Vertical Seismic Profiling) data concurrently with an OBS survey in a deep water environment in the Gulf of Mexico. A work flow was developed to process the 3D DAS-VSP data and use the VSP first arrival times to diagnose the velocity models which were derived from previous surface seismic surveys for selecting a suitable initial velocity model to be updated with the travel time tomography inversion. The VSP diagnosis method was applied again to the updated VTI-inversion model to ensure the velocity updating effort is in the right track. Both DAS-VSP and OBS data were migrated with the initial VTI velocity model and the updated VTI-inversion model. It is found that both borehole and surface seismic images generated with the VTIinversion model are significantly improved from those obtained with the initial model, especially for the seismic amplitudes at the target event. In this paper, we will share our processing procedure and some learning from our practices of processing marine 3D DAS-VSP data. Processing results from borehole seismic data acquired in Well 1 will be presented as examples.
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