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4D simultaneous PP-PS pre-stack inversion: the Edvard Grieg field, Norwegian North Sea
Held, Marihelen M.
Held, Marihelen M.
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2023
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Abstract
The Edvard Grieg field was discovered in 2007 in the Norwegian North Sea and in an active development phase as operator Aker BP and partners, OMV and Wintershall DEA, monitor the production and injection. Production began in November 2015, and water injection began in July 2016. Three OBC 4C seismic surveys were acquired in 2016, 2018, and 2020.
The oil bearing reservoir is composed of composed of aeolian sands, fluvial sands, alluvial conglomerates, and shallow marine sands and bounded by unconformities as most of deposition occurred subareally. The reservoir is capped by a regionally extensive chalk unit. This thesis conducts conventional PP pre-stack inversion and joint PP-PS pre-stack inversion on time-lapse seismic data to improve the S-impedance. Introducing the converted wave improves both P- and S-impedances in our final results as joint inversion shows that the S-impedance is not sensitive to saturation changes.
We introduce a new workflow which converts impedances to reservoir dynamic properties. We extracted the maximum values from 4D effects to calibrate to pressure and saturation changes. By calibrating the results of inversion to reservoir dynamic properties, this can lead to more detailed conversations among asset teams as they determine reservoir compartmentalization, monitor injected fluids, identify un-swept areas of the field, and alter depletion plans.
From the calibration process, there were four identified 4D changes: saturation increase, saturation decrease, pressure increase, and pressure decrease. Using five specified cases, the observed 4D effects were classified as one of the 4D changes. Between 2016 and 2020, the water saturation (saturation increase) of the field has increased on average from a maximum of 38% to 45%. The pressure decrease responds to production and grows in relation to development over the years.
With the addition of a new seismic vintage since this project was last analyzed, the geologic interpretations were updated. Previously, the interpretation identified a baffle blocking water flow between two injectors. This research found contradictory evidence as the 2020 survey shows water breakthrough, indicating this area is a zone of lower permeability than the surrounding areas.
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