Characterisation

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Elastic seismic inversion Elastic seismic inversion is an established industry technique used to measure parameters related to the density and strength of rocks when subjected to compression and shearing forces. These parameters depend on the minerals and structure of the rock, and the fluids it contains. Starting with seismic images of waves that reflect from the rock layers at different angles, elastic inversion builds a 3D picture of the rock parameters under the surface. Maersk Oil has a long history of using elastic seismic inversion. For example, in the Danish chalk fields, standard seismic images may not show the crucial top chalk structure clearly under certain circumstances. However, the chalk has characteristic elastic properties, and elastic seismic inversion is the key to extracting that information from the data. Since the development of Jigsaw, Maersk Oil has carried out large numbers of elastic seismic inversion projects, helping to map out potential reservoir rocks, and the fluids they contain. We have used elastic seismic inversion in fields around the world, including but not limited to Qatar, Angola, the Gulf of Mexico and the North Sea. The physical principles behind elastic inversion are equally applicable in different geological settings, whether the imaging target is tight limestone and chalk, or sandfilled turbidite channels. The next generation of seismic inversion technology is already under development by our subsurface technology researchers. This new method makes use of sophisticated mathematical techniques to take our inversion results beyond elastic properties and to the real geological parameters in the subsurface. This development is done in collaboration with the Norwegian University of Science and Technology (NTNU), where Maersk Oil is part of a research consortium focused on Uncertainties in Reservoir Evaluation. Inversion ... is a process that estimates something we can’t measure directly, such as the 3D subsurface distribution of elastic parameters, by analysing a related quantity that we can measure, such as a seismic image. The subsurface is complex, and although estimates of the subsurface properties can be computed, they will always be uncertain. The inversion approach allows many different types of data and constraints to be combined, minimising this uncertainty and improving the reliability of estimates.
4D Seismic Imaging Maersk Oil uses 4D seismic imaging as a key reservoir monitoring and production tool. Our recent survey of the Halfdan field in the Danish North Sea has produced what are arguably some of the best 4D images in the industry to date, and our proprietary technology has been responsible for a large part of that success. The fourth dimension in 4D seismic is time. When we repeat 3D seismic surveys, months or years apart, we see changes in the seismic response, caused by fluid movements and changes in pressure due to the hydrocarbon production process, which in turn change the elastic properties of the rock. Changes in the subsurface cause both changes in the strength of reflections and subtle vertical shifts between seismic images. Rather than ignore the vertical shift, or merely correct for it, Jigsaw combines it with the reflection strength to enhance the 4D image. Integrating geostatistical constraints allows Jigsaw to reduce noise and produce coherent images which our engineers use to track processes such as where water goes when it is injected into the ground, and which parts of the reservoir benefit from the pressure support that water injection provides. Maersk Oil planned and executed the acquisition and processing of the data and performed all of the analysis and inversion projects for the 2005- 2012 Danish Underground Consortium 4D project, which was used for 30 inversion studies over 4 major oil fields. Map of Upper Tor Formation proportional acoustic impedance change from 1993- 2005 (left) and proportional acoustic impedance change from 2005-2012 (right) - Halfdan field. ~400ft Top Tor Intra Tor AI Increase, hardening Top Tor Intra Tor AI Increase, hardening ~400ft AI Decrease, softening AI Decrease, softening
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Characterisation

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