[1] A moderate size seismic event on 7 May 2001 was strongly felt on platforms in the Ekofisk oil field, in the southern North Sea, but did not cause damage to platforms or wells. We combined near-and far-field observations to develop a consistent source model and to determine whether the event was induced. Seismic data placed the epicenter inside the Ekofisk field and suggested a shallow source depth based on spectral and moment tensor analysis. GPS data from the Ekofisk platforms displayed permanent vertical and horizontal movement due to the event. A topographic bulge in the sea bottom, revealed by differential bathymetry data, and overpressure in the overburden in the northeastern part of the field, detected only after the event, had been caused by unintentional water injection that started in 1999. The injection pressure and rate were sufficient to raise the overburden. Pressure gauge and compaction data ruled out that the event occurred at reservoir level, which was further supported by unaffected production rates and absence of well failure. We therefore conclude that the event occurred in the overburden, at less than 3 km depth. Initially, this appeared unlikely on account of very low shear strength of the overburden clay-rich shale and mud rocks. The seismic event was induced owing to stress changes caused by water injection. The event possibly initiated on the northern flank of the field near the water injector and may have involved flexure of the overburden into the depression bowl in the rest of the field. Moment tensor analysis is consistent with a pure double-couple source. We suggest that slip occurred on the near-horizontal rather than along the near-vertical nodal plane. Stress drop was low, and owing to the low overburden shear strength, the event released less energy than a typical stress drop event with similar source dimensions.Citation: Ottemöller, L
A complete series of 3D walkaway borehole profiles have been collected for the Ekofisk Field that are expected to image most of the crestal area of the field. This area has gas present in the overburden which obscures the seismic imaging using conventional surface seismic data. Good compressional images are obtained by undershooting the gas zones and taking advantage of the small Fresnel zones using the 3D walkaway technique. Considerable compressional to shear wave conversion was observed at the reservoir level in these profiles. Shear waves may be particularly suitable for imaging because they are less affected by gas than the compressional waves. Several 3D walkaway borehole profile surveys have been used to derive a 3D up-going shear image. This paper describes the results of this shear wave work from one of these surveys.
The Ekofisk Field is currently undergoing a major field re-development in which 45 new wells will be drilled before the end of 1998. This requires that the most comprehensive and detailed reservoir description and geological and fluid flow models be used as the basis for the planning of such a redevelopment. This situation, as well as new developments in hardware and software and multidisciplinary database and applications integration, led to the decision in 1994 to completely re-evaluate the reservoir characterization of the field. A major multi-disciplinary effort involving geoscience, petrophysical and reservoir engineering work was initiated through the Ekofisk reservoir characterization (ERC) project. The objective of the reservoir characterization project was to improve the existing reservoir description using all available data through the application of new techniques and technology, and to construct and history match a new 3D reservoir fluid flow model using this updated, detailed reservoir description. Greater demands are being made on geoscientists and engineers to model and manage processes taking place within the reservoir. At the same time systems are evolving which allow large and complex models to be developed and modeled in simulators. The development of these models, and asset management based on these models, are becoming requirements for effective reservoir management. Our biggest challenge is to maintain, update and interchange data between the large 3D reservoir description models and fluid flow models through integration of new data. Up-scaling of the detailed description produced the highest resolution model that is computationally manageable. The resulting history-matched fluid flow model provides the primary reservoir management tool for the field re-development programme and for the evaluation of reservoir and geoscience monitoring technologies. Down-scaled reservoir parameters are currently being integrated with petrophysical data and laboratory core analysis to drive seismic forward modelling of present and future reservoir conditions. These seismic simulations are being used to evaluate the implementation of a time-lapse seismic (4D) monitoring programme for the field.
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