Tu N Maximising the Value of Seismic Data for Continued Mature Field Development in the East Shetland Basin

Transcription

1 Tu N Maximising the Value of Seismic Data for Continued Mature Field Development in the East Shetland Basin P. Mitchell (TAQA Bratani Limited), J. Raffle* (ION GXT), P. Brown (ION GXT), I. Humberstone (Axis Well Technology) & T. Malloch (formerly of TAQA Bratani Ltd.) SUMMARY Careful reprocessing of seven vintage surveys covering oil fields in the East Shetland Basin was undertaken, eventually leading to a high quality presdm deliverable. Attention to preserving all available long-offset information was especially beneficial in improving velocity resolution. In the case of this dataset, structure and stratigraphy are more clearly imaged and the fields can be seen in their broader geologic context. The data have already been used to guide development and appraisal well drilling as well as confirming and updating existing geological models.

2 Introduction TAQA operates a number of giant, but mature, oil fields in the East Shetland Basin (ESB) of the Northern North Sea including North and South Cormorant, Tern, Pelican, Eider and Otter. It has also developed a new field in the area called Falcon, and has two new field developments in progress at Cladhan and Cormorant East. TAQA currently produces approximately 50,000bopd from these assets. Acquisition of new seismic data in these areas is difficult due to existing infrastructure and the close proximity of platforms can make undershoot impossible. In 2012 TAQA embarked on a re-processing project of multi-survey vintage data covering TAQA s major North Sea assets. Inputs comprised vintages between 1995 and 2012 and included the merge of conventional narrow towed streamer and broadband data acquired with PGS Geostreamer, the intention being to use vintage data to fill the holes in the modern acquisition. Recent enhancements in shallow water demultiple techniques and noise removal methods allowed a re-working of the pre-processing sequence, resulting in cleaner gathers with useful information at their longest offsets. The use of 3D regularisation, interpolation and careful trace selection prior to migration allowed the generation of a contiguous merged volume that has already provided significant value, enabling positioning of a new and successful development well. Input Data Quality The data covered some 2900 km 2 and comprised seven vintages of mixed azimuth data: Tern-Eider (1995, 90 o Azimuth, 3600m Offset); North Cormorant (2002, 0 o Azimuth, 4050m Offset); South Cormorant 1 (2002, 90 o Azimuth, 4050m offset, single source); South Cormorant 2 (2002, 90 o Azimuth, 4050m offset, dual source)esb09 (2009, o Azimuth, 4800m offset); ESB11 (2011, 159 o Azimuth, 7050m offset); and EOK12 (2011, 137 o Azimuth, 6000m offset). Data quality was variable due to acquisition from different vintages and times of year. Careful consideration of denoise for each input survey was required to ensure optimal denoise. Figure 1 shows representative data from the ESB09 survey before and after denoise. Figure 1 Left: ESB09 shot gathers before and Right: after denoise including removal of seismic interference. Maximum offset is 4800 m.

3 Advanced Demultiple and Pre-processing Water depths of m resulted in a significant amount of short period multiple. In order to target this, ION-GXT s proprietary shallow water demultiple was used. Long period multiples were targeted with a delayed-start 3DSRME and residual multiple then attenuated using a high resolution Radon filter Figure 2 shows the results of the overall demultiple sequence. Figure 2 Left: ESB09 stack before demultiple; Right: Stack following application of full demultiple route. 3D regularisation of each input survey on its native grid was performed along with an interpolation of traces to give a finer trace spacing of 12.5x12.5m. This helped to de-alias the input and ensure data was similarly sampled across the inputs. An offset based FXY deconvolution helped further attenuate random noise across the surveys to give a clean input to migration. Intelligent selection of traces on each offset ensured that optimal traces were used to fill holes and within overlap zones. Survey merging The ESB09 survey was used as the master survey and so all other inputs were matched to this. Matching comprised application of an amplitude scalar and phase/timing shifts. Further to this a convolutional matching filter was applied to the conventionally acquired data to match the broader bandwidth of the ESB11 and EOK12 surveys (Figure 3). Figure 3 Left: ESB09 Stack before match; Centre: ESB09 Stack after match; Right: ESB11 stack (regridded to ESB09 grid).

4 Model Building Five iterations of hybrid gridded tomography (Jones, 2010) were utilised in the model building update. Calibration against several well markers was made throughout the processing to help guide anisotropy parameters. The use of TTI anisotropic migration further enhanced the image. The six surveys were migrated together and following RMO correction and further trimming to remove migration swings that would have degraded the stack response, a final stack was created. In addition, to assist in interpretation, each input survey was also migrated separately so as to assess azimuthal variation in illumination. In Figure 4 is shown a comparison of the migrated images (plus colour overlay of the interval velocity models) for the initial and final stages of the velocity model building. It is clear that the final velocity model conforms to the structure (as expected in this environment), leading to a clearer image. Figure 4 Left: Initial starting velocity and migration. Right: Final results after 5 iterations of hybrid gridded tomography. In Figure 5 is shown the comparison between the vintage PreSTM and final PreSDM image for a crossline involving the ESB09 and ESB11 data. The deeper imaging, especially fault definition, is significantly improved. Figure 5 Left: Vintage PreSTM (Cross-line); Right: ION-GXT final PreSDM showing merge of ESB09 to ESB11 (converted to time).

5 Figure 6 shows the vintage PreSTM versus final comparison PreSDM images for an inline involving the Tern-Eider data. Again, the deeper imaging is significantly improved. Figure 6 Left: Vintage PreSTM (in-line); Right: ION-GXT final PreSDM showing Benefit of including well matched Tern-Eider Data (converted to time). Conclusions It is well known that reprocessing seismic data after a few years can often result in uplift in the data quality. The benefits of enhanced demultiple technology and attention to detail in the pre-processing can allow the successful merge of data acquired decades apart. In addition, if the uplift in the prestack data quality is significant, especially in preserving the longest available offsets, then subsequent velocity model building can be enhanced resulting in greatly improved imaging. In the case of this dataset, structure and stratigraphy are more clearly imaged and the fields can be seen in their broader geologic context. The data have already been used to guide development and appraisal well drilling as well as confirming and updating existing geological models. Acknowledgements We thank TAQA and partners, and PGS for permission and encouragement to present this work, Ian Jones for help in preparing this manuscript, and to ION GXT for permission to present the processing technology. References Jones, I.F. [2010] An introduction to velocity model building. EAGE, ISBN , 296.