C009 Wide Azimuth 3D 4C OBC A Key Breakthrough to Lead to the Development of Hild Field

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1 C009 Wide Azimuth 3D 4C OBC A Key Breakthrough to Lead to the Development of Hild Field D. Vaxelaire* (Total SA), K. Kravik (Total E&P Norge), F. Bertini (Total E&P Norge) & J.M. Mougenot (Total SA) SUMMARY Seismic imaging is a major challenge for some complex fields in the North Sea. That is the case for the Hild Brent field discovery (Total E & P Norge operator) not yet developed due to inconsistent dynamic data and poor seismic image resulting from a seismic obscured area created by gas dismigration, strong energy multiples and a highly faulted reservoir. Conventional 3D streamer data, despite several reprocessing did not allow reliable mapping of the reservoir geometry. Total decided to acquire in 2003 a multi-azimuth 3D streamer survey, following positive experiences in the nearby UK fields (Dunbar/Alwyn) and a successful example on Varg field (Hegna and Gaus, 2003). The combined PSDM processing provided a significant improvement, however it was not considered as good enough to properly assess the main uncertainties related to the drilling of development wells. As demonstrated on a similar OBC survey (Boelle et al. 2006), a high density wide-azimuth OBC geometry was considered as the most efficient solution and a 3D 4C OBC survey was acquired in In this paper on Hild, we present the preliminary results and analysis based on fast track PSDM which proved the great potential of high density wide azimuth OBC 3D for imaging challenging targets.

2 Introduction Seismic imaging is a major challenge for some fields in the North Sea where multiples interfere with a strongly faulted reservoir. That is the case for the Hild complex field where conventional towed streamer 3D could not properly image the Brent reservoir despite several attempts at reprocessing. Innovative seismic methods were then attempted to improve target illumination and seismic imaging. A high density 3D multi-azimuth towed streamer programme was first acquired in 2003, but did not deliver sufficient improvements. A wide azimuth 3D using Ocean Bottom Cable (OBC) survey was later acquired in In this paper we present the first results improvements from Hild 3D 4C OBC compared to multi-azimuths streamer, to highlight the benefits of wide azimuth OBC in a complex environment. Context Hild field is operated by Total E&P Norge AS and consists of three Brent group gascondensate field reservoirs which were discovered 30 years ago but are not yet developed. The field is located in the Norwegian sector of the North Sea along the UK border, in water depths ranging from 90 to 130 m. The main structure is a strongly faulted horst, trending a NNW-SSE. The Middle Jurassic Brent reservoirs are characterized by their high pressure (750 bars) and high temperature (127 C) and sealed by Upper Jurassic shales. Different pressure regimes, fluid contacts and faults were encountered by the drilled wells, confirming the compartmentalization of the reservoirs. There are risks and uncertainties related to the poor seismic imaging. Two towed conventional streamers surveys acquired respectively in 1982 and 1991 did not allow a reliable mapping of the reservoirs. Within the main accumulation (Hild East), the strongly faulted Brent reservoir is poorly imaged due to interfering multiples generated by the Base Cretaceous Unconformity (BCU) and to a Seismic Obscured Area (SOA) created by gas dismigration in the overlaying Cretaceous series (Figure 1). High fold multi-azimuth streamer 3D attempt Following Total s experience in the nearby UK fields of Dunbar/Alwyn and a successfully published example from Varg field (Hegna and Gaus, 2003), a high-density and multiazimuth 3D streamer seismic acquisition was considered as a cost effective solution to enhance the seismic image. West East Shadow from tertiary sands Azimuth 1 N 166 E Azimuth 2 N 038 E 1.0 sec Strong Lower Cretaceous reflectors Dismigrated Gas Multiples Area of interest BRENT reservoir BCU Structural Complexity Seismic Obscured Area (SOA) 1999 Azimuth 3 N 098 E Figure 1: Hild imaging issues Figure 2: Multi-azimuth towed streamer.

3 For this purpose two new surveys were acquired in 2003 in such a way that the two new data sets and an existing 3D 1991 made an angle of 60 degrees with respect to each other (Figure 2). The combined PSDM processing of the three surveys resulted in an improved seismic image at target level (Kravik et al. 2005). The reservoir structural description confirmed the reservoir volumes and the main structural segments. However it was not regarded as good enough to properly address the main uncertainties related to the drilling of development wells within Brent reservoirs (Figure 4 a). Wide-azimuth high density OBC 3D solution The encouraging results from a 2D OBC test and examples from recent OBC surveys acquired in the North Sea with similar Brent objectives (Kvitebjørn, Volve and Bruce fields) convinced Total and its partners to consider an high-density wide-azimuth OBC 3D acquisition as the most probable solution to best improve the seismic image within the SOA. As demonstrated in a similar OBC survey (Boelle et al. 2006), such geometry would make the following improvements possible: Improved S/N ratio resulting from higher fold and from receivers being placed on the quieter seabed. Improved illumination of the target due to wider azimuth sampling. Improved water-layer multiple attenuation resulting from a combination of the hydrophone (P) and vertical geophone (Z) components (PZ summation). Improved resolution as the improved S/N ratio will enable spectral widening of the data. Survey design and Acquisition highlights The nominal recording swath geometry consisted of: - 4 receiver lines of 9 km length separated by 400 m, with 4 Components receivers every 25 m (more than 5000 channels were recorded for each shot), shot lines of m length separated by 50 m, with one shot every 50 m of dual sources shooting parallel to the receiver cable. The acquisition geometry was designed in such a way that each receiver records data from a Shot Sail line 12600m Shot pattern area width 8400m 3600m 9000m 400m 3600m Shot line Shot grid 50m X 50m Figure 3: High fold and wide azimuth geometry. Receiver cable Common receiver gather = 42,336 SPs dense, long offset and full-azimuth grid of shots points (at least 4000 m along the inline direction and 3600 m along the crossline direction). More than shots were recorded by each receiver. (Figure 3). Such a dense shot grid (50 m by 50 m) enables a real 3D pre-processing in the common receiver domain for a better noise attenuation (Soudani et al. 2006). 26 receiver lines were required to cover 76.5 km 2 while the shot area was about 160 km². The survey was acquired in 5 swaths: 4 conventional swaths and a so-called Super Swath. A very high fold was achieved (more than 600 within a m offset range) compared to a

4 total fold of about 250 resulting from the combination of the 3 narrow-azimuths streamer surveys. Another indicator highlighting the huge operational effort is the trace density as summarized in Table 1. Narrowazimuth Streamer M ulti-azimuth Streamer m Wide-azimuth OBC Offset range Fold Coverage (40 to) Trace density per km² Table 1: Trace density comparison wide azimuth OBC versus narrow and multi- azimuth streamer At the time, this OBC acquisition was a world record in terms of cable length and number of recorded channels per shot. During the acquisition of the last super-swath, 10 receiver lines representing 65.6 km of cables and active channels were recorded for each shot. The super-swath improved the coverage and the fold distribution in the far offsets and reduced the overall acquisition duration by 12 days and (Pope et al., 2006). Another innovation during this acquisition was the use of a High-Density acoustic network to position the cable. This was achieved with a 25 m acoustic receiver interval along the cable and an acoustic positioning performed at the same time as seismic shooting. Final receiver cable position was then derived from both acoustic positioning and seismic records first break picking. The data acquisition started on July 24 th 2005 and lasted 101 days. The operations were performed by WesternGeco with their Q Sea Bed technology. The use of three large vessels for data recording, cable laying and source, was a key point of the good overall performance and to achieve an average daily production of 58 km shot sail line despite time share operations and adverse weather conditions. Processing highlights The final PSDM processing is not completed at the date of this abstract. Yet a fast track PSDM processing has been delivered to the interpreter. Particular care was given during the time pre-processing regarding the PZ summation, calibration process and the coherent/random noise removal process, and including the following main steps: Source de-signature, using a calibrated computed far-field signature derived for each shot from the near-field hydrophone records. Prior to PZ summation, the Z receiver gathers (geophones) were calibrated on P receiver gathers (hydrophones) by using the up-going wave-field which gives better results than direct wave event. Noise attenuation of the direct arrival based on Tau-P modeling and subtraction. The results were very beneficial for far offsets to define velocities and anisotropy parameters. Scholte waves and mud-roll removals. A noise model was derived using a FK 2D dip filter in the low-frequency domain and subtracted to the shot points. The second phase of the OBC processing (currently in progress) will focus on: Merging the final PSDM OBC data with the three existing streamer datasets Real 3D pre-processing to attenuate the direct arrival in the common receiver domain Converted wave processing of the 4 components.

5 Results and Conclusion The interpretation is still ongoing, yet the step change in data quality brought by this wide azimuth OBC acquisition exceeds our expectations. It allows better identification and mapping of faults, tracking the top of the Brent reservoir and consequently better understanding of field compartmentalization. A first comparison with the original multi-azimuths streamer survey shows significant improvements as illustrated in a North-South seismic section crossing the SOA within the Hild East structure (Figures 4 a & b).: The bandwidth has been increased by a better restitution of the Low Frequencies. The amplitude attenuation due to the gas cloud is partially compensated and the S/N is higher. The reservoir interval is better imaged and easier to track. The imaging of faults is enhanced with sharper geological events, as shown also from coherency attributes (Figures 5 a & b). The continuity of the deepest geological events controlling the structural pattern at Brent level is improved. The first results and analysis of the Hild 3D OBC already demonstrate the high potential of high density wide azimuth OBC 3D for imaging challenging seismic targets. (a) (b) TopBRENT Reservoir Top BRENT Reservoir BCU BCU 1000 m Base BRENT Reservoir 1000 m Base BRENT Reservoir Figure 4: PSDM stack section zoomed at target, (a) Multi azimuth streamer Final PSDM, (b) Wide azimuth OBC Fast track PSDM.

6 (a) (b) Figure 5: Structural Coherency attribute zoomed at level close to Top Brent Reservoir (BCU + 120m). Comparison between Multi-azimuths Streamer survey (a) and Wide azimuth OBC survey (b). Acknowledgements The authors would like to thank Total and its partners of PL 040/043 for permission to publish this paper. They are also grateful to their Total colleagues Thierry Castex and Jean-Luc Boelle for their technical support. References Kravik K., Sexton P., Lemaistre L., Aubin V., Riou A. and Bertini F. [2005], Hild Multi Azimuth seismic experiment. EAGE 67th Conference & Exhibition, Madrid, June Boelle J-L., Ricarte P., Suiter J., [2005], Sparse receiver and multi-azimuthal simulations from a high fold OBC campaign in the UK North Sea, 75th Ann. Internat. SEG., Expanded Abstracts. Hegna S. and Gaus D., [2003], Improved Imaging by Pre-Stack Depth Migration of Multi Azimuth towed streamer seismic data, EAGE 65th Conference & Exhibition Stavanger, Norway, June Pope J., Vieira C., Vaxelaire D., Kravik K., [2006], Benefits of the 10,000 Channel OBC Acquisition Technique Hild 4C Survey, Petroleum Exploration Society of Great Britain PETEX November Soudani M.T.A., Boelle J.L., Hugonnet P., Grandi A., [2006], 3D methodology for OBC pre-processing, EAGE 68th Conference & Exhibition Vienna, Austria, June 2006.