G034 Improvements in 4D Seismic Processing - Foinaven 4 Years On

Transcription

1 G034 Improvements in 4D Seismic Processing - Foinaven 4 Years On C. Lacombe* (CGGVeritas UK), S. Campbell (BP Aberdeen) & S. White (CGGVeritas UK) SUMMARY Using a case history from West of Shetlands, the impact of improvements in seismic processing technology over the last 4 years on 3D imaging and 4D seismic repeatability is demonstrated. We show that the quality of the 4D dataset has been greatly improved, with a reduction of background noise and therefore a better stand-out of the 4D difference. NRMS levels have decreased from 0.31 to 0.14 with this new processing sequence. A combination of processes is contributing to the repeatability improvement. The quality of the final result is therefore due to a mix of new technology, the use of previous processing results and the continuous work of fine-tuning the 4D sequence.

2 Introduction The Foinaven-Schiehallion-Loyal area, West of Shetlands on the UKCS, is a significant production area for BP and its co-venturers. 4D seismic has been successfully used in this region for infill drilling and reservoir management since the late 1990 s, with a new monitor survey acquired every two years. Whilst the earliest monitor surveys were acquired without 4D specification, all later monitors are designed for 4D (Campbell et al., 2005). In parallel, a large effort has been made in the 4D processing of these data in order to improve the quality and repeatability and so facilitate the interpretation and reservoir monitoring. The West of Shetlands 4D towed streamer datasets have now been reprocessed three times during the last decade with each new processing sequence integrating the latest technology solutions. Key processing challenges from this area are: water column variations, which can introduce sail-line to sail-line time shifts of the order of ms (Wombell (1996)), the strong multiples which overlap the reservoir signal and the low level of primary energy below the Top Balder. Using data from Foinaven, one of the three BP-operated fields in the area, this paper shows how improvements in processing technology over the last 4 years have enhanced both the quality of the 4D seismic repeatability and 3D imaging. Acquisition geometry The Foinaven baseline was acquired in 1993, before production started. It was followed by 4D monitor acquisitions in 1999, 2000 and In 2004, a dense 4D monitor was acquired by attempting to repeat source positions, making feather predictions from current observations and using overlapping swaths to closely repeat the receiver positions and to minimize infill (Campbell et al, 2005). Further monitor surveys using the same acquisition strategy have been acquired in 2006, 2008 and 2010, giving a total of 6 vintages to process in 2010 (1999/2000 were not reprocessed). Processing history The Foinaven datasets have been reprocessed three times during the last decade, each reprocessing sequence taking advantage of the latest available technologies. The 1993 and 2002 vintages were first processed in 2002 using a DMO/Stolt time migration style sequence. At the time, we did not use any simultaneous processing algorithms and the only true 4D steps integrated within the sequence were global matching, signature matching and 4D differential statics D processing In 2004, the 4D processing sequence was changed to incorporate full offset Kirchhoff pre-stack time migration, high density velocities, improvements in water column correction and regularization. The data were parallel (or co-) processed, with deterministic wavelet matching and new simultaneous processing algorithms such as 4D binning and acquisition footprint removal. Between 2004 and 2006, 4 vintages (1993, 2002, 2004 and 2006) were reprocessed with this flow. Hoeber et al. (2005) showed the improvements in the image of a 4D vintage processed with this 4D parallel processing, versus the same data processed with the DMO/Stolt sequence used previously. They showed that with careful 4D processing improvements in seismic repeatability (NRMS) of 20-25% were achieved when compared to the previous processing D processing In 2007, an opportunity to incorporate further developments in processing technology was recognized and a new processing sequence was designed. It had the potential to improve the 3D and to allow delivery of higher quality 4D products for the planned 2008 and 2010 acquisitions. In this new sequence, deterministic processing is employed whenever possible, therefore avoiding statistical methods which could be adversely impacted by noise. These processes use source and receiver depth

3 for static correction and recorded or modeled signatures for shot-by-shot correction and directional designature respectively (Lacombe et al 2008). Improved 3D processes, which are important for obtaining reliable 4D signals have been integrated in the sequence: better water column correction with GPS measured tide correction (Lacombe et al. 2009), improved demultiple (albeit using the same Radon-based algorithm as in processing), better amplitude correction and gather flattening, improved 3D regularization and 3D denoise (Whitcombe et al, 2008). The use of true 4D processes, such as 4D binning, matching, differential statics and co-filtering techniques (Zamorouev et al, 2006), ensures that the vintages and their 4D S/N are progressively improved. Finally the time migration used in has been replaced by a Kirchhoff pre-stack depth migration. The 1993 and 2006 vintages were first processed as a pair and each additional vintage was consecutively reprocessed as a 4D repeat relative to the 1993 base. This means that multi-vintage algorithms have seen limited use in this processing flow. Figure 1 shows the effect of the water column correction on CDP gathers and on the time-shift maps. Prior to correction (Figure 1, Left), the variations in water column (velocity and tide) and source and receiver depths manifest themselves as jitters on CDP gathers and sequence related stripes on time-shift maps. After correction (Figure 1, Right), jitter is reduced and 4D timeshifts are more laterally consistent. Improvement in final results Before Correction CDP gather 4D time-shifts Hoeber et al. (2005) compared the first two processing sequences ( and ) and concluded there was significant improvement in the 3D image and in 4D repeatability. We compare here the results from the latest two processing sequences ( and ). After Correction CDP gather 4D time-shifts -4ms 8ms -4ms 8ms Figure 1: Left Example of CDP gather and 4d timeshift maps before water column correction. Variations in water column between sail-lines result in jitters in the gathers and lateral time-shift variation. Right Gather and map after correction. Jitters have been reduced and resulting time-shift is more laterally consistent. 3D comparison: improvement of imaging We first compare the improvement in 3D imaging between the two flows, keeping in mind that a 4D parallel processing may not be optimum for 3D imaging and that the imaging algorithm has changed between the two flows (pre-stack time migration for the previous processing vs. pre-stack depth migration for the latest ). Figure 2 shows a crossline comparison for the two flows and we see a marked improvement in quality with our new sequence. The figure shows a clear enhancement in resolution with more continuity in the events. Stripes Figure 2 3D section obtained with the previous (top) and new processing (bottom). We see an improvement in resolution and a stripe reduction.

4 as well as residual static errors are reduced with the new processing. 4D comparison: improvement of repeatability Figure 3 shows a 4D difference for the previous (top) and new processing (bottom). This difference, used by the interpreter, has been coloured inverted, meaning that the spectrum of the seismic has been matched to the amplitude spectra of the impedance from the well logs (Lancaster and Whitcombe, 2000). This results in a boosting of the low frequencies. After coloured inversion data have been filtered back to the useable frequencies. Clearly, the new processing greatly reduces the background noise which leads to a much better stand-out of the 4D production effect. The resolution of the 4D difference is also improved, with better event continuity and fault delineation. Figure 4 shows NRMS maps between the 1993 and 2006 acquisition for the old processing (left) and the latest one (right). NRMS has been calculated on a time window located above the reservoir (therefore does not show the 4D difference from Figure 3). A clear overall reduction of NRMS can be observed, with a median NRMS value decreasing from 0.31 to Even though some striping, inherent in towed streamer acquisition, is still visible, the magnitude of the striping has been reduced. The low repeatability area observed at the bottom left of the map (highlighted in black) is a genuine 4D difference created by gas injection. The apparently very highly repeatable area in the middle of the map (red circle) is due to the presence of the Foinaven FPSO, which was padded with the base data at an early stage of the processing (no undershoot was acquired). Figure 3 4D difference obtained with the previous (top) and new processing (bottom). We see a reduction in background noise and improvement in resolution Figure 4 NRMS maps for the previous (left) and new processing (right). The overall level of NRMS has decreased, with low repeatability stripes being reduced. The black line highlights the location of a gas injector and the red circle the position of the Foinaven FPSO. Evolution of NRMS throughout the processing sequence The progression of NRMS throughout the processing sequence for 1993 and 2006 vintages is shown in Figure 5. NRMS decreases steadily throughout the processing, with the most effective steps being the pre-processing (including water column correction and demultiple) and regularization. Figure 5 also shows that the gather processing (RMO, demultiple, time-variant static) as well as the post-stack processing (including frequency dependant structurally comfortable filtering) play an

5 important part into the reduction of NRMS to a level of 0.14 and that this reduction has not been achieved at the cost of 4D resolution, as visible in Figure 3. Discussion / Conclusion This paper shows how improvements in processing the Foinaven data over the last 4 years have enhanced the quality of the 4D seismic repeatability and 3D imaging. The 4D difference exhibits low background noise and the NRMS reaches values of 0.14, which is half the NRMS of the previous processing. Improved water column correction, demultiple and regularisation play a key part in this. However, all processing steps are reducing the NRMS. The repeatability improvement in this latest 4D sequence is therefore significantly impacted by continued development and Figure 5 NRMS reduction throughout the processing sequence for the previous (red) and new (blue) processing. refinement over the past 10 years by BP and the CGGVeritas processing teams in the dedicated inhouse processing centre in Aberdeen. In other words, the processing of the Foinaven data is benefitting from being treated in a similar manner as a Life of Field dataset. Lessons from previous processing have been learned, key data (stacks, QC) are kept and can be referred to at all major processing steps. Looking ahead, we might wonder which level of seismic repeatability can be achieved with towed streamer acquisition on Foinaven data. Some areas of processing could still be improved (the undershoot areas have lower repeatability, acquisition related striping is still an issue) and some technology is already flagged for application on the next round of reprocessing (use of SRME, change of binning strategy). Acknowledgements This paper is being published with permission of CGGVeritas and BP. We thank the Foinaven partners Marathon and Marubeni Oil & Gas (North Sea) Limited for the permission to show the data. The authors would like to thank their colleagues at CGGVeritas and BP, particularly everyone involved with CGGVeritas in-house centre in BP Aberdeen, and the collaborative technology development effort. References Campbell, S., T.A. Ricketts, D.M. Davies, C.P. Slater, G.G. Lilley, J. Brain, J. Stammeijer and A.C. Evans [2005], Improved 4D seismic repeatability a west of Shetland towed streamer acquisition case history, 75 th Annual International Meeting, SEG, Expanded Abstracts, 24, Hoeber, H., S. Butt, D. Davies, S. Campbell and T. Ricketts [2005] Improved 4D seismic processing: Foinaven case study, 75 th Annual International Meeting, SEG, Expanded Abstracts, 24, Lacombe, C., S. Butt, G. Mackenzie, M. Schons and R. Bornard [2009], Correction for water column variation, The Leading Edge, 28(2), Lacombe, C., H. Hoeber, S. Campbell and S. Butt [2008] Target oriented directive designature, 70 th EAGE Conference and Exhibition, Extended abstracts, H012. Lancaster, S and D.N. Whitcombe [2000], Fast Track coloured inversion, 70 th Annual International Meeting, SEG, Expanded Abstracts, 19, Whitcombe, D.N., L. Hodgson, H. Hoeber and Z. Yu [2008] Frequency dependent structurally comfortable filtering, 78 th Annual International Meeting, SEG, Expanded Abstracts, 27, Wombell, R. [1996], Water velocity variations in 3D seismic processing, SEG, Expanded Abstracts, 15, Zamorouev A., D.N. Whitcombe, M. Dyce and L.Hodgson [2006] A simple methodology for 4D noise reduction and repeatability improvements, 76 th Annual International Meeting, SEG, Expanded Abstracts, 25,