Stephen Pharez
Anthony Day
Petroleum Geo-Services
A time-lapse 3D or 4D project involves repeat 3D seismic surveys over a producing hydrocarbon reservoir, where a change in the physical state of the reservoir has occurred because of production. These surveys are used to optimize producer and injector well placement.
The first survey is the “baseline” and each successive survey is a “monitor.” Ideally, the acquisition geometry and hardware are repeated exactly from survey to survey, as are environmental conditions, so any observable difference or 4D signal derives entirely from physical changes in the reservoir.
Variations in acquisition geometry naturally invoke variations in target illumination and variations in wavefield sampling, and will contribute errors to the 4D signal. Variations in acquisition hardware and/or environmental conditions invoke changes in the signal-to-noise content, signal fidelity, and the seismic wavelet to also contribute errors to the 4D signal.
For conventional marine acquisition (by towed streamers) to record the pressure wavefield, the requirement to repeat the acquisition geometry means that all monitor surveys must be acquired at the same acquisition depth as the base survey. If a dual-sensor streamer is used, this requirement can be relaxed.
Dual-sensor streamer technology uses collocated sensors to measure the pressure, as in a conventional streamer, and also the vertical component of the particle velocity. This allows the wavefield to be separated into up- and down-going parts, which can be redatumed independently to emulate acquisition at any recording depth.
Thus, the streamer may be towed at any depth, which usually means a deeper towing depth than for conventional acquisition. This takes advantage of the quieter recording environment, and increases the data low-frequency content. .
To validate these findings, a time-lapse acquisition experiment was conducted in the North Sea. Dual-sensor streamer data were acquired over five adjacent sail lines that had been acquired earlier in the year using a conventional streamer.
The data were processed and analyzed using a modern time-lapse processing sequence. Given that only a few months had elapsed between baseline and monitor surveys and there had been no production in the area, minimal differences between the two datasets were expected
PGS has proven the universal data quality benefits of its dual-sensor GeoStreamer since the first commercial 2D survey in early 2008. As of October 2010, more than 120,000 km (74,565 mi) of 2D and 19,000 sq km (7,336 sq mi) of 3D data had been acquired worldwide. The final chapter was to progress to 4D. Expected benefits included:
- Higher signal-to-noise content on baseline and monitor surveys processed to yield the up-going pressure wavefield to improve 4D repeatability.
- Deep streamer towing (between 15 and 25 m, or 49 and 82 ft) is less subject to surface-related noise and current effects that impact receiver spread control, acoustic positioning has less noise/uncertainty, and there is less ambient noise.
- The unique ability to independently extrapolate the up-going and down-going pressure wavefields to any depth in processing allows matching dual-sensor streamer monitor surveys with conventional baseline surveys acquired at different depths.
A full 4D repeatability trial was conducted in Quad 26 of the North Sea. A 3D survey acquired with theOcean Explorer towing conventional hydrophone-only solid streamers at a depth of 8 m (26 ft) in April 2009 was the baseline survey.
The survey aimed to optimize coverage rather than ease of repeatability, which is usually the case for a baseline survey. Five adjacent sail lines were selected as having moderate feathering and shot positions which facilitated repetition of source and receiver positions in the monitor survey. TheAtlantic Explorer then reacquired in June 2009 part of the survey with dual-sensor streamers towed at a depth of 15 m (49 ft). These five adjacent sail lines constituted the monitor survey used here to quantify the 4D repeatability yielded by dual-sensor streamer technology.
Pre-survey modeling of 4D acquisition parameters included an acknowledgement that the dual-sensor streamer would be less affected by swell-related noise. The source parameters were identical for the baseline and monitor surveys: 3,090 cu in source arrays towed at 6 m (19.5 ft) depth, in dual-source shooting mode. Both the baseline and monitor surveys were acquired with 6 x 5,100-m streamers at 100 m streamer separation. Source positions were matched quite well in the monitor survey, though time-sharing constraints did not permit optimal feather matching.
4D processing
Ideally, the receiver ghost is removed from both the baseline and monitor surveys prior to 4D differencing in processing. Thus, the significant “noise” component from the receiver ghost (down-going pressure) wavefield that normally is embedded into seismic data in a continuous and destructive manner is absent. The conventional hydrophone-only streamer data, containing the receiver ghost is being differenced against dual-sensor streamer data.
Comparison of total pressure data (top) with up-going pressure data from the dual-sensor streamer (middle) with corresponding amplitude spectra (bottom). As expected, the removal of the receiver ghost improves resolution in all areas and at all depths. Thus, 4D projects with GeoStreamer baseline and monitor surveys will have the optimum combination of high repeatability and high resolution.
It is necessary to recreate the receiver ghost in the dual-sensor streamer data so total pressure data are differenced against total pressure data. The difference primarily is the 4D signal (given appropriate acquisition geometry and processing). If total pressure data are differenced against up-going pressure data, the difference will be the receiver ghost wavefield, plus the 4D signal, plus various non-4D noises. The “ideal” 4D project would use dual-sensor streamers for both baseline and monitor surveys; if the data are demonstrably 4D-compliant in terms of amplitude preservation. The latter point was of interest to PGS, as an excellent 4D repeatability result would be final verification that the 3D GeoStreamer processing toolbox and workflow was optimal.
Wavefield separation was performed for the dual-sensor streamer monitor data, and the up-going and down-going pressure wavefields were extrapolated independently from the true streamer depth of 15 m (49 ft) to the baseline streamer depth of 8 m (26 ft). The wavefields were then summed to recreate the total-pressure wavefield at an effective streamer depth of 8 m. This process is fully 3D compliant, and correctly handles the amplitudes for all emergence angles. A deterministic matching filter was applied to correct for the differences in the instrument response of the GeoStreamer and conventional recording filters.
Both the baseline and monitor datasets were then taken through a state-of-the-art 4D processing sequence, including multiple removal, 4D binning, 3D wavefield regularization, pre-stack time migration, and post-migration global matching with a long design window. Detailed 4D QC was pursued at each step in the 4D processing flow using PGS’ proprietary holoSeis immersive visualization technology and established 4D QC procedures. The 4D QC attributes used include Normalized RMS Difference (NRMSD) in amplitude, Normalized Cross-Correlation, Time shift, and Phase Rotation.
The inherent signal-to-noise content of the dual-sensor streamer monitor data was higher than for the conventional streamer baseline data, an advantage which is preserved after extrapolation to an effective receiver depth of 8 m, and reconstruction of the total-pressure wavefield. This is a universally observed consequence of towing the streamer much deeper, in an acoustically quieter environment. The acoustic ranging and positioning errors also are smaller.
Results
The final NRMSD result was only 11%; an outstanding result that is fully compatible with industry requirements. There is negligible energy remaining after 4D differencing. There is an excellent match in all details of the amplitude spectrum within the target window, including at 50 Hz (arrowed) where the majority of the energy for the monitor survey is derived from the particle velocity sensor due to a notch in the hydrophone spectrum at this frequency.
This result verifies that dual-sensor streamer data is sound fundamentally, and all the processing steps are robust and amplitude preserving since such a small difference could not be obtained otherwise.
Although the small 4D difference indicates that the baseline acquisition geometry was adequately repeated, it is clear that further improvements could be made. In particular, the use of PGS source array steering technology for both baseline and monitor surveys would have been beneficial.
From 2011, inline streamer steering will be available using the eBird developed by Kongsberg Seatex specifically for GeoStreamer 3D and 4D operations. The final point to note is that as expected, the up-going pressure version of the monitor survey (receiver ghost removed) had stronger low and high frequency amplitudes than the total-pressure version; the frequency bandwidth was considerably greater; and resolution at all depths was higher. Dual-sensor streamers represent the ultimate 4D solution.
These results confirm that 3D GeoStreamer is fully backwards-compatible with conventional (hydrophone-only) streamer data for 4D differencing, and that its 3D processing flow is fully amplitude preserving and high fidelity.
Offshore Articles Archives
View Oil and Gas Articles on PennEnergy.com
