Case Study: 4D Seismic Processing & Imaging you can trust
At the beginning of 2021 (see Figure 1 below), Shearwater was well positioned to become a partner of choice for time-lapse processing projects. This was the result of significant investment in R&D to develop the tools and flows needed for such projects, plus a smart and diversified recruitment of experts in the domain.
However, through discussions with our clients, it was clear that despite having a great car (our proprietary software REVEAL) and great drivers (our experts), we were lacking experience on the track (running a full time-lapse project with REVEAL) to be fully qualified and considered as a service provider of choice for these projects.
Figure 1 – Status at Shearwater Early 2021.
Therefore, we made the decision to undertake an exercise of processing a full time-lapse project in order to be fully qualified and have a proven track record, while at the same time fine-tuning our tools and flows for optimum results.
We opted to purchase the Pluto 4D 2015 data from Geoscience Australia for the purpose of running this project.
The Pluto gas field is located in the North Carnarvon Basin, NW Australia, as shown in Figure 2 and is operated by Woodside. The field was discovered in 2005 and production started in 2012. The baseline is a 2004 conventional acquisition survey and the first monitor survey was shot in 2016 using the PGS Geostreamer technology. Overall, the baseline and the first monitor are around 500km2. The water bottom over the survey is highly rugose, going from shallow to deep through a shelf break, as shown in Figure 3. We purchased the raw navigation merge data for the baseline along with the P-UP data from the monitor survey. We also acquired the legacy data (final stacks for base and monitor along with all the velocity models) and the Legacy processing report. For reference, the project was processed at the time of acquisition in Perth with a leading seismic service provider.
Several papers have been published by Woodside over the year, before and after the first monitor survey. Figure 4 shows some comments extracted from a paper by Benjamin Peterson and Andre Gerhardt (Woodside) – The Leading Edge July 2020 – detailing that a fairly strong 4D signal is expected around the GWC (Gas-Water Contact).
Figure 4 – comment extracted from a paper from Benjamin Peterson and Andre Gerhardt (Woodside) – The Leading Edge July 2020
The processing was performed internally between our Perth and UK office, with strong involvement from our R&D group.
The processing sequence applied is presented in Figure 5. As you will see the sequence is fairly “simple” and “4D dedicated”. You will notice that only 2 passes of Global Matching (M to B) has been applied – one pre-migration and one post migration. This means that the 3D sequence applied to both baseline and monitor was doing a great job removing the non-repeatable events and limiting the need for more intensive and potentially “artificial” local matching.
Figure 5 – 4D processing sequence applied
It is worth noting that our progress was followed locally by 2 major oil companies, which allowed us to validate our sequence and tools.
One key aspect of the time-lapse processing is of course the QC procedures in place. At Shearwater, we have designed an extensive set of QCs both in the data and image domains. These QCs are run at each production step to ensure utmost results and catch any issue quickly. These QC procedures have also been used during the testing phase to some extent. Figure 6 presents the QC procedures put in place during this internal project.
The QC procedures allowed us to track and monitor the evolution of the different 4D metric attributes along with the 4D signal through the life of the project. Examples for the NRMS and the 4D signal at intermediate stages are presented in Figures 7 and 8.
Figure 7 – Evolution of NRMS to an intermediate stage
Figure 8 – Evolution of 4D signal to an intermediate stage
Our results were compared against the 2016 legacy ones at each stage and at the end of the processing. We had access to the legacy final stacks so we were able to generate identical QCs between the Legacy processing and our results.
It is worth noting that the legacy results were judged to be very good and had a big impact of the field development.
The following displays (Figure 9 to 13) will show you the comparison between the legacy and our results for NRMS, 4D signal (IL/XL/Depth slice) and amplitude at the GWC. All of these will confirm that we have achieved superior results compared to the legacy with great reduction of the 4D noise while preserving a good 4D signal.
Figure 9 – Final NRMS comparison over the reservoir – left = Legacy, right SW 2021 – the Pluto field outlined in black
Figure 10 – Final 4D signal Depth slice around the GWC (3156m) – left = Legacy, right SW 2021 – the Pluto field outlines in red
Figure 11 – Final 4D signal for 1 XL annotated in blue – left = Legacy, right SW 2021
Figure 12 – Final 4D signal for 1 IL annotated in blue – left = Legacy, right SW 2021
Previous 4D Differences as input
Phase rotation 90°
Amplitude Extraction in a ±50m window centered around GWC horizon
Figure 13 – Final 4D Signal – Summed positive amplitude around the GWC – left = Legacy, right SW 2021
In conclusion, the compared results with the Legacy are explicit, proving that Shearwater has the expertise and technology to successfully process time-lapse projects. Moreover, we can offer proven 4D Seismic Processing & Imaging you can trust.
Following the completion of this internal project, Shearwater has been awarded a large 4D project, the Wheatstone Field in Australia by Chevron – please see the press release for more details.