As introduced in a previous Seismic Tech-Notes article on Separated Wavefield Imaging [‘SWIM’], SWIM uses both the up-going pressure wavefield (‘P-UP’) and down-going pressure wavefield (‘P-DWN’) from the dual-sensor wavefield separation of GeoStreamer 3D data to fundamentally change the way that seismic migration can image the subsurface, the first 1 km or so below the surface in particular. Broadband and continuous seismic images all the way up to, and including, the seafloor seismic event—even in areas with very shallow water and where the 3D seismic surveys have towed a very large streamer spread to optimise survey efficiency. Every receiver from every streamer becomes a ‘virtual source’, thereby greatly extending the spatial extent of the migrated images, most famously mitigating the cross-line acquisition footprint associated with all 3D streamer surveys, notably those in shallow water, using large streamer spreads and with shallow targets of interest. A common vernacular has also been to describe SWIM as imaging ‘All orders of surface multiples’, which it does, but a better description is that ‘The complete seismic wavefield is being imaged’.
I discuss the power of SWIM to contribute to shallow velocity model building, imaging and interpretation below, building a platform to discuss seismic inversion and quantitative interpretation (QI) in a future article.
Shallow Velocity Model Building
Figure 1 compares shallow image gathers imaged from GeoStreamer data with Kirchhoff PSDM versus SWIM. Anyone familiar with seismic processing will understand that the fold on shallow arrival times/depths is always very small because of the combination of the 3D multi-streamer acquisition geometry and the outer trace mute applied in processing. In the worst case at the outer streamer locations for wide-tow geometry and/or shallow water areas there will in fact be zero fold for shallow depths and a very strong acquisition footprint will corrupt the shallow seismic data. The upper part of Figure 1 displays this well-known lack of shallow fold using Kirchhoff pre-stack depth migration (PSDM). Almost unbelievably, the lower part of Figure 1 illustrates how the introduction of P-DWN into a modified form of depth migration (SWIM) enables all offsets for all depths to contribute useful information using the same (GeoStreamer) survey data.
Whilst the events in the upper part of Figure 1 appear to be ‘flat’ and would satisfy a seismic processing expert picking velocities, anyone can see that it is very challenging trying to assess the ‘flatness’ of such short seismic events. In contrast, every event in the lower part of Figure 1 enables a very accurate assessment of the offset-dependent quality of the velocity model at all two-way time (TWT) or depth. Indeed, it is apparent in the SWIM gathers that the velocity model used is in fact slightly inaccurate at the far offsets (‘undercorrected’), and more sophisticated high-order/multi-offset/anisotropic velocity corrections can be applied, tested and refined with confidence. The use of SWIM in ‘Complete Wavefield Imaging’ (CWI) velocity model building has now become commonplace.
It is worth reminding ourselves that the ‘structural stability’ of depth imaging is rather useless without very accurate shallow velocity control. The 3D spatial integrity of the structures imaged, the juxtaposition of events across faults, the pull-up/push-down influence of near-surface velocity heterogeneities upon seismic-to-well ties, the ability to accurately quantify the volumes of prospects, and so on, all depend upon the seismic depth range where we historically have the worst understanding—the first 1 km below the surface.
Near Angle Illumination
It is worth noting that all SWIM events are naturally zero phase at all depths, and the angle coverage/illumination is substantially improved by comparison to conventional image gathers—particularly in terms of near-angle coverage. These considerations contribute to the spatial resolution of shallow features associated with SWIM depth slices. Whilst minimum angles in excess of 25° are routinely observed in the first 0.5 seconds on conventional angle gathers in shallow water depths (less than 300 m), the minimum angles observed on SWIM angle gathers are routinely less than 5°. This becomes particularly relevant for seismic inversion and AVO studies. We also observe in the lower part of Figure 1 that stable far angle information is available past 42° in this particular example—both because the velocity model used to convert offset to angle can be more accurate and because significantly larger offsets are imaged.
Velocity model building in complex geological regimes is increasingly moving towards using angle domain common image gathers (ADCIGs) as the preferred platform rather than traditional offset domain gathers. The challenges observed in Figure 2 are therefore also relevant to shallow velocity model building in addition to seismic inversion where small minimum incidence angles are critical to derive accurate AVO incidence and gradient information—the application for angle stacks discussed in a future article. As we will see, SWIM image gathers have densely sampled angle information for all angles of interest and all depths of interest.
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Headline crude prices for the week beginning 10 December 2018 – Brent: US$62/b; WTI: US$52/b
Headlines of the week
The Permian is in desperate need of pipelines. That much is true. There is so much shale liquids sloshing underneath the Permian formation in Texas and New Mexico, that even though it has already upended global crude market and turned the USA into the world’s largest crude producer, there is still so much of it trapped inland, unable to make the 800km journey to the Gulf Coast that would take them to the big wider world.
The stakes are high. Even though the US is poised to reach some 12 mmb/d of crude oil production next year – more than half of that coming from shale oil formations – it could be producing a lot more. This has already caused the Brent-WTI spread to widen to a constant US$10/b since mid-2018 – when the Permian’s pipeline bottlenecks first became critical – from an average of US$4/b prior to that. It is even more dramatic in the Permian itself, where crude is selling at a US$10-16/b discount to Houston WTI, with trends pointing to the spread going as wide as US$20/b soon. Estimates suggest that a record 3,722 wells were drilled in the Permian this year but never opened because the oil could not be brought to market. This is part of the reason why the US active rig count hasn’t increased as much as would have been expected when crude prices were trending towards US$80/b – there’s no point in drilling if you can’t sell.
Assistance is on the way. Between now and 2020, estimates suggest that some 2.6 mmb/d of pipeline capacity across several projects will come onstream, with an additional 1 mmb/d in the planning stages. Add this to the existing 3.1 mmb/d of takeaway capacity (and 300,000 b/d of local refining) and Permian shale oil output currently dammed away by a wall of fixed capacity could double in size when freed to make it to market.
And more pipelines keep getting announced. In the last two weeks, Jupiter Energy Group announced a 90-day open season seeking binding commitments for a planned 1 mmb/d, 1050km long Jupiter Pipeline – which could connect the Permian to all three of Texas’ deepwater ports, Houston, Corpus Christi and Brownsville. Plains All American is launching its 500,000 b/d Sunrise Pipeline, connecting the Permian to Cushing, Oklahoma. Wolf Midstream has also launched an open season, seeking interest for its 120,000 b/d Red Wolf Crude Connector branch, connecting to its existing terminal and infrastructure in Colorado City.
Current estimates suggest that Permian output numbered around 3.5 mmb/d in October. At maximum capacity, that’s still about 100,000 b/d of shale oil trapped inland. As planned pipelines come online over the next two years, that trickle could turn into a flood. Consider this. Even at the current maxing out of Permian infrastructure, the US is already on the cusp on 12 mmb/d crude production. By 2021, it could go as high as 15 mmb/d – crude prices, permitting, of course.
As recently reported in the WSJ; “For years, the companies behind the U.S. oil-and-gas boom, including Noble Energy Inc. and Whiting Petroleum Corp. have promised shareholders they have thousands of prospective wells they can drill profitably even at $40 a barrel. Some have even said they can generate returns on investment of 30%. But most shale drillers haven’t made much, if any, money at those prices. From 2012 to 2017, the 30 biggest shale producers lost more than $50 billion. Last year, when oil prices averaged about $50 a barrel, the group as a whole was barely in the black, with profits of about $1.7 billion, or roughly 1.3% of revenue, according to FactSet.”
The immense growth experienced in the Permian has consequences for the entire oil supply chain, from refining balances – shale oil is more suitable for lighter ends like gasoline, but the world is heading for a gasoline glut and is more interested in cracking gasoil for the IMO’s strict marine fuels sulphur levels coming up in 2020 – to geopolitics, by diminishing OPEC’s power and particularly Saudi Arabia’s role as a swing producer. For now, the walls keeping a Permian flood in are still standing. In two years, they won’t, with new pipeline infrastructure in place. And so the oil world has two years to prepare for the coming tsunami, but only if crude prices stay on course.
Recent Announced Permian Pipeline Projects
Headline crude prices for the week beginning 3 December 2018 – Brent: US$61/b; WTI: US$52/b
Headlines of the week