Atlantis Field Research Articles

Atlantis — 20 years of seismic innovation finally removes the shroud of mystery

The Atlantis Field has gone through more than two decades of continuous seismic imaging efforts, during which time many innovative technologies were incubated, the most recent one being the successful application of full-waveform inversion (FWI) in salt environments. This technique led to a significant improvement in the subsalt image. However, imaging challenges remain for the Atlantis reservoirs, primarily due to the complex overburden salt geometries and the highly compartmentalized reservoir. Even with an improved velocity model from FWI, the conventional reverse time migration (RTM) images still suffer from illumination issues and contain strong migration swings that hinder the subsalt imaging and subsequent interpretations. Furthermore, early versions of FWI employed an acoustic assumption, leading to visible salt halos at the salt boundaries in the velocity model, which adversely impacted the reservoir imaging. In the last 12 months, elastic time-lag FWI (TLFWI) and FWI-derived reflectivity (FDR) imaging using long-offset ocean-bottom node data have minimized these imaging issues at Atlantis, providing another step change in subsalt understanding. Although the 3D RTM images using the elastic FWI velocity model are similar overall to their acoustic counterparts, the 4D time-lapse RTM images at Atlantis show noticeable improvements. Furthermore, FDR images derived from elastic FWI velocities show obvious benefits over the acoustic ones. With a more accurate modeling engine that allows for better match between synthetic and real data, FDR imaging shows improved illumination, higher signal-to-noise ratio, and better reservoir details over acoustic FDR imaging. This recent advancement in using elastic TLFWI has had immediate positive effects in facilitating the Atlantis Field's current and future development.

The Wolfspar experience with low-frequency seismic source field data: Motivation, processing, and implications

The promise of fully automatic full-waveform inversion (FWI) — a (seismic) data-driven velocity model building process — has proven elusive in complex geologic settings, with impactful examples using field data unavailable until recently. In 2015, success with FWI at the Atlantis Field in the U.S. Gulf of Mexico demonstrated that semiautomatic velocity model building is possible, but it also raised the question of what more might be possible if seismic data tailor-made for FWI were available (e.g., with increased source-receiver offsets and bespoke low-frequency seismic sources). Motivated by the initial value case for FWI in settings such as the Gulf of Mexico, beginning in 2007 and continuing into 2021 BP designed, built, and field tested Wolfspar, an ultralow-frequency seismic source designed to produce seismic data tailor-made for FWI. A 3D field trial of Wolfspar was conducted over the Mad Dog Field in the Gulf of Mexico in 2017–2018. Low-frequency source (LFS) data were shot on a sparse grid (280 m inline, 2 to 4 km crossline) and recorded into ocean-bottom nodes simultaneously with air gun sources shooting on a conventional dense grid (50 m inline, 50 m crossline). Using the LFS data with FWI to improve the velocity model for imaging produced only incremental uplift in the subsalt image of the reservoir, albeit with image improvements at depths greater than 25,000 ft (approximately 7620 m). To better understand this, reprocessing and further analyses were conducted. We found that (1) the LFS achieved its design signal-to-noise ratio (S/N) goals over its frequency range; (2) the wave-extrapolation and imaging operators built into FWI and migration are very effective at suppressing low-frequency noise, so that densely sampled air gun data with a low S/N can still produce useable model updates with low frequencies; and (3) data density becomes less important at wider offsets. These results may have significant implications for future acquisition designs with low-frequency seismic sources going forward.

Ten years of time-lapse seismic on Atlantis Field

In 2019, the fourth ocean-bottom-node survey was acquired over Atlantis Field. This survey was quickly processed to provide useful time-lapse (4D) observations two months after the end of the acquisition. The time-lapse observations were immediately valuable in placing wells, refining final drilling target locations, updating well prioritization, and sequencing production and water-injection wells. These data are indispensable pieces of information that bring geophysicists and reservoir engineers together and focus the conversation on key remaining uncertainties such as fault transmissibilities and drainage areas. Time-lapse observations can confirm the key conceptional models already in place but are even more valuable when they highlight alternative models that have not yet been considered. The lessons learned from the acquisition, processing, analysis, interpretation, and integration of the data are shared. Some of these lessons are reiterations of previous work, but several new lessons originated from the latest 2019 acquisition. This was the first survey in which independent simultaneous sources were successfully deployed to collect a time-lapse survey. This resulted in a much faster and less expensive acquisition. In addition, full-waveform inversion was used as the main tool to update the velocity model, enabling a much faster turnaround in processing. The fast turnaround enabled incorporation of the latest acquisition to better constrain the velocity model update. The updated velocity model was used for the final time-lapse migration. In the integration part, the 4D-assisted history-match workflow was engaged to update the reservoir model history match. All of the upgrades led to an overall faster, less expensive, and better way to incorporate the acquired data in the final business decisions.

Full-waveform inversion for salt: A coming of age

Full-waveform inversion (FWI), proposed by Lailly and Tarantola in the 1980s, is considered to be the most promising data-driven tool for automatically building velocity models. Many successful examples have been reported using FWI to update shallow sediments, gas pockets, and mud volcanoes. However, successful applications of FWI to update salt structures had almost only been seen on synthetic data until recent progress at the Atlantis Field in the Gulf of Mexico. We revisited some aspects of FWI algorithms to minimize cycle-skipping and amplitude discrepancy issues and derived an FWI algorithm that is able to build complex salt velocity models. We applied this algorithm to a variety of data sets, including wide-azimuth and full-azimuth (FAZ) streamer data as well as ocean-bottom-node data, with different geologic settings in order to demonstrate the effectiveness of the method for salt velocity updates and to examine some fundamentals of the salt problem. We observed that, in multiple cases, salt velocity models from this FWI algorithm produced subsalt images of superior quality. We demonstrate with one FAZ streamer data example in Keathley Canyon that we do not necessarily need very high frequency in FWI for subsalt imaging purposes. Based on this observation, we envision that sparse node for velocity acquisition may provide appropriate data to handle large and complex salt bodies with FWI. We believe the combination of advanced FWI algorithms and appropriate data acquisition will bring a step change to subsalt imaging.

Full-waveform inversion: The next leap forward in subsalt imaging

Subsalt imaging has been a long-term challenge for the oil and gas industry. The substantial progress made in data acquisition and imaging since the late 1990s has made some subsalt imaging problems tractable, but building earth models that enable imaging under complex salt remains a challenge. Labor-intensive workflows remain industry standard practice. Not only are these costly and time consuming, they have also performed poorly in many areas of economic interest. Various automatic model-building tools have been proposed to overcome these disadvantages. One such tool, full-waveform inversion (FWI), has already revolutionized velocity-model building in areas with shallow gas. Prior to 2006, imaging in these areas had been considered challenging and labor intensive, just as imaging under complex salt remains today. Modeling indicates that low frequencies and wide offsets may be the key to success when building velocity models using FWI. Just how low and how wide that may be required for FWI success depends on the particular problem. At the Atlantis Field in the deepwater Gulf of Mexico we recently acquired wide-offset ocean-bottom-node data with conventional airguns. By taking care during the acquisition, we recorded usable signal down to a lower frequency than previously achieved. We then applied FWI to the resulting data set and used the resulting velocity model, unmodified, to reverse time migrate the seismic data. It produced some of the best subsalt images of the Atlantis reservoir structure ever seen. Furthermore, the FWI velocity model revealed several major interpretation errors in the legacy salt model; thus the FWI result also offered an excellent basis for updating the salt model with the conventional workflow. These results demonstrate that with appropriate seismic data to support it, and with due care taken during processing and inversion, FWI truly offers a paradigm shift in model building and imaging in areas of complex salt.

Generating value from 4D through efficient integration

The use of time-lapse (4D) seismic at BP now has a track record stretching back more than 20 years. In the North Sea, reservoir types such as Tertiary turbidites and Cretaceous chalks produced clear 4D signals, and uptake increased rapidly in the late 1990s. In the Gulf of Mexico, 4D was generally more challenging due to greater subsurface complexity, but the industry made breakthroughs with modern acquisition systems such as ocean-bottom nodes (OBN). The challenge now is to maximize the value of existing and new surveys by better integration of 4D into reservoir management. As part of this effort, the authors implemented fit-for-purpose workflows for 4D-assisted history matching (4DAHM) and 4D geomechanical modeling. In an example from west of the Shetland Isles, UK, running the 4DAHM match-quality tool on multiple monitors from the Clair Field allows the field team to quickly see how changes in the reservoir simulation model affect the match to the 4D and identify areas of concern. The 4D geomechanics workflow utilizes the Geertsma equation to speed up modeling of effective stress changes due to compaction. It is demonstrated on the Gulf of Mexico Atlantis Field, where the authors were able to resolve a significant mismatch between the reservoir simulation model and observed seismic amplitude responses by modeling a strong stress-arching effect due to reservoir depletion.

Observation of shear‐wave splitting in the multicomponent node data from Atlantis field, Gulf of Mexico

ABSTRACTIn 2005, a multicomponent ocean bottom node data set was collected by BP and BHP Billiton in the Atlantis field in the Gulf of Mexico. Our results are based on data from a few sparse nodes with millions of shots that were analysed as common receiver azimuthal gathers. A first‐order look at P‐wave arrivals on a common receiver gather at a constant offset reveals variation of P‐wave arrival time as a function of azimuth indicating the presence of azimuthal anisotropy at the top few layers. This prompted us to investigate shear arrivals on the horizontal component data. After preliminary processing, including a static correction, the data were optimally rotated to radial (R) and transverse (T) components. The R component shows azimuthal variation of traveltime indicating variation of velocity with azimuth; the corresponding T component shows azimuthal variation of amplitude and phase (polarity reversal). The observed shear‐wave (S‐wave) splitting, previously observed azimuthal P‐wave velocity variation and azimuthal P‐wave amplitude variation, all indicate the occurrence of anisotropy in the shallow (just below the seafloor) subsea sediment in the area. From the radial component azimuthal gather, we analysed the PP‐ and PS‐wave amplitude variation for the first few layers and determined corresponding anisotropy parameter and VP/VS values. Since fracture at this depth is not likely to occur, we attribute the observed azimuthal anisotropy to the presence of microcracks and grain boundary orientation due to stress. The evidence of anisotropy is ubiquitous in this data set and thus it argues strongly in favour of considering anisotropy in depth imaging for obtaining realistic subsurface images, at the least.

Processing of a novel deepwater, wide-azimuth node seismic survey

In the fall of 2005, BP commissioned Fairfield Industries to conduct a large, wide-azimuth survey over Atlantis Field in deepwater Gulf of Mexico. The purpose of the survey was wide-azimuth, subsalt imaging using P-waves. The seafloor topography is complex with scoured furrows at the base of the escarpment. A shallow salt body covers a large part of the survey area and rafts the seafloor at the escarpment, leading to very strong velocity contrasts. The water depth in the area varies from approximately 1300 m above the Sigsbee escarpment to 2200 m below it.

Field design and operation of a novel deepwater, wide-azimuth node seismic survey

Many new deepwater, subsalt Gulf of Mexico developments are afflicted by poor imaging caused by the complex nature of their salt overburdens. The Atlantis Field is an example; a poor image of its northern flank hindered development. The ocean-bottom seismic (OBS) autonomous node method was applied with the goal of substantially improving the subsalt reservoir image.

Atlantis, the Quest for Stray Shallow-Water-Flow Sands

This article, written by Technology Editor Dennis Denney, contains highlights of paper OTC 17658, "Atlantis, the Quest for its Stray Shallow-Water-Flow Sands," by H. Mannaerts, J. Oldroyd, M. Williams, R. Fisher, E. Radjef, R. Day, J. Faulkerson, A. Pere, R. Sanders, and J. Thomson, BP plc, and K. Mix, BHP Billiton Petroleum , prepared for the 2005 Offshore Technology Conference, Houston, 2-5 May. The presence of shallow overpressured sand bodies in the Atlantis field led to the abandonment of one of the appraisal wells and created challenges to predicting, monitoring, and controlling shallow water flow (SWF). Drilling the shallow sections of all wells in one batch enabled optimization of offshore SWF monitoring and reporting. The SWF severity-classification scheme was used successfully during the program, and it has been adopted by the U.S. Minerals Management Service (MMS) as a guideline for offshore SWF monitoring at the wellhead by a remotely operated vehicle (ROV). Introduction The Atlantis field, discovered in 1998, is in U.S. Gulf of Mexico (GOM) Green Canyon Block 743, approximately 150 miles south of New Orleans. Atlantis is in water depths varying from 4,500 ft to greater than 6,800 ft. Hydrocarbons from the field will be produced from a drill center at the base of the Sigsbee escarpment in Green Canyon Block 743. The water depth at the drill center is approximately 6,800 ft. The tophole sections of 15 of the 16 production wells were batch-set (drilled) over a period of 3 months during the first half of 2004. A previously existing well constitutes the 16th production well. Appraisal Drilling Extensive shallow-geohazard assessment at the drill center confirmed the presence of mass-transport complexes (MTCs) at various depths in the tophole section and the absence of significant shallow gas hazards. Exploration-well drilling in the Atlantis area did not encounter SWF, suggesting that the area had an overall low potential for encountering SWF despite the presence of sandy turbidites and MTCs. The shallow-geohazard assessment for appraisal Well GC 743-4, drilled 2 years before the batch-set, was favorable. However, while the 20-in.-casing interval was being drilled, an unexpected 100-ft-thick sand was encountered within an MTC between 1,365 and 1,665 ft below mudline (Unit F). The sand flowed undetected, creating a ledge in the wellbore. Drilling continued to the proposed casing depth of 3,615 ft below mudline. As a result of borehole instability associated with the SWF sand, the 20-in. casing encountered a ledge while running to bottom, buckled, and failed. Consequently, the well was abandoned. The appraisal well was respudded 200 ft southwest of the original location. Over-burden conditions for this well were expected to be similar, if not identical, to those of the abandoned well. However, the SWF sand associated with the MTC in Unit F, which caused the previous appraisal well to fail, was absent.

Measurements and modeling of the shallow pore pressure regime at the Sigsbee Escarpment: Successful prediction of overpressure and ground-truthing with borehold measurements

Editor's note: The material in this paper was prepared and presented (OTC Paper 15201) at the 2003 Offshore Technology Conference, 5–8 May in Houston, Texas, U.S., and is published with permission. Shallow overpressures and shallow water flow (SWF) are significant hazards to deepwater drilling and facilities. In this paper we demonstrate how a combination of geomorphology, hydrogeology, and seismic interpretation has been successfully used in infer shallow overpressures, to develop a testable hypothesis of pore pressure distribution, and to delineate where to collect in-situ data. In-situ pore pressure measurements provide ground-truth data that can validate the model and be used to evaluate the hazards that internally driven failure might pose for facilities development. Several slope failures on the Sigsbee Escarpment in the Mad Dog and Atlantis field areas show a geomorphology distinct from “top-down” slope failures. Specifically, these slumps have steep, amphitheater-shaped headscarps, shallowly sloping bases, sharp inflection points with the surrounding seafloor at both the top and bottom of the headscarp, a linear trend in map view, and linear side walls. These geomorphic characteristics suggest that the slumps are formed by internally driven failure, and that the slumps grow retrogressively by headward migration. Within the Mad Dog and Atlantis field areas, Mad Dog Slump 8 and Atlantis Slump A are the best examples of this morphology; in this paper we will focus our attention on Mad Dog Slump 8 due to its proximity to planned facilities. The types of slope failures discussed in this paper are found on many margins worldwide. Their characteristic morphology indicates that the primary mechanism of slope failure is due to internal forcing (overpressure, weakening), rather than by downslope erosional flow. Internally driven slope failures can provide clues about the shallow overpressure regime, and provide exploration targets. Because the ocean is at constant head, and …