Prestack time migrated section of the 40-block test area, displaying significant imaging distortion due to salt and waterbottom changes.
As exploration advances into ever-deeper offshore environments accompanied by substantially higher exploration costs, understanding the exploration and development risk is paramount for a successful program. As evidenced by recent significant discoveries at the Mad Dog and Crazy Horse prospects in the US Gulf of Mexico, the presence of economically viable hydrocarbon accumulations in ultra-deepwater is beginning to be realized.
Prestack depth migration of the 40 block test area showing significant improvement.
One of the many challenges companies have had to overcome in harvesting the potential of the deepwater Gulf of Mexico has been the imaging problem presented by the pervasive salt features. These usually mask or distort exploration objectives. Depth imaging has played a significant role in mitigating risk in exploration and development projects in the past and will continue to play a significant role in future discoveries and their associated development.
In 1991, Exxon's Mickey discovery in Mississippi Canyon marked the first significant deepwater subsalt find and foreshadowed the potential hidden beneath the vast salt sheets covering the deepwater Gulf of Mexico. The next few years brought a string of subsequent discoveries that revolutionized industry thinking on salt emplacement, reservoir development, and technology fears associated with seismic imaging and drilling through salt in deepwater environments.
Shell's deepwater development success at Auger highlighted not only the technological feasibility of deepwater developments, but also the phenomenal deliverability of deepwater reservoirs. The combination of advances in subsalt imaging and high quality deepwater reservoirs were key contributing factors in the exploration activity boom in the deepwater Gulf in the mid-1990s. The only thing missing was 3D data, and it was not long in coming.
The mid-1990s saw unprecedented 3D activity covering the deepwater Gulf, with programs covering thousands of OCS blocks in just a few years. Companies were hungry for data; there were leases to acquire and wells to be drilled. As records for deepwater leasing were being broken, exploration and production companies were also committing to build expensive deepwater drillships to meet the demand for rigs capable of drilling in water depths greater than 4,000 ft.
As the data and the rigs began to arrive, there remained the technical hurdle of depth migrating the data, to deal with the significant time mis-positioning and distortion, due to the dramatic velocity contrast between the salt masses and surrounding sediment. Imaging beneath and around salt was not an unexpected problem, and in fact had been the subject of immense effort and research by many of the major Gulf players throughout the 1990s, with varying degrees of success. The key factors in successful depth imaging were:
- Good velocity estimation techniques
- Good depth migration algorithms
- Accurate salt model interpretation
- Healthy dose of computing power, money, and time.
This was an expensive process that taxed the resources of even the biggest players.
Main visualization theater showing rear-projected screen.
Most of the seismic data acquired in the deepwater Gulf during during this period was non-exclusive, a practice that has gained wide acceptance and enthusiasm in the industry. By providing quality seismic data at a fraction of the proprietary price, exploration companies were able to explore larger areas more efficiently. More advanced processing of this data such as depth migration was a procedure most of the bigger players performed in-house, using multi-disciplinary teams of top processors and interpreters. Those that did not possess in-house resources teamed up with specialty contractors.
Either way, the process was expensive, time consuming and resource intensive. But, it was a necessary step, as many exploration targets required depth imaging to mitigate risks prior to drilling. With shrinking budgets and expensive newly built drillships looming on the horizon, the time was ripe for another paradigm shift.
The solution came in the form of large scale (>100 OCS block) non-exclusive depth migration projects. The economic benefits were compelling, but there were many technical concerns to overcome before this idea would become a reality. Assuming one could get agreement on the fundamental processing parameters and algorithms, questions regarding who would be responsible for the interpretation, and how effective a service company could be in integrating the interpretive ideas of the participants into a single model remained. For the project to be successful, collaboration and consensus would be critical. In order to launch such a large-scale 3D depth migration project, these concerns would have to be addressed.
Perspective views of the PSDM 3D volume permits quality control of large volumes of data.
To demonstrate the effectiveness of the proposed algorithms, parameters, and techniques, in addition to meeting the strict requirements of many of the participants' in-house models and benchmarks, Veritas embarked on a 40-block pre-stack depth migration (PSDM) test area to demonstrate the results on real data. The results helped to alleviate concerns about technical standards.
Concerns about throughput were addressed by the track record established in the three consecutive years of processing large multi-client surveys through pre-stack time migration using the three Veritas supercomputing facilities around the world. In order to address concerns about a contractor's ability to coordinate and manage the interpretation aspect of such a project, two hurdles had to be overcome:
- Question of interpretation expertise
- Method of developing consensus on the inter pretation.
The first issue was addressed by assembling a core team made up of experienced interpreters, recruited from major oil companies, who were seasoned in interpretation and visualization for depth imaging in the salt structures of the deepwater Gulf.
The issue of gaining consensus on complex interpretation problems was addressed by building a state of the art interpretation and visualization facility. Using the visualization center as a collaborative interpretation and quality control (QC) medium, concerns over the ability to incorporate client interpretations into the model building process quickly subsided.
The visualization center was designed to provide a collaborative 3D interpretation environment that enables multiple ideas to be exchanged and tested, resulting in a better product than produced in traditional workstation environments. The center has a main theatre with a rear-projection-viewing screen that is 18 ft wide by 7 ft high. The high intensity projectors allow lighting to be on in the workroom when the interpretation and processing teams are working through issues - which encourages interaction. Each participant has access to a keyboard and mouse and can take control of the screen as appropriate.
The theater workroom is surrounded by private "mini-vis" rooms that have large screen systems appropriate for smaller groups, and all rooms are tied into the SGI Onyx2 Reality Monster to access the appropriate graphics supercomputing power when needed. This setup is ideal to foster the collaboration needed, allowing interpretive geophysicists, processing geophysicists, and geologists working together as a team. Thirty-two gigabytes of memory and six terabytes of raided disk are available, along with the visualization software. The center allows the interpretation and processing staff, along with client personnel to interactively analyze and review large data volumes. This, in turn enables the teams to build an effective salt model and understand its impact on subsalt imaging.
As part of the speculative 3D PSDM effort, client participation was an integral part of the model building process. The PSDM effort requires several stages of interpretation, each involving client participation, to arrive at a common solution that meets all objectives. The PSDM interpretation staff would do the bulk of the interpretation, and work closely along the way with the participants to integrate each of the interpretation efforts into the final model. Client input varies, depending on specific areas of interest and acreage holdings. Participant sessions are kept confidential and solutions derived at each stage of the model building process are presented to representatives of each client company for final approval.
Although the velocity model building involved a multiphase solution with multiple iterations within each phase, the varying interpretation hypotheses could be worked through quickly, by making use of the visualization center to aid the integrated teams. Intimate and frequent involvement of the client companies is a significant factor in reducing the interpretation cycle time. Review meetings are collaborative discussions, rather than formal presentations. Work sessions in the theater revolve around:
- 3D perspective rendering for fast and effective volume quality control
- Co-rendering of the velocity model with the seismic volume to quality control model-fit to the seismic volume output
- Quality control of the common depth point gathers for residual normal move-out.
The true benefit of visualization in the work process is the ability to interactively probe and examine the 3D data volume fast and efficiently. Areas of concern or debate can be highlighted quickly and examined in details. The higher the confidence is in the model, the more likely the result will be successful - for example, comparing pre-stack time migration data (PSTM) to pre-stack depth migration data (PSDM).
Perspective views of the PSDM 3D volume co-rendered with the velocity model allows for quality control of the model.
The significant differences illustrated in the imaging of the PSDM data when compared to the PSTM data are not only structurally significant, but also stratigraphically significant. Time-section structural highs are often seen as lows on the PSDM data, and events that could not be correlated on the time data are now stratigraphically interpretable.
Prestack time migrated section from the full 3D cube in Walker Ridge.
The ability to manage the development of large pre-stack depth migration projects involving multiple clients requires expertise in processing, interpretation, and the ability to efficiently incorporate multiple solutions into the model-building process. This ability is aided by the collaborative environment created during the use of 3D visualization facilities, and the integrated geophysical and geological staff working on the projects.
Prestack depth migrated section from the full cube in Walker Ridge. Significant imaging improvements are visible, making geologic correlations more meaningful.
Frequent interaction with the client companies utilizing 3D visualization technologies, collaboration between in-house staff, and collaboration with clients during model building allows for communication of ideas and rapid project completion. By serving as the focus of the interpretation effort, the contractor is able to incorporate and integrate all client input, while maintaining confidentiality, whether the project is proprietary or non-exclusive.
With geologically sound interpretations feeding into the 3D PSDM, the chances of a successful result are increased. Non-exclusive depth migration is gaining acceptance as an attractive solution for companies exploring in the Gulf of Mexico, as confidence in the model and processing techniques grows.
Dale Bowering is President of Veritas Exploration Services.