Report from the First EAGE/PESGB Workshop in Velocities
The first EAGE/PESGB workshop on velocities took place from 22-23 February 2018. Some sixty delegates from operators, service companies and academia filled the central London venue, ensuring a lively and engaging workshop, the focus of which was on the latest technologies and workflows for velocity estimation in exploration, development and production workflows, with an emphasis on their practical application to regional challenges. A wide range of expertise and experiences were represented with delegates and presenters attending not only from across Europe, but from as far abroad as the USA and South America. The workshop was preceded by a one-day primer on velocity model building by Ian Jones.
The call for papers covered all aspects of velocities in E&P workflows, and the technical program included topics as diverse as anisotropy estimation, understanding uncertainty in velocity models, velocities in the drilling domain, the building of regional models and velocity estimation for multicomponent data. While several case studies discussed the challenges of the North Sea a broad range of geological environments were represented, including the Barents Sea, Brazil, Equatorial Guinea and the Gulf of Mexico among others. A significant proportion of the papers submitted, however, reflected the industry-wide interest in Full Waveform Inversion (FWI), and a significant part of the technical program concerned discussion of what value this emerging technology brings, and how, where and when it should be used.
The workshop kicked off with the keynote from Olga Zdraveva who discussed the key developments in model building over the years and emphasised the need for anisotropic earth models (not only velocity models) which explained all available data and reminded us that a key part of improving models is not just ‘better technology’, but the integration of ‘more and better data’ (both seismic and non-seismic) to constrain and validate our models – more well logs in the overburden, please!
There were three sessions dedicated to Full Waveform Inversion, where the technique was demonstrated to be in production use on large scale projects and adding value by improving the resolution of overburden velocity anomalies in complex settings. Case studies included the modelling of shallow channels in the North Sea, faulted overburden in the Barents Sea, imaging below gas offshore Equatorial Guinea and beneath complex ooze bodies in the Norwegian Sea. Case studies from the Gulf of Mexico were also shown. Common elements to several of the presentations included the implementation of cycle-skipping mitigation measures (allowing FWI to start with simpler models or with data without very low frequencies) and the use of reflections in FWI to allow velocity updates below the depth of penetration of diving waves. The inclusion of ‘Q’ in FWI was also discussed. While it was clear that FWI techniques are changing the way we build models, it was also noted that tomographic techniques still have an important part to play in model building workflows.
Two ocean-bottom seismic case studies from the North Sea, where injectites in the overburden disrupted imaging at the reservoir, further demonstrated the need for an integrated approach to model building including FWI, tomography, surface wave inversion and event registration. Converted wave reverse time migration was also seen to significantly improve images in this area. A third paper on converted wave techniques discussed elastic reflection FWI for the simultaneous inversion of pressure and shear wave velocities.
A number of papers discussed the generation of very large regional velocity models using velocity data from a variety of sources and the associated challenges of producing valid models from velocity information of different types. Geostatistical methods and the computation of scaling factors for regional consistency were discussed in this context.
To complete an interesting and diverse program, further insightful talks were given on topics including depth uncertainty estimation in the North Sea, the joint inversion of VSP data in the Gulf of Mexico, orthorhombic tomography offshore Cameroon, common reflection stacking for poor quality datasets, the estimation of velocities for drilling hazard mitigation, high resolution velocity attributes for interpretation, and the use of impedance inversion to update layered salt bodies offshore Brazil!
The technical committee would like to thank EAGE and PESGB for organizing this successful event, the presenters for sharing their work, the chairs for keeping things running to order and the attendees for their contribution to the discussion during an interactive and informative event.
We look forward to the next one!
Chairman Workshop Committee 2018