Graham Conroy

Full waveform inversion: A North Sea OBC case study

Full Waveform Inversion (FWI) aims to obtain superior velocity models by minimizing the difference between observed and modelled seismic waveforms. We apply FWI to a North Sea OBC field data set with wide azimuths and more than 10 km long offsets. We discuss the methodology used and the associated practical issues. Our FWI result has revealed detailed velocity features associated with thin, gas-charged layers and faulting in the shallow sections of the model. We demonstrate that this velocity update has improved the imaging of the deeper structures.

Production-induced stresses from time-lapse time shifts A geomechanics case study from Franklin and Elgin fields

Franklin and Elgin fields were discovered in 1986 and 1991, respectively, within the U.K. North Sea Central Graben blocks 22/30 and 29/5. The producing reservoirs are contained in the Jurassic Fulmar shallow-marine and Pentland fluvial formations at depths of 5100–5600 m subsea. The fields presented significant development challenges both in terms of seismic imaging complexity and being in a state of exceptionally high pressure/high temperature (HPHT). Addressing these challenges meant that production could not commence until 2001.

Full Waveform Inversion in a Complex Geological Setting - A Narrow Azimuth Towed Streamer Case Study from the Barents Sea

We apply Full Waveform Inversion (FWI) to a Narrow Azimuth (NAZ) towed streamer data acquired over the Samson Dome region in the Barents Sea which has complex structural geology. This case study demonstrates the improvement in seismic image quality resulting from dramatically increasing the wavenumber content in the velocity field using FWI. The FWI result shows a spatial consistency that was unexpected from a standard 3D NAZ dataset. It also demonstrates the potential use of FWI velocity models in geological interpretation and shallow geohazard detection, even though in this implementation the FWI is effectively solving a combination of velocity, anisotropy and density, rather than just P-wave velocity.

Using Full Waveform Inversion to Update Anisotropy - A North Sea Case Study

Full Waveform Inversion (FWI) aims to obtain high resolution velocity models by minimizing the misfit between observed and modelled data. While FWI algorithms that take into account anisotropy are often used in the industry, it is still common practice to update for vertical velocity only, keeping the anisotropic parameters fixed during the inversion. The main advantage of such an approach is mitigating the different sensitivity of the data to the parameters that characterize the subsurface. Nonetheless, fixing the anisotropic parameters imposes a constraint on the update of the vertical velocity, potentially leading to a sub-optimal solution of the inverse problem. Here we formulate the inverse problem within the Vertical Transverse Isotropy (VTI) model, and present an approach for updating both vertical and horizontal velocities. The necessary theory for the implementation of the algorithm is reviewed and developed within the scope of this work and a practical application to a North Sea field dataset is then presented. Our FWI result reveals velocity and anisotropy details associated with shallow channels and other features in the near surface geology.

Geomechanical stresses from 4D time shifts measured around the depleting Franklin and Elgin reservoirs

This paper is a geomechanics case study in which timelapse timeshifts have been accurately measured over the producing HP/HT Franklin and Elgin gas condensate fields then inverted to production-induced stress and strain changes throughout the reservoir, overburden and underburden. The interpretation of the result can be used to constrain the coupled reservoir-geomechanical model with the objective of enhancing its capacity to detect parts of the reservoirs that have not been depleted, explaining casing integrity problems and anticipating potential wellbore instability problems on infill wells.

Estimation of Production Induced Stress Changes From 4D Finite Offset Timeshifts

Summary Significant stress changes are generated when producing reservoirs compact due to large reductions in the reservoir pore-pressure. These stress changes are not confined to the reservoir. The stress and strain is redistributed to the surrounding formations, modifying both velocity and thicknesses in these formations. These changes often manifest themselves as significant timelapse time differences on migrated 4D images. Various authors (Hatchell et al, 2003 and Barkved et al, 2005) have used geomechanical modelling to explain these 4D timeshifts, thereby gaining valuable insight into the behaviour of the whole subsurface around some compacting reservoirs. This has so far been accomplished by assuming a simple relationship between thickness and velocity changes. The modelling is presumably repeated using various updated relationships until a match is obtained with the observed 4D timeshifts. We present an approach in which the 4D time differences are measured on prestack data. Without relying on any assumed relationship between velocity and thickness changes, we use the additional non-zero offset information combined with raytracing and linear least squares inversion techniques to derive the thickness and velocity changes. These resulting velocity and thickness changes combined with density and pore-pressure well data can then be converted to stress and strain changes. The technique should therefore help to close the loop between seismic 4D time differences and geomechanical stress and strain