Marianne Houbiers

3D full-waveform inversion at Mariner - a shallow North Sea reservoir

We applied isotropic acoustic 3D full-waveform inversion (FWI) to OBC data from the Mariner field, a shallow heavy-oil field in the North Sea. This resulted in a multiphase workflow that can be adapted to imaging challenges in similar geological settings. FWI improves the resolution of the velocity field compared to the benchmark velocity field from reflection tomography. The background trend of and details introduced in the velocity model correlate well with the geology from seismic data and with well logs down to reservoir level. Resulting depth images show significantly better well tie in the overburden and improved definition of sand bodies at reservoir level.

Marine full-azimuth field trial at Heidrun

SUMMARY Based on promising results from a full azimuth modeling study, a marine field trial with a coil shooting design was carried out at Heidrun. The survey design approximated a ‘dahlia’ pattern with 18 intersecting coils. This design provided full azimuth, high fold data over a 2.5 x 2.5 km target area, plus sufficient surrounding aperture to provide migrated data that can be compared with previously acquired conventional seismic data. The preliminary processed coil survey data show fewer dip conflicts and improved fault definition at the reservoir level. This indicates better illumination of dipping events and better noise and multiple attenuation.

Vent Complex at Heidrun

SUMMARY The Heidrun field in the Norwegian Sea contains an area in which seismic data is very disturbed. Because of its domeshaped structure with on lap sequences and transparent interior, it has been assumed that the anomalous feature is caused by a relatively small and slim salt diapir in the overburden of the Heidrun reservoir section. The source of the salt diapir is assumed to be the Middle and Upper Triassic evaporates. However, finite difference modeling shows that the seismic image of and below a small salt diapir should be clearer. The seismic data show several characteristic features above the Base Cre taceous Unconformity (BCU) that alternatively can be explained by a vent complex. The seismic volume has been correlated with well 6507/7-2 located 700 meters away from the center of the dome. The log data show several beds with rapid and strong variations in both acoustic velocity and density. These cause strong impedance contrasts which can be mapped out as circular shaped bodies around the apex of the dome. Analysis of carbonates from these beds show δ 13 C values around -30 ‰ VPDB, which indicate that the carbon originates from seeping hydrocarbon gases. This implies that the circular shaped bodies are most likely related to periodic hydrocarbon leakage over million of years. Based on these observations, we propose that the Heidrun dome is a vent complex.

Marine full‐azimuth field trial at Heidrun revisited

In 2008, a marine field trial with a coil shooting design was acquired at Heidrun. In the preliminary processing of the data, some noise filters were used that assume straight line geometries and thus do not take the 3D nature of the data properly into account. Moreover, due to sparseness of data outside the 7 km target area, a limited migration aperture was used. As a result, the lateral resolution of the data is less good than in the conventional streamer data, causing loss of some details of the geological structure.

Quantitative 4D analysis using business analytics techniques

A Reservoir Data Warehouse was successfully created where key subsurface information used for geophysical reservoir monitoring has been transformed, integrated, and stored into a geospatially registered relational database. Quantitative 4D analysis was performed on the data, utilising the power of the parallel processing architecture of the high-performance analytical database, and combining traditional 4D analysis and business analytics tools. The 4D analysis successfully demonstrated quantitative relationships between the different data. At its simplest the Reservoir Data Warehouse enables a more sustainable and easily accessible solution for data integration and co-visualization, and simplification in the data management realm. At its most powerful the Reservoir Data Warehouse will enable the creation of new subsurface workflows where large amounts of data from disparate origins can be analysed jointly and presented in existing software solutions, or in newly developed cross-disciplinary software solutions.

Reverse Time Migration Velocity Analysis - A Real Field Data Example

Depth migration by reverse time migration requires the knowledge of a smooth approximation to the seismic velocity field. This background velocity model can be estimated by wave equation migration velocity analysis (WEMVA), an automatic process based on minimizing the errors in the kinematics of the depth migrated image. In this paper we present a WEMVA method where we use a combination of semblance and differential semblance to measure the errors in the positioning of reverse time migrated images. The errors are then turned into velocity updates by a gradient based optimization scheme. We apply the method to a 2D line extracted from a 3D marine survey acquired over the Snorre field in the North Sea. The resulting WEMVA velocities obtained from the field data are compared to the background velocities obtained by traveltime tomography, and also to well logs.

Full Azimuth Modelling at Heidrun

A simulation study was initiated to investigate whether or not a marine full azimuth acquisition geometry improves the image of the subsurface at Heidrun. Full azimuth shot data was modelled with finite difference and one-way wave equation modelling, and images obtained from full, wide, and narrow azimuth survey geometries were compared with each other. The study shows that a full azimuth geometry leads to better suppression of noise, less migration artefacts, more consistent amplitudes along horizons, and sharper fault planes than a narrow azimuth design. Attenuation of multiple energy is present, but less than expected. The improvements in image quality can be obtained with a realistic 4-vessel wide azimuth design and a coil geometry, but the coil geometry has a smaller acquisition footprint in the shallower part. Based on the modelling results, a field trial with coil design was carried out at Heidrun.

Full Azimuth Seismic Modeling in the Norwegian Sea

SUMMARY One of the fields in the Norwegian Sea has been imaged several times over the past decades, both with conventiona l narrow azimuth seismic surveys as well as with ocean bottom seismic. The extensively faulted structure of the field and the possible presence of a salt diapir cause imaging problems in some areas. Therefore, a simulation study has been initiated to judge whether or not a marine full azimuth acquisition geometry improves the image of the subsurface. For this simulation study, simplified velocity and densit y models of the field were created, containing the main features characterizing it, as well as the problem areas. The simulation was done with 3D finite difference (FD) modeling. Data sets with and without free surface multiples were generated, and imaging from a full azimuth acquisition geometry was compared with imaging from a conventional narrow azimuth geometry. FD modeling shows that the full azimuth design generally leads to a better suppression of noise in the data, mainly due to increased fold. Depth sl ices show that fault edges are imaged sharper in a full azimuth geometry. Also, the image of and below the salt/limestone structure is improved. However, attenuation of multiple energy due to increased cross line fold is less than expected, except for the first seabed multiple. The low maximum frequency used in FDmodeling may have limited the increase in image quality with the full azimuth modeling.