Summary Seismic While Drilling is used effectively to identify the base of salt in a complex salt structure in order to minimize drilling risks in a highly uncertain environment.

Summary The Hejre oil and gas field is located in the Danish Central Graben, a shallow water part of the central North Sea. The reservoir lies at depths below 5 km in a high-temperature (172°C) and high-pressure (1 kbar) environment. Seismic resolution is relatively poor due to absorption in the overburden, multiples and distortion from a complex structure. Anisotropic depth processing improved the seismic quality but the anisotropy uncertainty remained high. In order to measure elastic anisotropy in-situ, reduce the velocity model uncertainties and allow improvements in anisotropic surface seismic processing, comprehensive Walkaway VSP and modern wireline sonic logging surveys were planned in a new deviated production well drilled in 2016. This project represents a first step in the velocity model calibration workflow and highlights the importance of integrating measurements taken at different scales: cores, sonic, borehole and surface seismic, in order to understand the elastic anisotropy of the rocks drilled and allow reducing the uncertainties in the seismic velocity models used for depth imaging.

Summary The Wisting discovery in the southwest Barents Sea is characterised by a shallow reservoir lying at as little as 250 m below the sea bed. At the time of this study three exploration wells had been drilled in the field. At the first two wells achieving a well tie between borehole and surface seismic measurements had proved challenging. At the latest well, the operators OMV acquired an extensive suite of borehole measurements including multi-azimuth Walk-Away VSP (WAVSP) and wireline sonic data to facilitate the well tie. Historically, borehole seismic and wireline acquired sonic data are processed separately to one another and to the surface seismic data with which they must ultimately tie. OMV chose to instead follow an integrated processing methodology with continuous calibration and validation of results across the different domains. This workflow results in consistent datasets which have been used to derive a 1D VTI elastic model of the subsurface at the well which ties all data across each domain. By processing both surface seismic and borehole data together in this manner a well tie has been achieved and a deeper understanding of key factors that influence well tie at the Wisting discovery has been gained.

The Australian Cooperative Research Centre for Greenhouse Gas Technologies (CO2CRC) is currently undertaking the Otway Project, which involves the injection and storage of 100,000 tones of carbon dioxide within the subsurface. CO2 injection will be into Naylor onshore depleted gas reservoir and, therefore, the project will provide important experience for monitoring and verification under these conditions. The overall complexities of the field, its deep, small size and in particular the presence of both free and residual gas zones present a serious challenge for time-lapse seismic monitoring. Borehole seismic has a strong advantage over surface seismic: energy crosses surface layers only once, and hence is much less sensitive to the variations in the weathered layer properties than surface seismic. Consequently a comprehensive borehole seismic observational program was designed at the Otway project. In the initial phase a Zero Offset VSP, an Offset VSP, and walkaway VSP data were acquired with a minivibroseis (6000 lb) seismic source in the Naylor-1 well in May 2006. In 2007, in the newly drilled injection well (CRC-1) a series of wireline logs, a Zero Offset, an Offset VSP and the first 3DVSP was acquired with a weight drop source simultaneously with 3D surface seismic. The results of VSP data analysis will be shown and discussed in light of its improved repeatability and image resolution in comparison to surface seismic data. We will also discuss evolving workflows developed to overcome inherently poor land seismic repeatability which is mainly related to changes in the near surface layer conditions.

The Otway Project conducted under the Australian Cooperative Research Centre for Greenhouse Gas Technologies (CO2CRC) is the first of its kind, where CO2 is injected into a depleted gas reservoir. The use of depleted fields for CO2 storage is likely to become widely adopted globally and, therefore, the project will provide important experience for monitoring under these conditions. However, such scenario is not favorable for the application of geophysical techniques for the purpose of CO2 monitoring and verification (M&V) because the injection of CO2 into a CH4 depleted reservoir is modeled to produce very subtle changes in elastic properties of the reservoir rock which may be very difficult to measure. Consequently geophysical program for the Otway site was design according to the expected time-lapse effects. It combines both surface and borehole seismic methods. Surface seismic should provide a global vision of the underground and an indirect confirmation of the CO2 containment by recording no differences between the successive time-lapse experiments. Vertical Seismic Profile (VSP) surveys are expected to provide an improved characterization of the reservoir and hopefully a direct indication of the fluid distribution and/or its potential upward migration along the reservoir bounding fault pattern. Indeed the results of the current analysis of both pre-base line (test) and base-line 2D and 3D VSP data are encouraging. The availability of vector wave field (three-component) data recorded in VSP surveys should significantly improve the outcomes of M&V program at Naylor site.

Injection of CO 2 into a depleted gas field, Otway basin, Australia, is expected to create very subtle changes in elastic properties of the reservoir. This is a serious challenge for the monitoring program at this site. Here, we perform a series of numerical experiments to evaluate the likelihood of detecting a weak 4D signal caused by CO 2 injection. We simulate seismic response changes due to variable near surface conditions. We also take into account the expected ambient noise level. To come to realistic input parameters a detailed analysis of borehole seismic data (several Vertical Seismic Profile, or VSP surveys) is performed. We then analyze the possibility of extracting 4D seismic signatures of CO 2 from the simulated low repeatability seismic data.