4D Petrophysical Seismic Inversion - Case Studies

Statoil and CGGVeritas have developed a methodology to directly follow fluid and pressure changes across seismic vintages. During this R&D collaboration program on 4D Petrophysical Seismic Inversion, we have accumulated experience, and some of the results are presented here, extracted from two case studies on different North Sea fields: Troll West and Snorre. These mature offshore fields consist of sandstone reservoirs divided in large rotated fault blocks monitored by several seismic vintages. In both cases, the 4D seismic signal is strongly impacted by gas appearing in the oil leg and below the OWC. It is gas out of solution on Troll West and the injected gas on Snorre. There are significant differences between the fields in terms of reservoir depth, heterogeneities of the reservoir quality as well as the amount of pressure changes. On Troll West, only a limited and monotonous pressure drop has occurred, while on Snorre, significant and potentially compartmentalized pressure changes have taken place during production with gas, water and WAG injection, during the last decade. Setting constraints on dynamic property behaviour is a critical aspect for obtaining quantitative estimates. A comparison of implemented constraints and of results obtained in both cases is made.