The application of three-component VSP data on the interpretation of the Vulcan Gas Field and its impact on field development

Abstract

Results of three-component Vertical Seismie Profile (VSP) surveys acquired during the initial phase of development drilling of the Vulcan Gas Field, situated in the UK sector of the Southern North Sea, are described. A main objective of the survey was to acquire high-resolution VSP seismic data along lines of varying azimuth, radiating from the wellbore. These lines would supplement the existing seismic coverage of the gasbearing Rotliegendes Sandstone reservoir and possibly highlight any potential drilling hazards or structural complexities, not previously mapped, which could have an impact on the location of later development wells. The three-component data were acquired in a near-vertical well using four orthogonally arranged, fixed-offset (2 km), energy sources. A three-component borehole geophone was used in conjunction with a gyro tool for measurement of horizontal-component geophone azimuth. The results of both conventional vertical component only and of three-component VSP data processing are compared. The impact of these data on the current interpretation of the Vulcan Field is discussed. In addition, the fixed-offset-source VSP data are directly compared with conventional (normal-incidence) VSP data subsequently acquired along deviated wells with trajectories similar to the original VSP seismie lines. It was concluded that, in this case, three-component VSP processing of the data was necessary to provide a useful sub-surface image. The fixed-offset VSP technique utilized in this case produced useful seismic data up to some distance from the wellbore and should therefore be recognized as a very useful location verification tool for future development drilling.