Techniques and Experiences Using Exploration 3D Seismic Data to Map Drilling Hazards

Abstract

Abstract This paper will review Statoil's experiences and techniques applied to the mapping of drilling hazards by using exploration 3D seismic data. During the period since first implementing thus approach to shallow gas evaluations, in 1989, the methodology has continued to develop. Attribute mapping techniques have improved the identification of shallow gas and shallow gm traps. Seabed topography may also be mapped using 3D data and attribute analysis. The observation of gas migration through the sediments has significance for the understanding of potential shallow gas accumulations. and different attributes are used to aid in identifying possible gas migration paths. The use of 3D data, modern interpretation techniques and correlation to an extensive well data base, covered by the 3D data, has led to a better understanding of the shallow geology and result in rapid and precise geohazards evaluations. Costs are reduced by minimizing the standard site survey acquisition programmed. and the high degree of accuracy of the predictions has consequences for well planning. leading to safer and less expensive drilling. Introduction The technique of using exploration 3D seismic data for detection of shallow gas was pioneered by Statoil during a shallow gas project lasting from 1986 to 1989'. Shallow gas evaluation based on 3D data has since been a standard procedure in Statoil, The technique has, to date, been performed for more than 50 Iocation, i.e. exploration wells, production wells and platform sites, About 150 wells have been used as correlation wells, Early results from Statoil soon encouraged other oil companies to try the same methods, which has had a major impact for site survey contractors who have since had to modify their services offered to accommodate thechanging technology. Later, the Norwegian authorities changed their requirements for shallow gas investigations accordingly to include the use of exploration 3D seismic data where available. The most significant aspect of using 3D data to map drilling hazards is the use of horizontal displays (time slices attribute maps), leading to a better understanding of shallow gas settings and greater reliability of predictions which SUICCimplemented has had an accuracy close to 100 %0. Cost savings by reducing the site survey programmes arc significant. In addition considerable amounts arc saved in the drilling operations due to the accuracy of the predictions which has a direct impact on well planning. Interpretation Techniques The method proposed by Gallagher et al,, 1989' and 199 I', was to perform shallow gas evaluations using exploration 3D data loaded to an interpretation station. The most important advantage of this method was the possibility of using time slices. which recvcled shallow gas traps, in particular buried iceberg ploughmarks.