Santi Randazzo

Ultra-deepwater 4-C offshore Brazil

This paper is the case history of an ultra-deepwater 4-C seismic program, recently acquired in the Campos and Santos basins, offshore Brazil. All aspects of the program will be discussed, including the feasibility study which first indicated the potential value of multicomponent technology to this region, to the survey objectives, to the illumination analysis used in designing the survey, to the field operations, to the fully processed results. Acquisition was completed on 19 April 2005, and the fully processed data were delivered in early November 2005.

Reverse Time Migration of P-wave and C-wave Data from a 3D VSP over a Deep Subsalt Prospect in the Gulf of Mexico

Summary Difficulties in imaging subsalt data in the Gulf of Mexico are well documented. Even with the lates t wide azimuth acquisition technology the quality of imaging subsalt is quite variable. 3D VSPs offer the potential to improve reservoir imaging as compared to surface data and to calibrate the reservoir to the surface seismic. The major drawbacks to 3D VSPs are that they are quite expensive due to the rig time required and by their very nature offer a narrow illumination window around the wellbore. To extract more information from 3D VSPs than standard Pwave processing two avenues were investigated: 1) The utilization of the reverse time migration (RTM) imaging algorithm which offers the possibility to increase the lateral coverage of 3D VSPs because it honors two-way propagation and all multiples in the model. 2) Processing for converted wave modes of energy.

Imaging and Characterization with PP and PS A feasibility study from the North Sea

The first step in successful application 4C technology is a feasibility study. The current study was done on the Sleipner field in the North Sea. The objective of the data processing and analysis was to investigate how 4C data can be used to improve imaging of structural geometry and aid in fluid and lithology discrimination. Then demonstrate that it can be achieved in a limited sense from existing 2D4C data. We emphasized in particular how 4C data can help delineate the Ty formation. The field has a gas saturated sandstone reservoir which is difficult to map with the 3D streamer data. The study was split into two main components; One modeling and analysis part based on well data and a one processing and interpretation part based on the 2D4C data available. The Ty formation was better imaged by converted wave data as suggested by the modelling. It was also concluded from the modelling that there is good separation between the sand and the surrounding shales in terms of Vp/Vs values suggesting that combined PP and PS data analysis should enable lithology discrimination. Well logs also suggest that Vp/Vs can be used to discriminate between water and gas saturated Ty sand. Well logs and modeling - Methodology In this study we had access to a few typical wells in the area. Well 15/9-26A coincided with the 2D4C line but did not have shear information whereas 15/9-D-1H had shear logs. The well log data available was used to cross-plot observations and also as a basis for forward modeling of PP and PS response. Since shear wave data was not available from all wells it was estimated based on the Krief approach (See Krief et.al 1990, Hilterman 1987 and Biot 1941). The mineral properties and the estimated shear wave response was calibrated in a well with shear observations. The synthetics were generated by doing Zoeppritz modeling based on calculated impedances and density maintaining well log resolution. The PP wavelet was derived from the seismic data and the PS wavelet was derived from the same with a stretch equivalent to a Vp/Vs ratio of 2. Well logs and modeling – Results Figure 1, shows the Vp/Vs vs. depth as a Vp/Vs semblance log plot, for the Well 15/9-D-1H, sampled over a 15 m interval. The plot highlights the separation between hydrocarbon sands and shales/wet sands along the Vp/Vs axis. Note the good separation between the gas-saturated sands (Ty) and the shales (Lista). Figure 2 shows the Vp/Vs versus porosity cross-plot for both wells (D-1H, 26-A) only for the Ty formation interval with the Z-axis as volumes of shale. The Vertical axis is indicating in colour the range for the three formations (Lista, Ty brine and Ty Gas). The synthetic PP and PS (Figures 3) show a stronger PS stack response than that of PP for the Ty formation.