4D Seismic on Gullfaks

SUMMARY New technologies are rapidly emerging helping to obtain optimal drainage of large reservoirs. 4D seismic is such a reservoir monitoring technique. The physical basis for the technique is to do repeated seismic measurements over a reservoir in production. The changes in the formation response can then be attributed to changes in the reservoir i.e. fluid saturation changes in the reservoir pore space. The implementation of 4D seismic includes developments of new tools to quantify the repeatability of the seismic experiments and new tools for analysing 4D seismic data. In 1995-1996 a 4D seismic study is being done over the northern part of the Gullfaks field in the North Sea. More than 50% of the estimated recoverable reserves in Gullfaks has been produced since the start-up in December 1986. The reservoir sands are Jurassic, shallow marine to fluvial sediments of the Brent group, Cook Formation and the Statfjord Formation. The structural setting of the field is very complex, representing the main factor of uncertainty in the spatial drainage of the field. The major objective of this study is to identify potentially undrained reservoir compartments. In addition to the seismic measurements extensive logging of the saturation distribution in the wells was performed at the time of the seismic experiment. 4D seismic results are dependent on the precision with which rock and fluid parameters changes can be estimated through time. The precise seismic data interpretation (flow barriers) at the reservoir scale requires new tools for accurate and rapid mapping of main and subtle faults. It also requires an extended set of seismic attributes for surface and volume seismic facies mapping and a classification tool able to integrate seismic and welllog data and establish relationships between petrophysical and seismic parameters. The calibration of the seismic data with the well log parameters is established by calculating all seismic attributes along the well log synthetic traces. Simulation of expected saturation responses are done on well-logs and transformed to seismic responses. This is done by perturbating the fluid-saturation on welllogs and produce the synthetic seismic response resulting from that model perturbation. CALIBRATION OF SEISMIC DATA Repeatability is the keyword for seismic reservoir monitoring. Every step in acquisition, processing, interpretation and modelling of a target area must be repeated from time lapse to time lapse to producerelyable results. Therefore, measurements must be calibrated between time lapses to compensate for inaccuracies. Calibration of seismic data consists of:raw pre-stack and post-stack migrated seismic data calibration by using 3D matching filters in order to obtain uniform response. Areas where no changes are expected (undrained reservoir or overburden) can be used as references for calibration.overburden calibration applied to the entire survey to obtain uniform illumination of the reservoir zone.quantitative calibration of seismic attributes to well log parameters through consistent well ties