Abstract

Interpretation of time-lapse (or 4D) seismic data in terms of reservoir changes due to production posed many challenges in the Norne Field as the field experienced intense production activity from 1997 to 2006. For some segments within the field, fluid movement and pressure changes have approximately the same degree of impact and possibly opposite effects on the seismic data. Moreover, hardening anomalies could be caused by the increase in water saturation or gas going back to solution, while softening anomalies could be related to the increase in pore pressure or the decrease in fluid bulk modulus following the injection of gas. Therefore, for time-lapse seismic analysis to be most effective and less erroneous, different seismic attributes must be addressed to infer reservoir changes caused by production activity such as seismic amplitude and impedance derived by seismic inversion. In the present work, we analyze the challenges of 4D seismic interpretation in the Norne benchmark case. Our study indicates that acoustic impedance differences derived by a 4D model-based inversion provide an increase in vertical resolution compared to standard seismic amplitude differences. We also present a comparison between results of 4D model-based and colored inversions to evaluate the confidence of inversion anomalies. As this is a benchmark case, this study can be considered to enrich the discussions over qualitative and quantitative time-lapse seismic interpretation and to improve reservoir characterization.

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