Rock Physics Calibration for Ensemble Based 4D Seismic History Matching

Abstract

To model the future of a complex system, it is first necessary to be able to model the past. Accurate and efficient reservoir modeling is crucial for production forecasting and reservoir development. In this work the potential of using 4D seismic data for ensemble based history matching of a reservoir model based on a North Sea oil field is studied. A general methodology for rock physics calibration for ensemble based history matching is presented. The patchy cement rock physics model is used to calculate synthetic seismic time strain values. The model is calibrated based on sensitivity studies. It is concluded that for this particular rock physics model the time strain is especially sensitive to the input parameter cement patchiness. This parameter determines the volume fraction of consolidated, pressure insensitive rock in a patchy cemented sandstone. An ensemble experiment varying this parameter between the minimum and maximum value is performed. This experiment quantifies the sensitivity and the resulting uncertainty in the time strain described by the ensemble span. The ensemble experiment reveal a broad ensemble span of time strain values for reservoir grid cells where pressure and saturation effects give significant time strain changes. However the ensemble span is in some cases not able to cover the observed time strain values. This is related to the fluid and pressure changes predicted by the reservoir model being inconsistent with what the observed time strain values are indicating. It is concluded that the cement patchiness affect the saturation effect on the compressional velocities and therefore the time strain. When a fluid substitution occurs the bulk density change and the bulk modulus change work in opposite directions. When the cement patchiness is set to high values the increase in the saturated bulk modulus make it less sensitive to fluid changes which might cause the density change to be dominant. The reservoir parameter transmissibility is varied to alter the saturation and pressure changes in the reservoir. It is established that varying both the cement patchiness and the transmissibility provide a broader ensemble span covering more of the observed time strain values. This increases the potential of getting a good 4D seismic history match for the studied area in the reservoir model. The study indicates that care must be taken in evaluating the saturation effect on synthetic time strain generated with the patchy cement model due to varying saturation effect. This research demonstrate a potential for using ensemble modeling varying only rock physics parameters to quantify how sensitive the synthetic seismic data is to the uncertain rock physics parameters. The results suggests that it can be important to include uncertain rock physics parameters in addition to reservoir parameters to be updated when performing 4D seismic history matching.