Rock Physics Calibration for Ensemble Based 4D Seismic History Matching
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To model the future of a complex system, it is first necessary to be able to model thepast. Accurate and efficient reservoir modeling is crucial for production forecastingand reservoir development. In this work the potential of using 4D seismic data forensemble based history matching of a reservoir model based on a North Sea oil fieldis studied.A general methodology for rock physics calibration for ensemble based historymatching is presented. The patchy cement rock physics model is used to calculatesynthetic seismic time strain values. The model is calibrated based on sensitivitystudies. It is concluded that for this particular rock physics model the time strainis especially sensitive to the input parameter cement patchiness. This parameterdetermines the volume fraction of consolidated, pressure insensitive rock in a patchycemented sandstone. An ensemble experiment varying this parameter between theminimum and maximum value is performed. This experiment quantifies the sensitivityand the resulting uncertainty in the time strain described by the ensemblespan. The ensemble experiment reveal a broad ensemble span of time strain valuesfor reservoir grid cells where pressure and saturation effects give significant timestrain changes. However the ensemble span is in some cases not able to cover theobserved time strain values. This is related to the fluid and pressure changes predictedby the reservoir model being inconsistent with what the observed time strainvalues are indicating. It is concluded that the cement patchiness affect the saturationeffect on the compressional velocities and therefore the time strain. When afluid substitution occurs the bulk density change and the bulk modulus change workin opposite directions. When the cement patchiness is set to high values the increasein the saturated bulk modulus make it less sensitive to fluid changes which mightcause the density change to be dominant. The reservoir parameter transmissibility is varied to alter the saturation and pressure changes in the reservoir. It is establishedthat varying both the cement patchiness and the transmissibility provide a broaderensemble span covering more of the observed time strain values. This increases thepotential of getting a good 4D seismic history match for the studied area in thereservoir model.The study indicates that care must be taken in evaluating the saturation effecton synthetic time strain generated with the patchy cement model due to varyingsaturation effect. This research demonstrate a potential for using ensemble modelingvarying only rock physics parameters to quantify how sensitive the synthetic seismicdata is to the uncertain rock physics parameters. The results suggests that it canbe important to include uncertain rock physics parameters in addition to reservoirparameters to be updated when performing 4D seismic history matching.