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Fig. 4. Coarse initial (left) and higher resolution (right) acoustic impedance modelsfor the carbonate reservoir (High=Blue; Low=Red).passing to wakestones and mudstones towards the south. Theoverall trends of the rocks can be seen in the seismic attributes.The reservoir coarsens upwards to the north and becomes finetextured towards the south. Dolomitization and other diageneticeffects have caused the reservoir to become less permeableon the flanks. The formation facies vary from peloidalskeletal mudstones to packstones. Variography was performedand the direction of anisotropy (i.e., trend) of the carbonateplatform bodies was identified following a general 120° azimuth.The extensive variability of the reservoir rocks and thediagenetic effects required a careful nonstationary model. Theseismic AI data was incorporated to handle this otherwise unpredictableheterogeneity.A seismic AI volume is of fairly low resolution, Fig. 3. Theobjective of the study was to improve the high-resolution spatialmodel distribution of porosity by integrating the negativelycorrelated seismic AI. Downscaling and integration were appliedafter the sampled point set (as previously explained) was combinedwith the synthetic log AI from the wells. Figure 4 showsthe downscaled acoustic impedance. The initial porosity modelwithout the seismic acoustic impedance data is too continuouslaterally, and could not represent the diagenesis effect. Thedownscaling prior to integration methodology allowed for amore realistic porosity model to be generated. A secondarytrend is extracted at the crest of the anticline from the acousticimpedance. This trend cannot be explained by carbonate deposition,but may be due to deterioration of the rock quality atthe flanks of the anticline due to diagenesis, Fig. 5. Figure 6shows examples of the coarse and fine resolution acoustic impedancefollowed by the porosity model from the integrationof data. The results were checked to ensure the data was properlymatching the spatial location and first and second orderparameters. In addition, history matching was performed (notshown here) in which the contribution of the integration ofdata was absolutely necessary.Fig. 6. Coarse (left) and downscaled (center) acoustic impedance models(High=Blue; Low=Red) and a porosity model (right) for the carbonate reservoir(High=Reddish/Yellow).Clastic Reservoir ExampleIn the second example, an offshore clastic reservoir model wasinitially constructed conditional to hundreds of wells. Therocks consist of a thicker main sand zone overlain by sandstonestringers intercalated with shales. Intermediate rockqualities are shaley sands and sandy shales, which are part offining upward sequences in tidal and distributary channels.The main sand is made up of staked fining upward sands.Some coarsening upward features appear as isolated sand bars.The challenge in this reservoir is to develop production in theupper stringer sands, which are less likely to be intercepted byvertical wells. The development strategy is therefore to drill deviatedwells to intersect the sand stringers, then plug back anddrill a horizontal production section along the stringers, withcompletion at MRC.Additional production drilling required the placement ofplatforms selected from the global reservoir model. The initialareal resolution of cells was 125 x 125 m 2 , and the vertical resolutionwas approximately 2 ft on average. The goal was toconstruct a detailed reservoir model with a 50 x 50 m 2 cell size.The model should contain the same heterogeneities as in theprior coarser well only model, including faults and stratigraphy.The initial efforts showed that downscaling using collocatedtechniques was disappointing; the final model showed blockyartifacts (i.e., too similar properties in neighboring cells) conformingto the coarse voxet and ignoring the new higher resolutiongrid. One of the reasons for the artifacts was that thecorrelation between the prior and the posterior models had tobe kept significant (as shown in the data) to assure that theflux boundary conditions from the coarser resolution model,to be applied during dynamic forecast, would still be valid forthe downscaled versions of the model.After the construction of high-resolution grids, the rockproperties were downscaled from the original coarse resolutionFig. 5. Coarse (left) and downscaled (center) acoustic impedance models, and a high resolution (right) porosity models (High=Reddish) for the carbonate reservoir.66 WINTER 2011 SAUDI ARAMCO JOURNAL OF TECHNOLOGY