Occidental is conducting a geophysical monitoring program to aid optimization of production from a<br>heavy oil field in central Oman. The Permian age Gharif reservoir consists of three stacked sandstone<br>units spread over a gross interval of about 50 meters with average porosities of 30%. Oil recovery is<br>stimulated by steam injection into each of the three reservoirs to lower oil viscosity. Steam injection<br>and production alter reservoir properties such as temperature, pressure and saturation. A 4D modeling<br>study was carried out to investigate the impact of these reservoir changes on compressibility and<br>rigidity of the rocks. Synthetic seismic models were derived from our understanding of the reservoir<br>rock properties combined with the history matched reservoir models. The modeling predicts a change<br>in the reservoir interval velocities of approximately 10-15%, resulting in a change in acoustic<br>impedance that should be large enough to observe in surface seismic data. Modeling also suggests that<br>changes in the reservoir properties will be localized close to the steam injectors and that these<br>anomalies could be strong enough to identify in surface seismic without time lapse differencing. A<br>crosswell tomography survey was acquired through a well that injects steam into all three reservoirs.<br>The cross well survey confirms a reduction in reservoir interval velocity by 10% associated with the<br>steam injection. Comparison of the crosswell tomography cross section with the equivalent predicted<br>velocity section from the reservoir simulation highlights differences between the reservoir simulation<br>prediction and how steam is actually affecting the reservoir. Petrophysical and production surveillance<br>data have helped our understanding of these differences. The crosswell tomography helped in<br>assessing the connectivity vertically between the three reservoirs and horizontally between the study<br>wells as the resolution is higher in the cross well than in the surface seismic. The crosswell tomography<br>and modeled seismic response also serve to calibrate the surface seismic response which is needed to<br>highlight field wide lateral reservoir changes. Information gained from geophysical monitoring that is<br>correlated with observation and production data is important for monitoring steam movement in the<br>subsurface and optimizing field production.


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