Crosswell Electromagnetic (EM) tomography is a recently developed technique to map the interwell<br>formation resistivity distribution. In this project, time-lapse crosswell EM surveys are used to monitor<br>saturation changes in a water injection pilot in the basal low-reservoir quality units of a giant carbonate<br>field in the Middle East. We demonstrate how EM results were used with seismic-derived structural<br>information, geological data, structural models, time-lapse cased hole logs, pressure transient and<br>production data to improve reservoir characterization and dynamic modeling.<br>The evolution of water saturation is derived from measured resistivity distributions, which are obtained<br>by inversion of the EM measurements with respect to an initial resistivity model. To obtain a detailed<br>image of saturation changes, this model must incorporate realistic representations of the small-scale<br>heterogeneities common to carbonate reservoirs. These include layer thickness variations and the<br>presence of thin dense layers interbedded in some reservoir units. Such details allowed improvements<br>in the quality and resolution of the EM results, leading to a better understanding of the reservoir<br>architecture and the behavior of the water flooding process.<br>Preliminary simulation results using simplified models predicted high vertical sweep across the<br>reservoir units, without encountering any flow barriers. This was inconsistent with the EM results,<br>which show that the injected water stays confined within the lower reservoir units, and the measured<br>injection pressures and flow volumes, which were different from those predicted from simulation.<br>Successive adjustments were therefore applied on the dynamic model to honor the EM and injection<br>pressure results. Adjusting the permeability contrast across some layers prevents the upward<br>movement of the injected water, and is consistent with geological interpretation of continuous stylolitic<br>dense layers within the lower reservoir. In addition, fracture corridors, identified on seismic attributes<br>and supported by PLT data and field-wide review of borehole image logs, are used to account for the<br>injected volume mismatch, yielding the correct injection pressure.<br>When properly constrained with seismic, geological and production data the EM results provide<br>important information on the location and behavior of the fluid front and identification of the required<br>amount of geological detail that needs to be preserved in the dynamic model.


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