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FEASIBILITY STUDY OF THE GRAVIMETRY DETECTION OF LARGE-SCALE BYPASSED OIL AREAS IN A RESERVOIR, USING NUMERICAL MODELLING
Abstract
Estimation of the Remaining Oil Saturation (ROS, the fraction of pore volume occupied by oil at a given location and time) in a reservoir is one of the key processes in oil field monitoring and exploitation. Because of the various geological, structural, and chemical heterogeneities that exist within the formations composing an oil field, the evolution of ROS as the reservoir gets drained will be heterogeneous as well. Such discrepancies can occur at different scales, from the microscopic (pore size) to the macroscopic (hundreds of meters). In the case of macroscopic heterogeneities, regions with low porosity can divert the flow of injected fluid, and create “shadow zones” where large volumes of oil can get bypassed. Knowing the locations of such zones is of utmost importance in order to better plan for enhanced oil recovery, during or after the field exploitation. The possibility of detecting such zones using gravimetry methods is investigated here. This method has the advantage of being fast, and easy to deploy over large areas. To do so, we compute the gravity anomaly generated by a modelled reservoir, before and after exploitation. “Shadow zones” of unaccessed oil, of varying width and thickness, are added to the model at various depths. The reservoir model itself produces anomalies of the order of magnitude 2*10-2 mGal. Time-lapse measurements, measuring the difference between anomalies caused by the oil filled reservoir and drained reservoir, ranges from 0 up to intensities of a few ?Gal. The anomaly variations produced by the shadow zones alone vary depending on the chosen parameters. We discuss the possibility to detect the gravity variations generated by different amounts of bypassed oil, and try to explore the parameters required for this method to function.
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