Abstract

Time-lapse seismic has established itself as a valuable tool to monitor dynamic changes in oil and gas reservoirs. These changes occur continuously in a reservoir due to production and/or injection operations and are manifested broadly in pore pressure and also water saturation changes. These changes might occur either independently or concurrently and affect seismic wave velocities. Therefore, this can be studied by 3D seismic surveys from different times or 4D seismic. The change in seismic response can be large or subtle depending on the magnitude of dynamic reservoir changes. Time-lapse feasibility modeling is the practice that realizes whether a detectable 4D signal can be captured or not. In this research, a 4D seismic feasibility model has been developed to assess the possibility of obtaining a 4D signal in an oil zone located in the Malaysian basins. This model is based on different production and water injection scenarios and a new rock physics workflow. Gassmann fluid substitution has been used to model the effect of different water saturation changes on seismic velocities. For pore pressure changes, Hertz-Mindlin geo-mechanical model in conjunction with pore space stiffness theory and the constant ratio approach for sandstone is used to simulate the effect of various pressure changes on seismic responses. This 4D feasibility model is followed by amplitude versus offset (AVO) modeling to exam the sensitivity of AVO intercept and gradient attributes resulting from different injection and production scenarios. In 1999, the AVO analysis method was presented to estimate fluid saturation and pressure changes from time lapse seismic data. This research has tested the AVO method in an oil zone located in the Malaysian basins. Finally, it is shown that AVO intercept and gradient cross plot is capable of distinguishing between water saturation and pore pressure changes in this zone. Therefore, this can be generalized in the Malaysian basins, where the AVO method can be used to differentiate between pore pressure and water saturation changes from 4D seismic data.

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