4D Simultaneous Inversion

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In the Gullfaks field time-lapse seismic technology plays a vital role in reservoir management. Due to increasing challenges associated with near 'end of field' lifecycle production, a quantitative approach to interpreting the time-lapse data has been adopted. This consists of a 4D simultaneous elastic inversion followed by a stochastic rock physics inversion into pore pressure and oil saturation changes (El Ouair and Stronen, 2006).

The 4D simultaneous AVO inversion method is unique and benefits from the following characteristics compared with more conventional 4D inversion algorithms and workflows:

• Independent wavelet estimations for each partial stack and each conventionally acquired survey. Because the wavelet estimations for the different seismic volumes are independent, any variations in amplitude and phase for the different seismic volumes will be captured by the wavelets.
• Inversion directly for desired rock properties such as acoustic impedance, shear impedance, Poisson's ratio, Vp/Vs and density and the corresponding changes in the desired rock properties. As 4D optimized simultaneous inversion inverts directly for the changes in rock properties, it allows for direct implementation of prior models for the changes in rock properties.
• Global optimization technique, where all partial stacks of all vintages of surveys and all samples within each partial stack are used simultaneously through the inversion algorithm. Because all seismic volumes are inverted simultaneously, the 4D optimized simultaneous inversion algorithm will find the model of the subsurface with rock properties and corresponding changes that optimally matches all partial stacks of all vintages of surveys.

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