ABSTRACT

We find that conventional time-lapse seismic pressure-saturation discrimination methods become unstable for high VP/VS ratios. Using first-order approximations in the amplitude variation with offset (AVO) gradient and intercept changes and in the rock-physics models increase the inaccuracy in pressure-saturation changes estimates. We propose a new method, based on a stepwise linear approximation to the intercept and gradient reflectivity changes, to estimate pressure and saturation changes. The applicability of the new method is tested on synthetic data over a range of 0%–50% gas saturation and 0–3.5 MPa pore-pressure changes. The new method is more consistent and provides better estimates compared with conventional methods. In the presence of random noise (up to 15%), the estimates are reasonably better for the new method. We use this new method to investigate the feasibility of pressure-saturation discrimination for a shallow unconsolidated sand reservoir in which an underground blowout occurred in 1989. We analyze a sand layer at 490-m depth that was charged by gas due to the blowout. Because of the shallow depth, the VP/VS is expected to be higher than 2 before the blowout. Near- and far-offset time-lapse seismic data sets from 1988 and 1990 are used as input to estimate changes in AVO intercept and gradient and then changes in pressure and saturation. We find that the new method estimates more realistic pressure-saturation changes compared to the conventional one.

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