Effect of Petrophysical Parameters on Seismic Waveform Signatures : Review of Theory with Case Study from Frigg Delta Oil Field, Norway

University essay from Uppsala universitet/Geofysik

Abstract: Conventional AVO analysis has been used for the past 4 decades to aid in locating oil and gas reservoirs for extraction. It is, however, not possible to use it to acquire information on the porosity of the rock, the fluid saturation or other important petrophysical parameters. In this thesis, I study the effects of attenuation on seismic waveform signatures, due to wave induced fluid-flow. In the first part of the thesis, 2 models were used to synthetically model the attenuation caused by the wave induced fluid-flow: White's model and the double-porosity dual-permeability (DPDP) model. The parameters used for modeling were both synthetic and acquired from real well data of a known oil field off the coast of Norway. White's model was found to model relatively high attenuation (5%) for intermediately consolidated gas reservoirs while oil saturated intermediately consolidated reservoirs showed such low attenuation (0.3%) that it is easy to say that for the real-world situation it would not be detected. The DPDP model seemed to be able to better describe the attenuation and gave attenuations up to 10% for an intermediately consolidated oil reservoir, but due to lack of parameters from well data it was not sufficiently able to model the real-world situation. The synthetic data, however, show interesting characteristics and it is therefore recommended that more and detailed well parameters be acquired if the research should continue. For the second part, Svenska Petroleum Exploration AB and Det Norske Oljeselskap ASA provided stacked seismic data that were spectrally analyzed for hints of attenuation variation with frequency (using Fourier Transform and Complex Spectral Decomposition). Twelve locations, on the stacked seismic cube, were analyzed; six oil saturated; and six (assumed) water saturated. At each location, a main trace was selected along with the two nearby traces on each side of it (five in total). The Complex Spectral Decomposition method seemed unable to correctly break down the stacked section's signal, which is why Fourier Transform was used for further analysis. The frequency analysis showed a peak at ~30 Hz for both oil and water saturated reservoirs which seems like a characteristic frequency of the source, but that was unfortunately not confirmed and not enough time was available to test the assumption. The Fourier transform seems to show some difference between oil and water saturated traces, but that could well be because of lithological differences and not the pore fluid. It is therefore recommended, if research is to be continued, that 4D seismic data is used to analyze the same location with respect to time. It is also recommended that pre-stack or shot data be used as information is lost in stacked data.

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