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Studying Seismic Signatures of Fluid Substitution in Reservoir Characterization

Abstract

Fluid substitution analysis is an integral part of seismic rock physics studies, which are efficient tools for reservoir identification and their quantification. Many theories for such fluid substitution processes have been formulated and discussed in terms of their advantages and limitations. In this work, we formulate the Gassmann theory for fluid substitution into a computational framework, and use it to analyze the effect of fluid substitution in a typical reservoir and the seismic signature study for various cases of fluid substitution. Initially, fluid replacement was done to obtain 100% water, oil, gas case. For more realistic case, we assumed (1) Gas reservoir with initial water saturation 0.2 and gradually depleted to 20% gas saturation (Sw=.8) and (2) Oil reservoir with initially Sw=0.2 to final water saturation =0.8. We compute synthetic seismogram using the modified (after fluid substitution) values of elastic parameters (Vp, Vs, Density). We further analyze AVO (Amplitude verses offset) to estimate the feasibility of reservoir characterization by seismic Inversion. To study the lithology influence on physical rock property several rock physics relationships were cross plotted for insitu reservoir fluids as well as for the perturbed fluid saturation after fluid replacement. In order to test the approach, we perform a similar analysis on a real borehole data. This study illustrates how rock and fluid property along with seismic amplitude can be used to estimate fluid type in a reservoir.