ANALYSIS OF TIME-LAPSE 3-D VSP DATA FOR SEISMIC MONITORING OF CO2 FLOOD IN WEYBURN FIELD, SASKATCHEWAN

Abstract

This study explores the potential of VSP data in reservoir studies and particularly in separating the effects of CO2 pore pressure and saturation after fluid injection into the reservoir. It helps establishing robust and reliable links between physical properties of hydrocarbon reservoirs and seismic data. It also reduces the uncertainties of the AVA analysis through constructing a detailed model of first-arrival amplitude decay combining geometric spreading, scattering, and inelastic dissipation. The inversion of seismic data reveals anisotropic variations of geometric attenuation (wavefront curvatures and scattering, denoted ) and the effective attenuation parameter () with depth. Statistical analysis of model uncertainties quantitatively measures the significance of these results. This model correctly predicts the observed frequency-dependent first-arrival amplitudes at all frequencies and can be used for reflected waves. Scattering and geometric spreading (focusing and defocusing of wave fronts) significantly affect seismic amplitudes at lower frequencies and shallower depths. Using of complete well logs, a model of P- and S-wave scattering is derived from direct-wave attenuation observations by numerical and analytical methods. Both approaches reveal fluctuations in the transmitted-energy flux within different depth intervals, and particularly at frequencies above 60 Hz. A randomization of well logs suggests that the upper envelope of the transmitted energy flux (corresponding to strongest transmission) is a reasonable estimate for random scattering. The lower envelope corresponding to the strongest reflectivity appears to be a useful characteristic of the fluctuations in the scattered wavefields. Once these ‘random’ and ‘fluctuation’ attenuations are modeled, they can be isolated from the intrinsic and geometric effects. Finally, three amplitude-based methods are used to separate the effects of CO2 pressure and saturation in the Weyburn reservoir. Based on these results, an area close to an injection well within the southern part of the study area is interpreted as having the highest pressure, and the area between the northern and eastern wells show the highest CO2 saturation. Near the center of the study area, the effects of CO2 saturation and pressure appear to be the weakest.