Well-seismic bandwidth and time-lapse seismic characterization related to CO₂ injection and fluid substitution: physical considerations

Yinbin Liu, Donald C. Lawton

Reservoirs are commonly heterogeneous. Injection of CO₂ (or other fluids) related to enhanced oil recovery (EOR) operations may cause strong lateral and depth-dependent changes of heterogeneity both within the reservoir and in the surrounding formations. A seismic signal propagating through the reservoir and the surrounding formations before and after the injection undergoes different velocity dispersion and amplitude attenuation, which result in time shifts and waveform distortion. This paper discusses the physical aspects of well log integration with seismic and time-lapse seismic characterization based on a thinly layered model (1D heterogeneity). The results show that discrete layering and interval multiple reflections (or scattering) within sedimentary sequences have a significant influence on synthetic seismograms. The velocity and density perturbations inside and outside the reservoir will mainly result in the time-lapse amplitude anomaly at the top of the reservoir and the local coda wave distortion from near the top of the reservoir to the strong basal reflection below the reservoir (BBR). The distortion of the coda wave is highly dependent on the magnitudes of medium perturbations. Large perturbations may cause time sag for the basal reflection as well as later events, which mainly include primary reflections. Those results have important implications for timelapse seismic monitoring.