Conventional processing of seismic time-lapse data is very valuable in gaining qualitative<br>insights into reservoir history. For example, this allows characterization of fluid front<br>displacements, identification of fluid migration pathway or detection of flow barriers and<br>compartments. Quantitative analyses are generally based on Amplitude Versus Offset (AVO)<br>and attribute generation techniques. However, AVO is fundamentally a 1D approach and is<br>generally applied using linear approximations. Integrating time-lapse seismic into historymatching<br>is another way to obtain quantitative conclusions. Nevertheless, in this case, seismic<br>forward modeling is often based on a simple 1D convolution model and a too large<br>computational load precludes any proper optimization loop. To overcome such limitations, we<br>propose to apply 2D elastic full waveform inversion to time-lapse seismic data. The method is<br>computer intensive but allows modeling the different propagation modes (reflections, wide<br>angles, multiples, converted) to achieve a rigorous non-linear inversion of the seismic data.<br>The approach is applied to reflection time-lapse data from monitoring surveys of the Sleipner<br>CO2 injection site, North Sea. CO2 separated from methane is injected into the Utsira Sands,<br>a deep saline aquifer. Inverted P-wave velocity variations are related to CO2 saturation using<br>Gassmann theory. Since gas injection into a water bearing formation is a drainage process,<br>saturation is likely to be patchy. Consequently, we also used the patchy Vp-saturation<br>relationship to determine the maximum possible saturations. Investigations about the level of<br>patchiness, with respect to the frequency bandwidth used for the inversion, is required to<br>determine the most likely CO2 saturation.


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