--> --> Investigation of Time-Lapse 4-D Seismic Attributes Across Discrete Frequencies to Monitor CO2-EOR in a Thin Carbonate Reservoir

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Investigation of Time-Lapse 4-D Seismic Attributes Across Discrete Frequencies to Monitor CO2-EOR in a Thin Carbonate Reservoir


Advancements, applications, and success of time-lapse (4D) seismic monitoring for carbonate reservoirs is limited by these systems' inherent heterogeneity and low compressibility. To contribute to the advancement of 4D seismic monitoring in carbonates, an investigation of seismic attributes across discrete frequencies was conducted on a high-resolution 4D seismic data set acquired in fine temporal intervals between monitor (n=8) surveys from 2003-2006 in the Hall-Gurney Field, Kansas. The shallow (approximately 900 m) Lansing-Kansas City Plattsburg ‘C Zone’ target reservoir is an oomoldic limestone which fluctuates around thin-bed thickness (approximately 5m). CO2 replacement of initial reservoir fluid generates a complex phenomenon with reduction and amplification of tuning effects at reservoir thickness above and below thin-bed thickness, respectively. Based on reservoir thickness estimations, spectral decomposition was employed to a range of isolated frequencies (50-70 Hz) expected to be susceptible to tuning effects. This included spectral ratios, normalization and differencing, and visual comparison between the monitor and baseline data. Observations of tuning effects across sub-bands of the data permitted geologic characterization in absence of well data, insight into the CO2 flood front, and validation of relative reservoir thickness based on interplay between tuning effects and reservoir thickness in zones invaded by CO2. Volume and horizon attributes, including RMS amplitude and amplitude envelope, were particularly robust in confirming subtle differences in the seismic character between surveys suggestive of CO2 fluid replacement. Investigation of seismic attributes at discrete frequencies enhanced the capacity to image and delineate the structural complexity of parabolic, sinuous, lobate, and linear ooid shoals in a depositional environment characterized by high-frequency, stacked cyclothems. Constraining these criteria through emphasis of the data's spectral content provided additional confidence in detecting CO2 migration. Detailed analysis of these 4D seismic data across discrete frequencies provide enhanced interpretation of seismic facies, identification of permeability barriers and potential thief zones, and estimation of reservoir thickness variation. These results suggest improved efficiency of CO2-EOR reservoir surveillance in carbonates, with implications to ensure optimal field planning and flood performance for analogous targets.